arXiv daily: Optics (physics.optics)

Authors:Rebecca Heilmann, Kristian Arjas, Tommi K. Hakala, Päivi Törmä

Abstract: Multicolour light sources can be used in applications such as lighting and multiplexing signals. In photonic and plasmonic systems, one way to achieve multicolour light is via multi-mode lasing. To achieve this, plasmonic nanoparticle arrays are typically arranged in superlattices that lead to multiple dispersions of the single arrays coupled via the superlattice Bragg modes. Here, we show an alternative way to enable multi-mode lasing in plasmonic nanoparticle arrays. We design a supercell in a square lattice by leaving part of the lattice sites empty. This results in multiple dispersive branches caused by the supercell period and hence creates additional band edges that can support lasing. We experimentally demonstrate multi-mode lasing in such a supercell array. Furthermore, we identify the lasing modes by calculating the dispersion by combining the structure factor of the array design with an empty lattice approximation. We conclude that the lasing modes are the 74th $\Gamma$- and 106th $X$-point of the supercell. By tuning the square lattice period with respect to the gain emission we can control the modes that lase. Finally, we show that the lasing modes exhibit a combination of transverse electric and transverse magnetic mode characteristics in polarization resolved measurements.

Authors:Olivia Y. Long, Cheng Guo, Shanhui Fan

Abstract: In photonics, band degeneracies at high-symmetry points in wavevector space have been shown to exhibit rich physical phenomena. However, obtaining degenerate bands away from such points is highly nontrivial. In this work, we achieve complex band degeneracy in a photonic crystal structure over a region of momentum space. We show that this band degeneracy corresponds to polarization-independent transmission, which can be harnessed for nonlocal metasurface design. Moreover, we find that the band degeneracy manifests as a topological singularity in the structural parameter space of the system. Our work highlights the importance of topological concepts in the design of polarization-independent photonic structures.

Authors:René Barczyk, L. Kuipers, Ewold Verhagen

Abstract: The control over light propagation and localization in photonic crystals offers wide applications from sensing and on-chip routing to lasing and quantum light-matter interfaces. While in electronic crystals magnetic fields can be used to induce a multitude of unique phenomena, the uncharged nature of photons necessitates alternative approaches to bring about similar control over photons at the nanoscale. Here, we experimentally realize pseudomagnetic fields in two-dimensional photonic crystals through engineered strain of the lattice. Analogous to strained graphene, this induces flat-band Landau levels at discrete energies. We study the spatial and spectral properties of these states in silicon photonic crystals at telecom wavelengths with far-field spectroscopy. Moreover, taking advantage of the photonic crystal's design freedom, we realize domains of opposite pseudomagnetic field and observe topological edge states at their interface. We reveal that the strain-induced states can achieve remarkably high quality factors despite being phase-matched to the radiation continuum. Together with the high density of states and high degeneracy associated with flat bands, this provides powerful prospects for enhancing light-matter interactions, and demonstrates a design principle to govern both on-chip and radiating light fields.

Authors:Mark Dong, Julia M. Boyle, Kevin J. Palm, Matthew Zimmermann, Alex Witte, Andrew J. Leenheer, Daniel Dominguez, Gerald Gilbert, Matt Eichenfield, Dirk Englund

Abstract: Programmable photonic integrated circuits (PICs) are emerging as powerful tools for the precise manipulation of light, with applications in quantum information processing, optical range finding, and artificial intelligence. The leading architecture for programmable PICs is the mesh of Mach-Zehnder interferometers (MZIs) embedded with reconfigurable optical phase shifters. Low-power implementations of these PICs involve micromechanical structures driven capacitively or piezoelectrically but are limited in modulation bandwidth by mechanical resonances and high operating voltages. However, circuits designed to operate exclusively at these mechanical resonances would reduce the necessary driving voltage from resonantly enhanced modulation as well as maintaining high actuation speeds. Here we introduce a synchronous, micromechanically resonant design architecture for programmable PICs, which exploits micromechanical eigenmodes for modulation enhancement. This approach combines high-frequency mechanical resonances and optically broadband phase shifters to increase the modulation response on the order of the mechanical quality factor $Q_m$, thereby reducing the PIC's power consumption, voltage-loss product, and footprint. The architecture is useful for broadly applicable circuits such as optical phased arrays, $1$ x $N$, and $N$ x $N$ photonic switches. We report a proof-of-principle programmable 1 x 8 switch with piezoelectric phase shifters at specifically targeted mechanical eigenfrequencies, showing a full switching cycle of all eight channels spaced by approximately 11 ns and operating at >3x average modulation enhancement across all on-chip modulators. By further leveraging micromechanical devices with high $Q_m$, which can exceed 1 million, our design architecture should enable a new class of low-voltage and high-speed programmable PICs.

Authors:S. V. U. Vedhanth, Shouvik Datta

Abstract: Momentum space distributions of photons coming out of any light emitting materials/devices provide critical information about its underlying physical origin. Conventional methods of determining such properties impose specific instrumentational difficulties for probing samples kept within a low temperature cryostat. There were past studies to measure one dimensional (1D) coherence function which could then be used for extracting momentum space information as well as reports of measurements of just two dimensional (2D) coherence function. However, all of those are associated with additional experimental complexities. So, here we propose a simpler, modified Michelson interferometer based optical setup kept at room temperature outside the cryostat to initially measure the 2D coherence function of emitted light, which can then be used to directly estimate the 2D in-plane momentum space distribution by calculating its fast Fourier transform. We will also discuss how this experimental method can overcome instrumentational difficulties encountered in past studies.

Authors:Tianyuan Liu, Wei Yan, Min Qiu

Abstract: Breaking parity (P) symmetry in C6 symmetric crystals is a common routine to implement a valley-topological phase. At an interface between two crystals of opposite valley phases, the so-called valley topological edge states emerge, and they have been proven useful for wave transport with robustness against 120 degree bending and a certain level of disorder. However, whether these attractive transport features are bound with the valley topology or due to topological-irrelevant mechanisms remains unclear. In this letter, we discuss this question by examining transport properties of photonic edge states with varied degrees of the P-breaking that tune the valley topology, and reveal that the edge states preserve their transport robustness insensitive to the topology even when the P-symmetry is recovered. Instead, a unique modal character of the edge states--with localized momentum hotspots around high-symmetric K (K') points--is recognized to play the key role, which only concerns the existence of the valleys in the bulk band structures, and has no special requirement on the topology. The "non-topological" notion of valley edge states is introduced to conceptualize this modal character, leading to a coherent understanding of bending immunity in a range of edge modes implemented in C3 symmetric crystals--such as valley topological edge states, topological edge states of 2D Zak phase, topological-trivial edge states and so on--, and to new designs in general rhombic lattices--with exemplified bending angle as large as 150 degree.

Authors:Kevin C. Zhou, Mark Harfouche, Maxwell Zheng, Joakim Jönsson, Kyung Chul Lee, Ron Appel, Paul Reamey, Thomas Doman, Veton Saliu, Gregor Horstmeyer, Roarke Horstmeyer

Abstract: We present a large-scale computational 3D topographic microscope that enables 6-gigapixel profilometric 3D imaging at micron-scale resolution across $>$110 cm$^2$ areas over multi-millimeter axial ranges. Our computational microscope, termed STARCAM (Scanning Topographic All-in-focus Reconstruction with a Computational Array Microscope), features a parallelized, 54-camera architecture with 3-axis translation to capture, for each sample of interest, a multi-dimensional, 2.1-terabyte (TB) dataset, consisting of a total of 224,640 9.4-megapixel images. We developed a self-supervised neural network-based algorithm for 3D reconstruction and stitching that jointly estimates an all-in-focus photometric composite and 3D height map across the entire field of view, using multi-view stereo information and image sharpness as a focal metric. The memory-efficient, compressed differentiable representation offered by the neural network effectively enables joint participation of the entire multi-TB dataset during the reconstruction process. To demonstrate the broad utility of our new computational microscope, we applied STARCAM to a variety of decimeter-scale objects, with applications ranging from cultural heritage to industrial inspection.

Authors:Shuiying Xiang, Yuechun Shi, Yahui Zhang, Xingxing Guo, Ling Zheng, Yanan Han, Yuna Zhang, Ziwei Song, Dianzhuang Zheng, Tao Zhang, Hailing Wang, Xiaojun Zhu, Xiangfei Chen, Min Qiu, Yichen Shen, Wanhua Zheng, Yue Hao

Abstract: Neuromorphic photonic computing has emerged as a competitive computing paradigm to overcome the bottlenecks of the von-Neumann architecture. Linear weighting and nonlinear spiking activation are two fundamental functions of a photonic spiking neural network (PSNN). However, they are separately implemented with different photonic materials and devices, hindering the large-scale integration of PSNN. Here, we propose, fabricate and experimentally demonstrate a photonic neuro-synaptic chip enabling the simultaneous implementation of linear weighting and nonlinear spiking activation based on a distributed feedback (DFB) laser with a saturable absorber (DFB-SA). A prototypical system is experimentally constructed to demonstrate the parallel weighted function and nonlinear spike activation. Furthermore, a four-channel DFB-SA array is fabricated for realizing matrix convolution of a spiking convolutional neural network, achieving a recognition accuracy of 87% for the MNIST dataset. The fabricated neuro-synaptic chip offers a fundamental building block to construct the large-scale integrated PSNN chip.

Authors:Henna Farheen, Suraj Joshi, J. Christoph Scheytt, Viktor Myroshnychenko, Jens Förstner

Abstract: Phased arrays are vital in communication systems and have received significant interest in the field of optoelectronics and photonics, enabling a wide range of applications such as LiDAR, holography, wireless communication, etc. In this work, we present a blazed grating antenna that is optimized to have upward radiation efficiency as high as 80% with a compact footprint of 3.5 {\mu}m \times 2 {\mu}m at an operational wavelength of 1.55 {\mu}m. Our numerical investigations demonstrate that this antenna in a 64 \times 64 phased array configuration is capable of producing desired far-field radiation patterns. Additionally, our antenna possesses a low side lobe level of -9.7 dB and a negligible reflection efficiency of under 1%, making it an attractive candidate for integrated optical phased arrays.

Authors:Navdeep Rana, Gopal Dixit

Abstract: Electrons in two-dimensional materials possess an additional quantum attribute, the valley pseudospin, labelled as $\mathbf{K}$ and $\mathbf{K}^{\prime}$ -- analogous to the spin up and spin down. The majority of research to achieve valley-selective excitations in valleytronics depends on resonant circularly-polarised light with a given helicity. Not only acquiring valley-selective electron excitation but also switching the excitation from one valley to another is quintessential for bringing valleytronics-based technologies in reality. Present work introduces a coherent control protocol to initiate valley-selective excitation, de-excitation, and switch the excitation from one valley to another on the fly within tens of femtoseconds -- a timescale faster than any valley decoherence time. Our protocol is equally applicable to {\it both} gapped and gapless two-dimensional materials. Monolayer graphene and molybdenum disulfide are used to test the universality. Moreover, the protocol is robust as it is insensitive to significant parameters of the protocol, such as dephasing times, wavelengths, and time delays of the laser pulses. Present work goes beyond the existing paradigm of valleytronics, and opens a new realm of valley switch at PetaHertz rate.

Authors:Graeme Neil Campbell, Lewis Hill, Pascal Del'Haye, Gian-Luca Oppo

Abstract: Ranges of existence and stability of dark cavity-soliton stationary states in a Fabry-Perot resonator with a Kerr nonlinear medium and normal dispersion are determined. The Fabry-Perot configuration introduces nonlocal coupling that shifts the cavity detuning by the round trip average power of the intracavity field. When compared with ring resonators described by the Lugiato-Lefever equation, nonlocal coupling leads to strongly detuned dark cavity solitons that exist over a wide range of detunings. This shift is a consequence of the counterpropagation of intracavity fields inherent to Fabry-Perot resonators. At difference with ring resonators, the existence and stability of dark soliton solutions are dependent on the size and number of solitons in the cavity. We investigate the effect of nonlocal coupling of Fabry-Perot resonators on multiple dark solitons and demonstrate long range interactions and synchronization of temporal oscillations.

Authors:Fernando Lorén, Cyriaque Genet, Luis Martín-Moreno

Abstract: We investigate the interaction between a monolayer of WS2 and a chiral plasmonic metasurface. WS2 possesses valley excitons that selectively couple with one-handed circularly polarised light. At the same time, the chiral plasmonic metasurface exhibits spin-momentum locking, leading to a robust polarisation response in the far field. Using a scattering formalism based on the coupled mode method, we analyse various optical properties of the WS2 monolayer. Specifically, we demonstrate the generation of circular dichroism in the transition metal dichalcogenide (TMD) by harnessing the excitation of surface plasmon polaritons (SPPs) in the metasurface. Moreover, we observe the emergence of other guided modes, opening up exciting possibilities for further exploration in TMD-based devices.

Authors:Michaël Lobet, Nathaniel Kinsey, Iñigo Liberal, Humeyra Caglayan, Paloma Arroyo-Huidobro, Emanuele Galiffi, Jorge Ricardo Mejía-Salazar, Giovanna Palermo, Zubin Jacob, Nicolò Maccaferri

Abstract: The engineering of the spatial and temporal properties of both the electric permittivity and the refractive index of materials is at the core of photonics. When vanishing to zero, those two variables provide new knobs to control light-matter interactions. This perspective aims at providing an overview of the state of the art and the challenges in emerging research areas where the use of near-zero refractive index and hyperbolic metamaterials is pivotal, in particular light and thermal emission, nonlinear optics, sensing applications and time-varying photonics.

Authors:Hui Yuan, Alvydas Lisauskas, Mark D. Thomson, Hartmut G. Roskos

Abstract: Fourier imaging is an indirect imaging method which records the diffraction pattern of the object scene coherently in the focal plane of the imaging system and reconstructs the image using computational resources. The spatial resolution, which can be reached, depends on one hand on the wavelength of the radiation, but also on the capability to measure - in the focal plane - Fourier components with high spatial wave-vectors. This leads to a conflicting situation at THz frequencies, because choosing a shorter wavelength for better resolution usually comes at the cost of less radiation power, concomitant with a loss of dynamic range, which limits the detection of higher Fourier components. Here, aiming at maintaining a high dynamic range and limiting the system costs, we adopt heterodyne detection at the 2nd sub-harmonic, working with continuous-wave (CW) radiation for object illumination at 600 GHz and local-oscillator (LO) radiation at 300 GHz. The detector is a single-pixel broad-band Si CMOS TeraFET equipped with substrate lenses on both the front- and backside for separate in-coupling of the waves. The entire scene is illuminated by the object wave, and the Fourier spectrum is recorded by raster scanning of the single detector unit through the focal plane. With only 56 uW of power of the 600-GHz radiation, a dynamic range of 60 dB is reached, sufficient to detect the entire accessible Fourier space spectrum in the test measurements. A lateral spatial resolution of better than 0.5 mm, at the diffraction limit, is reached.

Authors:Andrzej Opala, Magdalena Furman, Mateusz Król, Rafał Mirek, Krzysztof Tyszka, Bartłomiej Seredyński, Wojciech Pacuski, Jacek Szczytko, Michał Matuszewski, Barbara Piętka

Abstract: We observe natural exceptional points in the excitation spectrum of an exciton-polariton system by optically tuning the light-matter interactions. The observed exceptional points do not require any spatial or polarization degrees of freedom and result solely from the transition from weak to strong light-matter coupling. We demonstrate that they do not coincide with the threshold for photon lasing, confirming previous theoretical predictions [Phys. Rev. Lett. 122, 185301 (2019), Optica 7, 1015 (2020) ]. Using a technique where a strong coherent laser pump induces up-converted excitations, we encircle the exceptional point in the parameter space of coupling strength and particle momentum. Our method of local optical control of light-matter coupling paves the way to investigation of fundamental phenomena including dissipative phase transitions and non-Hermitian topological states.

Authors:Felix Ritzkowsky, Matthew Yeung, Engjell Bebeti, Thomas Gebert, Toru Matsuyama, Matthias Budden, Roland Mainz, Huseyin Cankaya, Karl Berggren, Giulio Rossi, Phillip Keathley, Franz Kärtner

Abstract: Attosecond science has demonstrated that electrons can be controlled on the sub-cycle time scale of an optical wave, paving the way toward optical frequency electronics. Using controlled few-cycle optical waveforms, the study of sub-cycle electron emission has enabled the generation of attosecond ultraviolet pulses and the control of attosecond currents inside of solids. However, these experiments rely on high-energy laser systems not suitable for integration with microcircuits. To move towards integrated optical frequency electronics, a system suitable for integration into microcircuits capable of generating detectable signals with low pulse energies is needed. While current from plasmonic nanoantenna emitters can be driven at optical frequencies, low charge yields have been a significant limitation. In this work we demonstrate that large-scale electrically-connected plasmonic nanoantenna networks, when driven in concert, enable a much higher charge yield sufficient for shot-to-shot carrier-envelope phase detection, which is a hallmark of the underlying sub-cycle processes. We use a tailored sub-2-cycle mid-infrared waveform of only tens of nanojoules of energy to drive in excess of 2000 carrier-envelope-phase-sensitive electrons from interconnected plasmonic nanoantenna arrays that we detect on a single-shot basis using conventional electronics. Our work shows that electronically integrated plasmonic nanoantennas are a viable approach to integrated optical frequency electronics. By engineering the nanoantennas to the particular use case, such as carrier-envelope phase detection, and optimizing the density and total amount, the output signals are fully controlled. This approach to optical frequency electronics will further enable many interesting applications, such as petahertz-bandwidth electric field sampling or the realization of logic gates operating at optical frequencies.

Authors:Konstantin Y. Bliokh

Abstract: We examine properties and propagation of the energy-density and photon-probability centroids of electromagnetic wavepackets in free space. In the second-order paraxial approximation, both of these centroids propagate with the same subluminal velocity because of the transverse confinement of the wavepacket and its diffraction. The tiny difference between the energy and probability centroid velocities appears only in the forth order. We consider three types of wavepackets: Gaussian, Bessel, and non-diffracting Bessel. In all these cases, the subluminal propagation is clearly visible in the intensity distributions and can be measured experimentally in both classical-light and single-photon regimes. For Gaussian wavepackets, the half-wavelength delay is accumulated after propagation over about 12 Rayleigh lengths.

Authors:Luca M. Berger, Martin Barkey, Stefan A. Maier, Andreas Tittl

Abstract: Bound states in the continuum (BICs) are localized electromagnetic modes within the continuous spectrum of radiating waves. Due to their infinite lifetimes without radiation losses, BICs are driving research directions in lasing, non-linear optical processes, and sensing. However, conventional methods for converting BICs into leaky resonances, or quasi-BICs, with high-quality factors typically rely on breaking the in-plane inversion symmetry of the metasurface and often result in resonances that are strongly dependent on the angle of the incident light, making them unsuitable for many practical applications. Here, we numerically analyze and experimentally demonstrate an emerging class of BIC-driven metasurfaces, where the coupling to the far field is controlled by the displacement of individual resonators. In particular, we investigate both all-dielectric and metallic as well as positive and inverse displacement-mediated metasurfaces sustaining angular-robust quasi-BICs in the mid-infrared spectral region. We explore their behavior with changes in the incidence angle of illumination and experimentally show their superior performance compared to two conventional alternatives: silicon-based tilted ellipses and cylindrical nanoholes in gold. We anticipate our findings to open exciting perspectives for bio-sensing, conformal optical devices, and photonic devices using focused light.

Authors:Caio Vaz Rimoli, Claudio Moretti, Fernando Soldevila, Enora Brémont, Cathie Ventalon, Sylvain Gigan

Abstract: Fiber photometry is a significantly less invasive method compared to other deep brain imaging microendoscopy approaches due to the use of thin multimode fibers (MMF diameter $<$ 500 $\mu$m). Nevertheless, the transmitted signals get scrambled upon propagation within the MMF, thus limiting the technique's potential in resolving temporal readouts with cellular resolution. Here, we demonstrate how to separate the time trace signals of several fluorescent sources probed by a thin ($\approx$ 200 $\mu$m) MMF with typical implantable length in a mouse brain. We disentangled several spatio-temporal fluorescence signals by using a general unconstrained non-negative matrix factorization (NMF) algorithm directly on the raw video data. Furthermore, we show that commercial and low-cost open-source miniscopes display enough sensitivity to image the same fluorescence patterns seen in our proof of principle experiment, suggesting that a whole new avenue for novel minimally invasive deep brain studies with multimode fibers in freely-behaving mice is possible.

Authors:Valerie Yoshioka, Jicheng Jin, Qiang Lin, Bo Zhen

Abstract: Generating widely tunable, continuous wave light at long wavelengths via difference frequency generation (DFG) remains challenging due to high absorption and dispersion. The relatively new platform of thin film lithium niobate enables high-confinement nonlinear waveguides, which could improve efficiency. We simulated DFG in thin film lithium niobate waveguides that are periodically poled for surface emission at 30 THz. Maximum efficiency for a 1 cm device is 9.16 $\times$ 10$^{-6}$ W$^{-1}$ assuming $d_{33}$ = 30 pm/V. The tuning range within 50$\%$ of efficiency at 30 THz is as wide as 25 THz, from 25 THz to 50 THz.

Authors:Roni Shaashoua, Lir Kasuker, Mor Kishner, Tal Levy, Barak Rotblat, Anat Ben-Zvi, Alberto Bilenca

Abstract: Optical imaging with mechanical contrast is critical for material and biological discovery since it allows contactless light-radiation force-excitation within the sample, as opposed to traditional mechanical imaging. Whilst optical microscopy based on stimulated Brillouin scattering (SBS) enables mechanical imaging of materials and living biological systems with high spectrospatial resolution, its temporal resolution remains limited. Here, we develop Brillouin gain microscopy (BGM) with a 200-fold higher temporal resolution by detecting the Brillouin gain at a mechanically contrasting frequency in the sample with high sensitivity. Using BGM, we demonstrate mechanical imaging of materials, living organisms and cells at high spectro-spatiotemporal resolution.

Authors:Roni Shaashoua, Tal Levy, Barak Rotblat, Alberto Bilenca

Abstract: Brillouin microscopy is an emerging technique for all-optical biomechanical imaging without the need for physical contact with the sample or for an external mechanical stimulus. However, Brillouin microscopy often retrieves a single, averaged Brillouin frequency shift of all the materials in the sampling volume, introducing significant spectral artifacts in the Brillouin shift images produced. To enable the identification between single- and multi-peak Brillouin signatures in the sample voxels, we developed here a new physics-driven model selection framework based on information theory and an overfit Brillouin water peak threshold. The model selection framework was applied to Brillouin data of NIH/3T3 cells measured by stimulated Brillouin scattering microscopy, facilitating the improved quantification of the Brillouin shift of different regions in the cells, and substantially minimizing spectral artifacts in their Brillouin shift images.

Authors:Chenghao Feng, Jiaqi Gu, Hanqing Zhu, Rongxing Tang, Shupeng Ning, May Hlaing, Jason Midkiff, Sourabh Jain, David Z. Pan, Ray T. Chen

Abstract: The optical neural network (ONN) is a promising hardware platform for next-generation neuromorphic computing due to its high parallelism, low latency, and low energy consumption. However, previous integrated photonic tensor cores (PTCs) consume numerous single-operand optical modulators for signal and weight encoding, leading to large area costs and high propagation loss to implement large tensor operations. This work proposes a scalable and efficient optical dot-product engine based on customized multi-operand photonic devices, namely multi-operand optical neurons (MOON). We experimentally demonstrate the utility of a MOON using a multi-operand-Mach-Zehnder-interferometer (MOMZI) in image recognition tasks. Specifically, our MOMZI-based ONN achieves a measured accuracy of 85.89% in the street view house number (SVHN) recognition dataset with 4-bit voltage control precision. Furthermore, our performance analysis reveals that a 128x128 MOMZI-based PTCs outperform their counterparts based on single-operand MZIs by one to two order-of-magnitudes in propagation loss, optical delay, and total device footprint, with comparable matrix expressivity.

Authors:Giovanni Morello Dipartimento di Matematica e Fisica 'E. De Giorgi'-Universita' del Salento, Maria Moffa Istituto Nanoscienze-CNR, Martina Montinaro Dipartimento di Matematica e Fisica 'E. De Giorgi'-Universita' del Salento, Annachiara Albanese Istituto Nanoscienze-CNR Dipartimento di Fisica-Universita' di Pisa, Karolis Kazlauskas Institute of Photonics and Nanotechnology-Vilnius University, Saulius Jursenas Institute of Photonics and Nanotechnology-Vilnius University, Ausra Tomkeviciene Department of Polymer Chemistry and Technology-Kaunas University of Technology, Juozas V. Grazulevicius Department of Polymer Chemistry and Technology-Kaunas University of Technology, Andrea Camposeo Istituto Nanoscienze-CNR, Dario Pisignano Dipartimento di Matematica e Fisica 'E. De Giorgi'-Universita' del Salento Istituto Nanoscienze-CNR Dipartimento di Fisica-Universita' di Pisa

Abstract: The optical gain of blue light-emitting electrospun polystyrene fibers doped with a linear multi-fragment molecular dye based on the combination of fluorine-carbazole functional units is investigated, with the aim of correlating emission properties and the specific material architecture made of either aligned or disordered fibers. Enhanced performance is found in aligned fibers, whose gain spectrum can be finely tuned by varying the dye concentration. Instead, randomly oriented fibers show a manifold spectral line narrowing, resulting in sharp laser peaks superimposed on top of a broad emission band, ascribable to random lasing. In these systems, the increase of the dye content turns out to be effective for both decreasing the lasing threshold by about a factor 6 and for varying the laser emission wavelength. These results make these arrays and disordered architectures of fibers valuable active media for variable-gain, broadband lasing, which is remarkably important in optical sensing and tunable microlaser devices.

Authors:Tengfei Wu, Marc Guillon, Gilles Tessier, Pascal Berto

Abstract: In astronomy or biological imaging, refractive index inhomogeneities of e.g. atmosphere or tissues induce optical aberrations which degrade the desired information hidden behind the medium. A standard approach consists in measuring these aberrations with a wavefront sensor (e.g Shack-Hartmann) located in the pupil plane, and compensating them either digitally or by adaptive optics with a wavefront shaper. However, in its usual implementation this strategy can only extract aberrations within a single isoplanatic patch, i.e. a region where the aberrations remain correlated. This limitation severely reduces the effective field-of-view in which the correction can be performed. Here, we propose a new wavefront sensing method capable of measuring, in a single shot, various pupil aberrations corresponding to multiple isoplanatic patches. The method, based on a thin diffuser (i.e a random phase mask), exploits the dissimilarity between different speckle regions to multiplex several wavefronts incoming from various incidence angles. We present proof-of-concept experiments carried out in wide-field fluorescence microscopy. A digital deconvolution procedure in each isoplanatic patch yields accurate aberration correction within an extended field-of-view. This approach is of interest for adaptive optics applications as well as diffractive optical tomography.

Authors:M. V. Belov, S. A. Koutovoi, V. A. Kozlov, N. V. Pestovskii, S. Yu. Savinov, A. I. Zagumennyi, Yu. D. Zavartsev, M. V. Zavertyaev

Abstract: Photoluminescence (PL) of Lu$_{2}$SiO$_{5}$ crystal and ceramics with a high concentration of oxygen vacancies (about ~0.5 at.%) is studied. Oxygen vacancies were created using two ways. The first method is a growth of crystal from the non-stoichiometric Lu$_{2}$Si$_{0.98}$O$_{4.96}$ melt and the second one is a doping of Lu$_{2}$SiO$_{5}$ matrix with divalent Ca$^{2+}$ ions at the concentration of 1 at.%. For the first time we observe a fairly bright room-temperature ultraviolet PL of both crystal grown from the Lu$_{2}$Si$_{0.98}$O$_{4.96}$ melt and Lu$_{2}$SiO$_{5}$ ceramics doped with Ca$^{2+}$ ions. A band at 290 nm in the PL spectrum of the crystal and a band at 283 nm in the PL spectrum of the calcium-doped ceramics are observed. The spectral and kinetic properties of these bands are close to each other. This fact indicates similar origins of these bands. The results of the work show that the studied ultraviolet luminescence is related to oxygen vacancies in lutetium oxyorthosilicate.

Authors:Avinash Kumar, Saurabh Kumar Singh, Pranav R. Shirhatti

Abstract: We describe the design and characterization of a versatile pulsed (5 ns, 10 Hz repetition rate) optical parametric oscillator and amplifier system capable of generating single longitudinal mode, narrow linewidth (0.01 $\rm{cm^{-1}}$) radiation in the wavelength range of 680 - 870 nm and 1380 - 4650 nm. Using a combination of power normalized photoacoustic signal and a wavemeter (based on a Fizeau interferometer), we are able to actively stabilize the output wavenumber to within 0.006 $\rm{cm^{-1}}$ (3$\sigma$) over a timescale of greater than 1500 seconds. We demonstrate an application of this system by performing ro-vibration state selected preparation of CO (v = 0 $\rightarrow 2$) and subsequent state selective detection in an internally cold molecular beam.

Authors:Chiara Devescovi, Antonio Morales-Pérez, Yoonseok Hwang, Mikel García-Díez, Iñigo Robredo, Juan Luis Mañes, Barry Bradlyn, Aitzol García-Etxarri, Maia G. Vergniory

Abstract: Axion insulators are 3D magnetic higher-order topological insulators protected by inversion-symmetry that exhibit hinge-localized chiral channels and induce quantized topological magnetoelectric effects. Recent research has suggested that axion insulators may be capable of detecting dark-matter axion-like particles by coupling to their axionic excitations. Beyond its fundamental theoretical interest, designing a photonic AXI offers the potential to enable the development of magnetically-tunable photonic switch devices through the manipulation of the axionic modes and their chiral propagation using external magnetic fields. Motivated by these facts, in this work, we propose a novel approach to induce axionic band topology in gyrotropic 3D Weyl photonic crystals gapped by supercell modulation. To quantize an axionic angle, we create domain-walls across inversion-symmetric photonic crystals, incorporating a phase-obstruction in the supercell modulation of their dielectric elements. This allows us to bind chiral channels on inversion-related hinges, ultimately leading to the realization of an axionic chiral channel of light. Moreover, by controlling the material gyrotropic response, we demonstrate a physically accessible way of manipulating the axionic modes through a small external magnetic bias, which provides an effective topological switch between different 1D chiral photonic fiber configurations. Remarkably, the unidirectional axionic hinge states supported by the photonic axion insulator are buried in a fully connected 3D dielectric structure, thereby being protected from radiation through the electromagnetic continuum. As a result, they are highly suitable for applications in guided-light communication, where the preservation and non-reciprocal propagation of photonic signals are of paramount importance.

Authors:Oscar Avalos-Ovando, Veronica Bahamondes, Lucas V. Besteiro, Artur Movsesyan, Zhiming Wang, Gil Markovich, Alexander O. Govorov

Abstract: Chirality, either of light or matter, has proved to be very practical in biosensing and nanophotonics. However, the fundamental understanding of its temporal dynamics still needs to be discovered. A realistic setup for this are the so-called metastructures, since they are optically active and are built massively, hence rendering an immediate potential candidate. Here we propose and study the electromagnetic-optical mechanism leading to chiral optical imprinting on metastructures. Induced photothermal responses create anisotropic permittivity modulations, different for left or right circularly polarized light, leading to temporal-dependent chiral imprinting of hot-spots, namely imprinting of chirality. The above effect has not been observed yet, but it is within reach of modern experimental approaches. The proposed nonlinear chiroptical effect is general and should appear in any anisotropic material; however, we need to design a particular geometry for this effect to be strong. These new chiral time-dependent metastructures may lead to a plethora of applications.

Authors:Dominic T. Meiers, Georg von Freymann

Abstract: Considering light transport in disordered media, the medium is often treated as an effective medium requiring accurate evaluation of an effective refractive index. Because of its simplicity, the Maxwell-Garnett (MG) mixing rule is widely used, although its restriction to particles much smaller than the wavelength is rarely satisfied. Using 3D finite-difference time-domain simulations, we show that the MG theory indeed fails for large particles. Systematic investigation of size effects reveals that the effective refractive index can be instead approximated by a quadratic polynomial whose coefficients are given by an empirical formula. Hence, a simple mixing rule is derived which clearly outperforms established mixing rules for composite media containing large particles, a common condition in natural disordered media.

Authors:Siqi Feng, Tingting Liu, Wenya Chen, Feng Wu, Shuyuan Xiao

Abstract: The miniaturization of nonlinear light sources is central to the integrated photonic platform, driving a quest for high-efficiency frequency generation and mixing at the nanoscale. In this quest, the high-quality ($Q$) resonant dielectric nanostructures hold great promise, as they enhance nonlinear effects through the resonantly local electromagnetic fields overlapping the chosen nonlinear materials. Here, we propose a method for the enhanced sum-frequency generation (SFG) from etcheless lithium niobate (LiNbO$_{3}$) by utilizing the dual quasi-bound states in the continuum (quasi-BICs) in a one-dimensional resonant grating waveguide structure. Two high-$Q$ guided mode resonances corresponding to the dual quasi-BICs are respectively excited by two near-infrared input beams, generating a strong visible SFG signal with a remarkably high conversion efficiency of $3.66\times10^{-2}$ (which is five orders of magnitude higher than that of LiNbO$_{3}$ films of the same thickness) and a small full-width at half-maximum less than 0.2 nm. The SFG efficiency can be tuned via adjusting the grating geometry parameter or choosing the input beam polarization combination. Furthermore, the generated SFG signal can be maintained at a fixed wavelength without the appreciable loss of efficiency by selectively exciting the angular-dependent quasi-BICs, even if the wavelengths of input beams are tuned within a broad spectral range. Our results provide a simple but robust paradigm of high-efficiency frequency conversion on an easy-fabricated platform, which may find applications in nonlinear light sources and quantum photonics.

Authors:J. Ortega, C. L. Folcia, J. Etxebarria

Abstract: We have developed a numerical method for calculating the second harmonic generation (SHG) generated by an anisotropic material whose optical properties present an arbitrary modulation in one dimension. The method is based on the Berreman 4x4 matrix formalism, which is generalized to include nonlinear optical phenomena. It can be used under oblique incidences of the input beam, and is valid even when the SHG frequency is close to photonic bands, where the usual slowly-varying-amplitude approximation breaks down. As an example of application, we have studied the SHG performance of ferroelectric and helielectric nematic liquid crystals. The latter present a helicoidal structure that can be distorted under electric field. In the different tests of the method we have analyzed the conditions for the most efficient SHG, and compared with previous results in the case there were any. The obtained results indicate that the present procedure may contribute to improve the structural design and enlarge the variety of nonlinear optical materials for their application in optical devices.

Authors:Suraj, Shankar Kumar Selvaraja

Abstract: We demonstrate, for the first time, sputtered PZT as a platform for the development of Si-based photonic devices such as rings, MZI, and electro-optic modulators. We report the optimization of PZT on MgO(002) substrate to obtain highly oriented PZT film oriented towards the (100) plane with a surface roughness of 2 nm. Si gratings were simulated for TE and TM mode with an efficiency of -2.2 dB/coupler -3 dB/coupler respectively with a polarization insensitive efficiency of 50% for both TE and TM mode. Si grating with an efficiency of around -10 dB/coupler and a 6 dB bandwidth of 30 nm was fabricated. DC Electro-optic characterization for MZI yielded a spectrum shift of 71 pm/V at the c-band.

Authors:Suraj, Shankar Kumar Selvaraja

Abstract: In this work, we have demonstrated a novel method to increase the electro-optic interaction in an intensity modulator at the C-band by optimizing the growth methodology of PZT with the metal (Ti/Pt) as a base material and the PZT poling architecture. Here, we have used a patterned Pt layer for PZT deposition instead of a buffer layer. By optimizing the PZT growth process, we have been able to do poling of the fabricated PZT film in an arbitrary direction as well as have achieved an enhanced electro-optic interaction, leading to a DC spectrum shift of 304 pm/V and a V{\pi} L{\pi} value of 0.6 V-cm on a Si-based MZI. For an electro-optic modulator, we are reporting the best values of DC spectrum shift and V{\pi} L{\pi} using perovskite as an active material. The high-speed measurement has yielded a tool-limited bandwidth of > 12GHz. The extrapolated bandwidth calculated using the slope of the modulation depth is 45 GHz. We also show via simulation an optimized gap of 4.5 {\mu}m and a PZT thickness of 1 {\mu}m that gives us a less than 1 V-dB.

Authors:Suraj, Shankar Kumar Selvaraja

Abstract: We report $\approx$400\% enhancement in PZT Pockels coefficient on DFT simulation of lattice strain due to phonon mode softening.The simulation showed a relation between the rumpling and the Pockels coefficient divergence that happens at -8\% and 25\% strain developed in PZT film.The simulation was verified experimentally by RF sputter deposited PZT film on Pt/SiO$_2$/Si layer.The strain developed in PZT varied from -0.04\% for film annealed at 530\degree C to -0.21\% for 600\degree C annealing temperature.The strain was insensitive to RF power with a value of -0.13\% for power varying between 70-130 W. Pockels coefficient enhancement was experimentally confirmed by Si Mach Zehnder interferometer loaded with PZT and probed with the co-planar electrode.An enhancement of $\approx$300\% in Pockels coefficient was observed from 2-8 pm/V with strain increasing from -0.04\% to -0.21\%. To the best of our knowledge, this is the first time study and demonstration of strain engineering on Pockels coefficient of PZT using DFT simulation, film deposition, and photonic device fabrication.

Authors:R. Flaschmann, C. Schmid, L. Zugliani, S. Strohauer, F. Wietschorke, S. Grotowski, B. Jonas, M. Müller, M. Althammer, R. Gross, J. J. Finley, K. Müller

Abstract: We investigate the growth conditions for thin (less than 200 nm) sputtered aluminum (Al) films. These coatings are needed for various applications, e.g. for advanced manufacturing processes in the aerospace industry or for nanostructures for quantum devices. Obtaining high-quality films, with low roughness, requires precise optimization of the deposition process. To this end, we tune various sputtering parameters such as the deposition rate, temperature, and power, which enables 50 nm thin films with a root mean square (RMS) roughness of less than 1 nm and high reflectivity. Finally, we confirm the high quality of the deposited films by realizing superconducting single-photon detectors integrated into multi-layer heterostructures consisting of an aluminum mirror and a silicon dioxide dielectric spacer. We achieve an improvement in detection efficiency at 780 nm from 40 % to 70 % by this integration approach.

Authors:Zhe Wang, Zhiwei Fang, Zhaoxiang Liu, Youting Liang, Jian Liu, Jianping Yu, Ting Huang, Yuan Zhou, Haisu Zhang, Min Wang, Ya Cheng

Abstract: We design an on-chip 8-channel TFLN AWG and fabricate the device using photolithography assisted chemo-mechanical etching (PLACE) technique. We experimentally measure the transmission of the fabricated TFLN AWG near the central wavelength of 1550 nm. We obtain an on-chip loss as low as 3.32 dB, a single-channel bandwidth of 1.6 nm and a total-channel bandwidth of 12.8 nm. The crosstalk between adjacent channels was measured to be below -7.01 dB within the wavelength range from 1543 nm to 1558 nm, and the crosstalk between non-adjacent channels was below -15 dB.

Authors:Antonio Morales-Pérez, Chiara Devescovi, Yoonseok Hwang, Mikel García-Díez, Barry Bradlyn, Juan Luis Mañes, Maia G. Vergniory, Aitzol García-Etxarri

Abstract: Tight-binding models can accurately predict the band structure and topology of crystalline systems and they have been heavily used in solid-state physics due to their versatility and low computational cost. It is quite straightforward to build an accurate tight-binding model of any crystalline system using the maximally localized Wannier functions of the crystal as a basis. In 1D and 2D photonic crystals, it is possible to express the wave equation as two decoupled scalar eigenproblems where finding a basis of maximally localized Wannier functions is feasible using standard Wannierization methods. Unfortunately, in 3D photonic crystals, the vectorial nature of the electromagnetic solutions cannot be avoided. This precludes the construction of a basis of maximally localized Wannier functions via usual techniques. In this work, we show how to overcome this problem by using topological quantum chemistry which will allow us to express the band structure of the photonic crystal as a difference of elementary band representations. This can be achieved by the introduction of a set of auxiliary modes, as recently proposed by Solja\v{c}i\'c et. al., which regularize the $\Gamma$-point obstruction arising from transversality constraint of the Maxwell equations. The decomposition into elementary band representations allows us to isolate a set of pseudo-orbitals that permit us to construct an accurate transversality-enforced tight-binding model (TETB) that matches the dispersion, symmetry content, and topology of the 3D photonic crystal under study. Moreover, we show how to introduce the effects of a gyrotropic bias in the framework, modeled via non-minimal coupling to a static magnetic field. Our work provides the first systematic method to analytically model the photonic bands of the lowest transverse modes over the entire BZ via a TETB model.

Authors:Fabio Bergamin, James Lough, Emil Schreiber, Hartmut Grote, Moritz Mehmet, Henning Vahlbruch, Christoph Affeldt, Tomislav Andric, Aparna Bisht, Marc Bringmann, Volker Kringel, Harald Lück, Nikhil Mukund, Severin Nadji, Borja Sorazu, Kenneth Strain, Michael Weinert, Karsten Danzmann

Abstract: Squeezed light is injected into the dark port of gravitational wave interferometers, in order to reduce the quantum noise. A fraction of the interferometer output light can reach the OPO due to sub-optimal isolation of the squeezing injection path. This backscattered light interacts with squeezed light generation process, introducing additional measurement noise. We present a theoretical description of the noise coupling mechanism. We propose a control scheme to achieve a de-amplification of the backscattered light inside the OPO with a consequent reduction of the noise caused by it. The scheme was implemented at the GEO 600 detector and has proven to be crucial in maintaining a good level of quantum noise reduction of the interferometer for high parametric gain of the OPO. In particular, the mitigation of the backscattered light noise helped in reaching 6dB of quantum noise reduction [Phys. Rev. Lett. 126, 041102 (2021)]. The impact of backscattered-light-induced noise on the squeezing performance is phenomenologically equivalent to increased phase noise of the squeezing angle control. The results discussed in this paper provide a way for a more accurate estimation of the residual phase noise of the squeezed light field.

Authors:Wenhao Li, Hooman Barati Sedeh, Willie J. Padilla, Simiao Ren, Jordan Malof, Natalia M. Litchinitser

Abstract: Electromagnetic multipole expansion theory underpins nanoscale light-matter interactions, particularly within subwavelength meta-atoms, paving the way for diverse and captivating optical phenomena. While conventionally brute force optimization methods, relying on the iterative exploration of various geometries and materials, are employed to obtain the desired multipolar moments, these approaches are computationally demanding and less effective for intricate designs. In this study, we unveil the potential of machine learning for designing dielectric meta-atoms with desired multipolar moments up to the octupole terms. Specifically, we develop forward prediction models to unravel the intricate relationship between the scattering response and the topological attributes of individual meta-atoms, and an inverse design model to reconstruct scatterers with the targeted multipolar moments. Utilizing a tandem network trained to tailor dielectric meta-atoms for generating intended multipolar moments across a broad spectral range, we further demonstrate the generation of uniquely shaped meta-atoms for exciting exclusive higher order magnetic response and establishing super-scattering regime of light-matter interaction. We also illustrate the accurate prediction of electric field distributions within the given scatterer. Our versatile methodology can be readily applied to existing datasets and seamlessly integrated with various network architectures and problem domains, making it a valuable tool for the design of different platforms at nanoscale.

Authors:Salim B. Ivars, Yaroslav V. Kartashov, Pedro Fernández de Córdoba, J. Alberto Conejero, Lluis Torner, Carles Milián

Abstract: Taming the instabilities inherent to many nonlinear optical phenomena is of paramount importance for modern photonics. In particular, the so-called snake instability is universally known to severely distort localized wave stripes, leading to the occurrence of transient, short-lived dynamical states that eventually decay. The phenomenon is ubiquitous in nonlinear science, from river meandering to superfluids, and to date it remains apparently uncontrollable. However, here we show that optical snake instabilities can be harnessed by a process that leads to the formation of stationary and robust two-dimensional zigzag states. We find that such new type of nonlinear waves exists in the hyperbolic regime of cylindrical micro-resonators and it naturally corresponds to two-dimensional frequency combs featuring spectral heterogeneity and intrinsic synchronization. We uncover the conditions of the existence of such spatiotemporal photonic snakes and confirm their remarkable robustness against perturbations. Our findings represent a new paradigm for frequency comb generation, thus opening the door to a whole range of applications in communications, metrology, and spectroscopy.

Authors:Anton V. Baranikov, Egor Khaidarov, Emmanuel Lassalle, Damien Eschimese, Joel Yeo, N. Duane Loh, Ramon Paniagua-Dominguez, Arseniy I. Kuznetsov

Abstract: Metalenses, in order to compete with conventional bulk optics in commercial imaging systems, often require large field of view (FOV) and broadband operation simultaneously. However, strong chromatic and coma aberrations present in common metalens designs have so far limited their widespread use. Stacking of metalenses as one of the possible solutions increases the overall complexity of the optical system and hinders the main benefit of reduced thickness and light weight. To tackle both issues, here we propose a single-layer imaging system utilizing a recently developed class of metalenses providing large field of view. Using it, we demonstrate full-color imaging with a FOV of 100 degrees. This approach, empowered by computational imaging techniques, produce high quality images, both in terms of color reproduction and sharpness. Suitable for real-time unpolarized light operation with the standard color filters present in prevalent camera systems, our results might enable a pathway for consumer electronics applications of this emerging technology.

Authors:Rabisankar Samanta, Romain Pierrat, Rémi Carminati, Sushil Mujumdar

Abstract: We report experimental and theoretical investigations on photon diffusion in a second-order nonlinear disordered medium under conditions of strong nonlinearity. Experimentally, photons at the fundamental wavelength ($\lambda=1064$ nm) are launched into the structure in the form of a cylindrical pellet, and the second-harmonic ($\lambda=532$ nm) photons are temporally analyzed in transmission. For comparison, separate experiments are carried out with incident green light at $\lambda=532$ nm. We observe that the second harmonic light peaks earlier compared to the incident green photons. Next, the sideways spatial scattering of the fundamental as well as second-harmonic photons is recorded. The spatial diffusion profiles of second-harmonic photons are seen to peak deeper inside the medium in comparison to both the fundamental and incident green photons. In order to give more physical insights into the experimental results, a theoretical model is derived from first principles. It is based on the coupling of transport equations. Solved numerically using a Monte Carlo algorithm and experimentally estimated transport parameters at both wavelengths, it gives excellent quantitative agreement with the experiments for both fundamental and second-harmonic light.

Authors:Matan Even Tzur, Oren Cohen

Abstract: The motion of laser-driven electrons quivers with an average energy termed pondermotive energy. We explore electron dynamics driven by bright squeezed vacuum (BSV), finding that BSV induces width oscillations, akin to electron quivering in laser light, with an equivalent ponderomotive energy. In the case of bound electrons, width oscillations may lead to tunnel ionization with noisy sub-cycle structure. Our results are foundational for strong-field and free-electron quantum optics, as they shed light on tunnel ionization, high harmonic generation, and nonlinear Compton scattering in BSV.

Authors:Jacek Piłka, Michał Kwaśny, Magdalena Czerniewicz, Mirosław Karpierz, Urszula Laudyn

Abstract: Vortex beams are a type of structured light characterized by phase rotation around the propagation axis, resulting in orbital angular momentum. Their properties make them useful in various applications such as high-resolution microscopy, optical tweezing, and telecommunications. This has led to a comprehensive development of methods for their generation, ranging from using single-purpose glass elements to utilizing computer-generated holograms using spatial light modulators. One of the most commonly used elements for vortex transformation is a vortex half-wave retarder called a q-plate, which can transform a Gaussian beam into a scalar vortex or vector beam depending on the input polarization. Although the commercially available ones are limited in the range of possible output topological charges, they can be stacked to perform arithmetic operations to expand them. However, changing the output or working wavelength requires rearranging the elements. We present an improvement to this method that solves these problems by introducing Q-modules, easy-to-fabricate, electrically tunable liquid crystal devices that combine the features of q-plates and half-wave plates and can be used as building blocks in modular assemblies. Electrical tuning makes it possible to change the working wavelength as well as the topological output charge or polarization order without the need to interact mechanically with the setup.

Authors:Athanasios Lekosiotis, Federico Belli, Christian Brahms, Mohammed Sabbah, Hesham Sakr, Ian A. Davidson, Francesco Poletti, John C. Travers

Abstract: We report the flexible on-target delivery of 800 nm wavelength, 5 GW peak power, 40 fs duration laser pulses through an evacuated and tightly coiled 10 m long hollow-core nested anti-resonant fiber by positively chirping the input pulses to compensate for the anomalous dispersion of the fiber. High output pulse quality and a guided peak intensity of 3 PW/cm2 were achieved by suppressing plasma effects in the residual gas by pre-pumping the fiber after evacuation. This appears to cause a long-term removal of molecules from the fiber core. Identifying the fluence at the fiber core-wall interface as the damage origin, we scaled the coupled energy to 1.8 mJ using a short piece of larger-core fiber to obtain 20 GW at the fiber output. This scheme can pave the way towards the integration of anti-resonant fibers in mJ-level nonlinear optical experiments and laser-source development.

Authors:Karim Achouri, Mintae Chung, Andrei Kiselev, Olivier J. F. Martin

Abstract: It has been observed that achiral nano-particles, such as flat helices, may be subjected to an optical torque even when illuminated by normally incident linearly polarized light. However, the origin of this fascinating phenomenon has so far remained mostly unexplained. We therefore propose an exhaustive discussion that provides a clear and rigorous explanation for the existence of such a torque. Using multipolar theory, and taking into account nonlocal interactions, we find that this torque stems from multipolar pseudochiral responses that generate both spin and orbital angular momenta. We also show that the nature of these peculiar responses makes them particularly dependent on the asymmetry of the particles. By elucidating the origin of this type of torque, this work may prove instrumental for the design of high-performance nano-rotors.

Authors:Dario Siebenkotten, Bernd Kaestner, Arne Hoehl, Shuhei Amakawa

Abstract: Scattering-type scanning near-field optical microscopy (s-SNOM) enables sub-diffraction spectroscopy, featuring high sensitivity to small spatial permittivity variations of the sample surface. However, due to the near-field probe-sample interaction, the quantitative extraction of the complex permittivity leads to a computationally demanding inverse problem, requiring further approximation of the system to an invertible model. Black-box calibration methods, similar to those applied to microwave vector network analysers, allow the extraction of the permittivity without detailed electromagnetic modelling of the probe-sample interaction. These methods, however, are typically designed for stationary setups. In contrast, the distance between the sample and the probe tip of the s-SNOM is slowly modulated, which is required for the lock-in detection used to extract the near-field interaction buried in the far-field background. Here we propose an improved calibration method that explicitly takes probe tapping into account. We validate our method for an s-SNOM operating in a mid-infrared spectral range by applying it to measurements of silicon microstructures of different but well characterised doping.

Authors:Mauricio Garcia-Vergara, Guillaume Demésy, André Nicolet, Frédéric Zolla

Abstract: In this paper, the electromagnetic radiation from an oscillating particle placed in the vicinity of an object of size comparable to the wavelength is studied. Although this problem may seem academic at first sight, the details of the calculations are presented throughout without any detail left under the carpet. A polyharmonic decomposition of the radiation sources allows the diffraction problem to be fully characterised while satisfying energy conservation. Finally, the source expressions obtained are suitable for use in a numerical code. A 3D illustration using finite elements is provided.

Authors:Ningzhi Xie, Quentin A. A. Tanguy, Johannes E. Fröch, Karl F. Böhringer, Arka Majumdar

Abstract: With the advent of neuroimaging and microsurgery, there is a rising need for capturing images through an optical fiber. We present an approach of imaging through a single fiber without mechanical scanning by implementing spatial-spectral encoding. The spectral encoding is achieved through a microfabricated spectral filter array, where light from different spatial pixels is coded with a highly orthogonal spectrum. The image is then computationally recovered via pseudo inverse of the encoding process. We demonstrate imaging of a $4 \times 4$ binary object at the proximity of the spectral filter array using $560-625nm$ wavelength band. The recovered image maintains an error rate of $<11\%$ when measured using a spectrometer with a spectral resolution of $1.5nm$. The image remains unchanged with fiber bending or moving. Thus our approach shows a more robust way to image through a single optical fiber, with potential applications in compact endoscopes and angioscopes.

Authors:Sebastián Bordakevich, Dudbil Pabón, Lorena Rebón, Silvia Ledesma

Abstract: Spatial light modulators (SLMs) are widely used to coherently control quantum states of light. When carrying out these experiments, some assumptions are made. For instance, it is supposed that the position-momentum correlations between twin photon pairs are not affected by the use of a liquid crystal display (LCD) as a SLM. Furthermore, it is assumed that the characterization of such devices performed with an intense laser source, is still valid in the single photon regime. In this work, we show that such assumptions are acceptable, within the experimental uncertainties, for a liquid crystal on silicon (LCoS) display. This is especially important when considering the use of this kind of displays for the coherent control of quantum states based on twin photon sources.

Authors:Geoffrey R. Harrison, Tobias Saule, R. Esteban Goetz, George N. Gibson, Anh-Thu Le, Carlos A. Trallero-Herrero

Abstract: We present a high precision, self-referencing, common path XUV interferometer setup to produce pairs of spatially separated and independently controllable XUV pulses that are locked in phase and time. The spatial separation is created by introducing two equal but opposite wavefront tilts or using superpositions of orbital angular momentum. In our approach, we can independently control the relative phase/delay of the two optical beams with a resolution of 52 zs (zs = zeptoseconds). In order to explore the level of entanglement between the non-local photons, we compare three different beam modes: Bessel-like, and Gaussian with or without added orbital angular momentum. By reconstructing interference patterns one or two photons at a time we conclude that the beams are not entangled, yet each photon in the attosecond pulse train contains information about the entire spectrum. Our technique generates non-local, quantum equivalent XUV photons with a temporal jitter of 3 zs, just below the Compton unit of time of 8 zs. We argue that this new level of temporal precision will open the door for new dynamical QED tests. We also discuss the potential impact on other areas, such as imaging, measurements of non-locality, and molecular quantum tomography.

Authors:Massimo Moccia, Giuseppe Castaldi, Andrea Alù, Vincenzo Galdi

Abstract: Time-harmonic electromagnetic plane waves in anisotropic media can exhibit complex-valued wavevectors (with nonzero real and imaginary parts) even in the absence of material dissipation. These peculiar modes, usually referred to as "ghost waves," hybridize the typical traits of conventional propagating and evanescent waves, displaying both phase accumulation and purely reactive exponential decay away from the direction of power flow. Their existence has been predicted in several scenarios, and has been recently observed experimentally in the form of surface phonon polaritons with complex-valued out-of-plane wavevectors propagating at the interface between air and a natural uniaxial crystal with slanted optical axis. Here, we demonstrate that ghost waves can arise also in lower-dimensional flat-optics scenarios, which are becoming increasingly relevant in the context of metasurfaces and in the field of polaritonics. Specifically, we show that planar junctions between isotropic and anisotropic metasurfaces can support "ghost line waves" that propagate unattenuated along the line interface, exhibiting phase oscillations combined with evanescent decay both in the plane of the metasurface (away from the interface) and out-of-plane n the surrounding medium. Our theoretical results, validated by finite-element numerical simulations, demonstrate a novel form of polaritonic waves with highly confined features, which may provide new opportunities for the control of light at the nanoscale, and may find potential applications in a variety of scenarios, including integrated waveguides, nonlinear optics, optical sensing and sub-diffraction imaging.

Authors:Sudip Shekhar, Wim Bogaerts, Lukas Chrostowski, John E. Bowers, Michael Hochberg, Richard Soref, Bhavin J. Shastri

Abstract: Silicon photonics has developed into a mainstream technology driven by advances in optical communications. The current generation has led to a proliferation of integrated photonic devices from thousands to millions - mainly in the form of communication transceivers for data centers. Products in many exciting applications, such as sensing and computing, are around the corner. What will it take to increase the proliferation of silicon photonics from millions to billions of units shipped? What will the next generation of silicon photonics look like? What are the common threads in the integration and fabrication bottlenecks that silicon photonic applications face, and which emerging technologies can solve them? This perspective article is an attempt to answer such questions. We chart the generational trends in silicon photonics technology, drawing parallels from the generational definitions of CMOS technology. We identify the crucial challenges that must be solved to make giant strides in CMOS-foundry-compatible devices, circuits, integration, and packaging. We identify challenges critical to the next generation of systems and applications - in communication, signal processing, and sensing. By identifying and summarizing such challenges and opportunities, we aim to stimulate further research on devices, circuits, and systems for the silicon photonics ecosystem.

Authors:Marco Saldutti, Yi Yu, Jesper Mørk

Abstract: Nanolasers based on emerging dielectric cavities with deep sub-wavelength confinement of light offer a large light-matter coupling rate and a near-unity spontaneous emission factor, $\beta$. These features call for reconsidering the standard approach to identifying the lasing threshold. Here, we suggest a new threshold definition, taking into account the recycling process of photons when $\beta$ is large. This threshold with photon recycling reduces to the classical balance between gain and loss in the limit of macroscopic lasers, but qualitative as well as quantitative differences emerge as $\beta$ approaches unity. We analyze the evolution of the photon statistics with increasing current by utilizing a standard Langevin approach and a more fundamental stochastic simulation scheme. We show that the threshold with photon recycling consistently marks the onset of the change in the second-order intensity correlation, $g^{(2)}(0)$, toward coherent laser light, irrespective of the laser size and down to the case of a single emitter. In contrast, other threshold definitions may well predict lasing in light-emitting diodes. These results address the fundamental question of the transition to lasing all the way from the macro- to the nanoscale and provide a unified overview of the long-lasting debate on the lasing threshold.

Authors:Suraj, Shashwat Rathkanthiwar, Srinivasan Raghavan, Shankar Kumar Selvaraja

Abstract: In this work, we report the realization of a polarization-insensitive grating coupler, single-mode waveguide, and ring resonator in the GaN-on-Sapphire platform. We provide a detailed demonstration of the material characterization, device simulation, and experimental results. We achieve a grating coupler efficiency of -5.2 dB/coupler with a 1dB and 3dB bandwidth of 40 nm and 80 nm, respectively. We measure a single-mode waveguide loss of -6 dB/cm. The losses measured here are the lowest in a GaN-on-Sapphire photonic circuit. This demonstration provides opportunities for the development of on-chip linear and non-linear optical processes using the GaN-on-Sapphire platform. To the best of our knowledge, this is the first demonstration of an integrated photonic device using a GaN HEMT stack with 2D electron gas.

Authors:Vinzenz Stummer, Tobias Flöry, Matthias Schneller, Edgar Kaksis, Markus Zeiler, Audrius Pugžlys, Andrius Baltuška

Abstract: Multi-photon resonant spectroscopies require tunable narrowband excitation to deliver spectral selectivity and, simultaneously, high temporal intensity to drive a nonlinear-optical process. These contradictory requirements are achievable with bursts of ultrashort pulses, which provides both high intensity and tunable narrowband peaks in the frequency domain arising from spectral interference. However, femtosecond pulse bursts need special attention during their amplification [Optica 7, 1758 (2020)], which requires spectral peak suppression to increase the energy safely extractable from a chirped-pulse amplifier (CPA). Here, we present a method combining safe laser CPA, relying on spectral scrambling, with a parametric frequency converter that automatically restores the desired spectral peak structure and delivers narrow linewidths for bursts of ultrashort pulses at microjoule energies. The shown results pave the way to new high-energy ultrafast laser sources with controllable spectral selectivity.

Authors:Philip Dienstbier, Lennart Seiffert, Timo Paschen, Andreas Liehl, Alfred Leitenstorfer, Thomas Fennel, Peter Hommelhoff

Abstract: Solids exposed to intense electric fields release electrons through tunnelling. This fundamental quantum process lies at the heart of various applications, ranging from high brightness electron sources in DC operation to petahertz vacuum electronics in laser-driven operation. In the latter process, the electron wavepacket undergoes semiclassical dynamics in the strong oscillating laser field, similar to strong-field and attosecond physics in the gas phase. There, the sub-cycle electron dynamics has been determined with a stunning precision of tens of attoseconds, but at solids the quantum dynamics including the emission time window has so far not been measured. Here we show that two-colour modulation spectroscopy of backscattering electrons uncovers the sub-optical-cycle strong-field emission dynamics from nanostructures, with attosecond precision. In our experiment, photoelectron spectra of electrons emitted from a sharp metallic tip are measured as function of the relative phase between the two colours. Projecting the solution of the time-dependent Schr\"odinger equation onto classical trajectories relates phase-dependent signatures in the spectra to the emission dynamics and yield an emission duration of $710\pm30$ attoseconds by matching the quantum model to the experiment. Our results open the door to the quantitative timing and precise active control of strong-field photoemission in solid state and other systems and have direct ramifications for diverse fields such as ultrafast electron sources, quantum degeneracy studies and sub-Poissonian electron beams, nanoplasmonics and petahertz electronics.

Authors:Hongtao Hu, Tobias Flöry, Vinzenz Stummer, Audrius Pugzlys, Markus Kitzler-Zeiler, Xinhua Xie, Alexei Zheltikov, Andrius Baltuška

Abstract: We present a novel approach to achieve hyper spectral resolution, high sensitive detection, and high speed data acquisition Stimulated Raman Spectroscopy by employing amplified offset-phase controlled fs-pulse bursts. In this approach, the Raman-shift spectrum is obtained through the direct mapping between the bursts offset phase and the Raman-shift frequency, which requires neither wavelength-detuning as in the long-pulse method nor precise dispersion management and delay scanning with movable parts as in the spectral focusing technique. This method is demonstrated numerically by solving the coupled non-linear Schroedinger equations and the properties of this approach are systematically investigated. The product of the spectral resolution and the pixel dwell time in this work is below 2 microsiemens/centimeter, which is at least an order of magnitude lower than previous methods. This previously untouched area will greatly expand the applications of SRS and holds the potential for discovering new science.

Authors:Haixin Liu, Grant M. Brodnik, Jizhao Zang, David R. Carlson, Jennifer A. Black, Scott B. Papp

Abstract: We explore optical parametric oscillation (OPO) in nanophotonic resonators, enabling arbitrary, nonlinear phase-matching and nearly lossless control of energy conversion. Such pristine OPO laser converters are determined by nonlinear light-matter interactions, making them both technologically flexible and broadly reconfigurable. We utilize a nanostructured inner-wall modulation in the resonator to achieve universal phase-matching for OPO-laser conversion, but coherent backscattering also induces a counterpropagating pump laser. This depletes the intra-resonator optical power in either direction, increasing the OPO threshold power and limiting laser-conversion efficiency, the ratio of optical power in target signal and idler frequencies to the pump. We develop an analytical model of this system that emphasizes an understanding of optimal laser conversion and threshold behaviors, and we use the model to guide experiments with nanostructured-resonator OPO laser-conversion circuits, fully integrated on chip and unlimited by group-velocity dispersion. Our work demonstrates the fundamental connection between OPO laser-conversion efficiency and the resonator coupling rate, subject to the relative phase and power of counterpropagating pump fields. We achieve $(40\pm4)$ mW of on-chip power, corresponding to $(41\pm4)$% conversion efficiency, and discover a path toward near-unity OPO laser conversion efficiency.

Authors:Andrea Cordaro, Ralph Müller, Stefan Tabernig, Nico Tucher, Patrick Schygulla, Oliver Höhn, Benedikt Bläsi, Albert Polman

Abstract: Multi-junction solar cells provide a path to overcome the efficiency limits of standard silicon solar cells by harvesting more efficiently a broader range of the solar spectrum. However, Si-based multi-junction architectures are hindered by incomplete harvesting in the near-infrared (near-IR) spectral range, as Si sub-cells have weak absorption close to the band gap. Here, we introduce an integrated near-field/far-field light trapping scheme to enhance the efficiency of silicon-based multi-junction solar cells in the near-IR range. To achieve this, we design a nanopatterned diffractive silver back-reflector featuring a scattering matrix that optimizes trapping of multiply-scattered light into a range of diffraction angles. We minimize reflection to the 0th-order and parasitic plasmonic absorption in the silver by engineering destructive interference in the patterned back contact. Numerical and experimental assessment of the optimal design on the performance of single-junction Si TOPCon solar cells highlights an improved external quantum efficiency (EQE) over a planar back-reflector (+1.52 mA/cm2). Nanopatterned metagrating back-reflectors are fabricated on GaInP/GaInAsP//Si two-terminal triple-junction solar cells via Substrate Conformal Imprint Lithography (SCIL) and characterized optically and electronically, demonstrating a power conversion efficiency improvement of +0.9%abs over the planar reference. Overall, our work demonstrates the potential of nanophotonic light trapping for enhancing the efficiency of silicon-based multi-junction solar cells, paving the way for more efficient and sustainable solar energy technologies.

Authors:Anton Pakhomov, Mikhail Arkhipov, Nikolay Rosanov, Rostislav Arkhipov

Abstract: The generalization of the area theorem is derived for the case of a pulse circulating inside a ring laser cavity. In contrast to the standard area theorem, which is valid for a single pass of a traveling pulse through a resonant medium, the obtained generalized area theorem takes into account the medium-assisted nonlinear self-action effects through the medium excitation left by the pulse at the previous round-trip in the cavity. The generalized area theorem was then applied to the theoretical description of the dynamics of a single-section ring-cavity laser and the steady solutions for the pulse area and for the medium parameters were found both in the limit of a lumped model and for a spatially-extended system. The derived area theorem can be used for the convenient analytical description of different coherent photonic devices, like coherently mode-locked lasers or pulse compressors, as well as for the analysis of the photon echo formation in cavity-based setups.

Authors:Mauro David, Ismael C. Doganlar, Daniele Nazzari, Elena Arigliani, Dominik Wacht, Masiar Sistani, Hermann Detz, Georg Ramer, Bernhard Lendl, Walter M. Weber, Gottfried Strasser, Borislav Hinkov

Abstract: Liquid spectroscopy in the mid-infrared spectral range is a very powerful, yet premature technique for selective and sensitive molecule detection. Due to the lack of suitable concepts and materials for versatile miniaturized sensors, it is often still limited to bulky systems and offline analytics. Mid-infrared plasmonics is a promising field of current research for such compact and surface-sensitive structures, enabling new pathways for much-needed photonic integrated sensors. In this work, we focus on extending the concept of Ge/Au-based mid-infrared plasmonic waveguides to enable broadband liquid detection. Through the implementation of high-quality dielectric passivation layers deposited by atomic layer deposition (ALD), we cover the weak and water-soluble Ge native oxide. We show that approximately 10 nm of e.g. Al2O3 or ZrO2 can already protect the plasmonic waveguides for up to 90 min of direct water exposure. This unlocks integrated sensing schemes for broadband molecule detection based on mid-infrared plasmonics. In a proof-of-concept experiment, we further demonstrate that the ZrO2 coated waveguides can be activated by surface functionalization, allowing the selective measurement of diethyl ether at a wavelength of 9.38 {\mu}m.

Authors:Mahmoud Jalali Mehrabad, Sunil Mittal, Mohammad Hafezi

Abstract: Topological photonics is emerging as a new paradigm for the development of both classical and quantum photonic architectures. What makes topological photonics remarkably intriguing is the built-in protection as well as intrinsic unidirectionality of light propagation, which originates from the robustness of global topological invariants. In this Perspective, we present an intuitive and concise pedagogical overview of fundamental concepts in topological photonics. Then, we review the recent developments of the main activity areas of this field, categorized into linear, nonlinear, and quantum regimes. For each section, we discuss both current and potential future directions, as well as remaining challenges and elusive questions regarding the implementation of topological ideas in photonics systems.

Authors:Jan Helbing, Peter Hamm

Abstract: Several ways to electronically synchronize different types of amplified femtosecond laser systems are presented, based on a single freely programmable electronics hardware: Arbitrary-detuning asynchronous optical sampling, as well as actively locking two femtosecond laser oscillators, albeit not necessarily to the same round-trip frequency. They allow us to rapidly probe a very wide range of timescales, from picoseconds to potentially seconds, in a single transient absorption experiment without the need to move any delay stage. Experiments become possible that address a largely unexplored aspect of many photochemical reactions, in particular in the context of photo-catalysis as well as photoactive proteins, where an initial femtosecond trigger very often initiates a long-lasting cascade of follow-up processes. The approach is very versatile, and allows us to synchronize very different lasers, such as a Ti:Sa amplifier and a 100~kHz Yb-laser system. The jitter of the synchronisation, and therewith the time-resolution in the transient experiment, lies in the range from 1~ps to 3~ps, depending on the method. For illustration, transient IR measurements of the excited state solvation and decay of a metal carbonyl complex as well as the full reaction cycle of bacteriorhodopsin are shown. The pros and cons of the various methods are discussed, with regard to the scientific question one might want to address, and also with regard to the laser systems that might be already existent in a laser lab.

Authors:Gil Weinberg, Uri Weiss, Ori Katz

Abstract: Fiber-based confocal endomicroscopy has shown great promise for minimally-invasive deep-tissue imaging. Despite its advantages, confocal fiber-bundle endoscopy inherently suffers from undersampling due to the spacing between fiber cores, and low collection efficiency when the target is not in proximity to the distal fiber facet. Here, we demonstrate an adaptation of image-scanning microscopy (ISM) to lensless fiber bundle endoscopy, doubling the spatial sampling frequency and significantly improving collection efficiency. Our approach only requires replacing the confocal detector with a camera. It improves the spatial resolution for targets placed at a distance from the fiber tip, and addresses the fundamental challenge of aliasing/pixelization artifacts.

Authors:Rui-Qian Li, Yi-Wei Shen, Bao-De Lin, Jingyi Yu, Xuming He, Cheng Wang

Abstract: Photonic reservoir computing (PRC) is a special hardware recurrent neural network, which is featured with fast training speed and low training cost. This work shows a wavelength-multiplexing PRC architecture, taking advantage of the numerous longitudinal modes in a Fabry-Perot semiconductor laser. These modes construct connected physical neurons in parallel, while an optical feedback loop provides interactive virtual neurons in series. We experimentally demonstrate a four-channel wavelength-multiplexing PRC, which runs four times faster than the single-channel case. It is proved that the multiplexing PRC exhibits superior performance on the task of signal equalization in an optical fiber communication link. Particularly, this scheme is highly scalable owing to the rich mode resources in Fabry-Perot lasers.

Authors:Jacob B Khurgin

Abstract: Photonic Time crystals (PTC) arise in time-modulated media when the frequency of modulation of permittivity is on the order of twice the frequency of light and are manifested by the generation and amplification of so-called time reversed waves propagating in the direction opposite to the incoming light. Superficially, the observed phenomenon bears resemblance to the widely known phenomena of optical parametric generation (OPG) and amplification (OPA) using second or third order optical nonlinearities. I show that while indeed the same physical mechanism underpins both PTC and OPA , the difference arises from the boundary conditions. Thus , while dispersion for both PTC and OPA exhibit the same bandgap in momentum space, only in the case of PTC can one have propagation in that bandgap with exponential amplification. I also show that PTC can be engineered with both second and third order nonlinearities, and that rather unexpectedly, modulating permittivity on the ultrafast (few fs) rate is not a necessity, and that one can emulate all the PTC features using materials with a few picoseconds response time commensurate with the propagation time through the medium.

Authors:Taiping Zhang

Abstract: In this thesis, we propose to tackle this important issue by designing and realizing a novel nano-optical device based on the use of a photonic crystal (PC) structure to generate an efficient coupling between the external source and a NA. In this dissertation, the content is arranged into three charpters. Chapter 1 introduces the theoritical background of this research including surface plasmon and photonic crystal concepts. This chapter also shows the design of the hybrid devices and demonstrates the numerical simulation of their optical properties. Chapter 2 mainly describes the process and the fabricated samples. The nanodevices are fabricated on an InP membrane substrate. The critical technology for the fabrication is complex electron beam lithography. With this technology the alignment of the positions of PC structure and NA is well controlled. Chapter 3 demonstrates the optical characterizations of the hybrid nanodevices including far-field characterizations and near-field characterizations. The far-field measurement is performed by micro-photoluminescence spectroscopy at room temperature. The results show that for the defect PC cavities, the presence of the NA influences the optical properties of the laser, such as lasing threshold and laser wavelength. The near-field measurement is performed by near-field scanning microscopy, at room temperature also. The investigation shows that the NA modifies the optical field distribution of the laser mode. The modification depends on the position and direction of the NA and it is sensitive to the polarization of the optical field.

Authors:Siying Liu, Saleh Alfarhan, Wenbo Wang, Shuai Feng, Yuxiang Zhu, Luyang Liu, Kenan Song, Sui Yang, Kailong Jin, Xiangfan Chen

Abstract: The emerging Internet of Things (IoTs) invokes increasing security demands that require robust encryption or anti-counterfeiting technologies. Albeit being acknowledged as efficacious solutions in processing elaborate graphical information via multiple degrees of freedom, optical data encryption and anti-counterfeiting techniques are typically inept in delivering satisfactory performance without compromising the desired ease-of-processibility or compatibility, thus leading to the exploration of novel materials and devices that are competent. Here, a robust optical data encryption technique is demonstrated utilizing polymer-stabilized-liquid-crystals (PSLCs) combined with projection photoalignment and photopatterning methods. The PSLCs possess implicit optical patterns encoded via photoalignment, as well as explicit geometries produced via photopatterning. Furthermore, the PSLCs demonstrate improved robustness against harsh chemical environments and thermal stability, and can be directly deployed onto various rigid and flexible substrates. Based on this, it is demonstrated that single PSLC is apt to carry intricate information, or serve as exclusive watermark with both implicit features and explicit geometries. Moreover, a novel, generalized design strategy is developed, for the first time, to encode intricate and exclusive information with enhanced security by spatially programming the photoalignment patterns of a pair of cascade PSLCs, which further illustrates the promising capabilies of PSLCs in optical data encryption and anti-counterfeiting.

Authors:Victor P. Ruban

Abstract: The coupled nonlinear Schroedinger equations for paraxial optics with two circular light polarizations, in a defocusing Kerr medium with anomalous dispersion, coincide in form with the Gross-Pitaevskii equations for a binary Bose-Einstein condensate of cold atoms in the phase separation regime. A helical symmetry of optical waveguide corresponds to rotation of transverse potential confining the condensate. The ``centrifugal force'' makes essential effect on propagation of light wave in such system. Numerical simulations for a waveguide of elliptical cross-section revealed previously unknown in optics, specific structures consisting of quantized vortices and domain walls between two polarizations.

Authors:Mitradeep Sarkar, Michael T. Enders, Mehrdad Shokooh-Saremi, Kenji Watanabe, Takashi Taniguchi, Hanan Herzig Sheinfux, Frank H. L. Koppens, Georgia Theano Papadakis

Abstract: High-quality low-dimensional layered and van der Waals materials are typically exfoliated, with sample cross sectional areas on the order of tens to hundreds of microns. The small size of flakes makes the experimental characterization of their dielectric properties unsuitable with conventional spectroscopic ellipsometry, due to beam-sample size mismatch and non-uniformities of the crystal axes. Previously, the experimental measurement of the dielectrirc permittivity of such microcrystals was carried out with near-field tip-based scanning probes. These measurements are sensitive to external conditions like vibrations and temperature, and require non-deterministic numerical fitting to some a priori known model. We present an alternative method to extract the in-plane dielectric permittivity of van der Waals microcrystals, based on identifying reflectance minima in spectroscopic measurements. Our method does not require complex fitting algorithms nor near field tip-based measurements and accommodates for small-area samples. We demonstrate the robustness of our method using hexagonal boron nitride and {\alpha}-MoO3, and recover their dielectric permittivities that are close to literature values.

Authors:Kiumars Aryana, Hyun Jung Kim, Cosmin-Constantin Popescu, Steven Vitale, Hyung Bin Bae, Taewoo Lee, Tian Gu, Juejun Hu

Abstract: Reconfigurable or programmable photonic devices are rapidly growing and have become an integral part of many optical systems. The ability to selectively modulate electromagnetic waves through electrical stimuli is crucial in the advancement of a variety of applications from data communication and computing devices to environmental science and space explorations. Chalcogenide-based phase change materials (PCMs) are one of the most promising material candidates for reconfigurable photonics due to their large optical contrast between their different solid-state structural phases. Although significant efforts have been devoted to accurate simulation of PCM-based devices, in this paper, we highlight three important aspects which have often evaded prior models yet having significant impacts on the thermal and phase transition behavior of these devices: the enthalpy of fusion, the heat capacity change upon glass transition, as well as the thermal conductivity of liquid-phase PCMs. We further investigated the important topic of switching energy scaling in PCM devices, which also helps explain why the three above-mentioned effects have long been overlooked in electronic PCM memories but only become important in photonics. Our findings offer insight to facilitate accurate modeling of PCM-based photonic devices and can inform the development of more efficient reconfigurable optics.

Authors:Dmitry Svintsov, Georgy Alymov

Abstract: Despite numerous applications of two-dimensional plasmons for electromagnetic energy manipulation at the nanoscale, their quantitative refraction and reflection laws (analogs of Fresnel formulas in optics) have not yet been established. This fact can be traced down to the strong non-locality of equations governing the 2d plasmon propagation. Here, we tackle this difficulty by direct solution of plasmon scattering problem with Wiener-Hopf technique. We obtain the reflection and transmission coefficients for 2d plasmons at the discontinuity of 2d conductivity at arbitrary incidence angle, for both gated and non-gated 2d systems. At a certain incidence angle, the absolute reflectivity has a pronounced dip reaching zero for gated plasmons. The dip is associated with wave passage causing no dynamic charge accumulation at the boundary. For all incidence angles, the reflection has a non-trivial phase different from zero and $\pi$.

Authors:Zhuoran Fang, Bassem Tossoun, Antoine Descos, Di Liang, Xue Huang, Geza Kurczveil, Arka Majumdar, Raymond G. Beausoleil

Abstract: Silicon photonics has evolved from lab research to commercial products in the past decade as it plays an increasingly crucial role in data communication for next-generation data centers and high performance computing1. Recently, programmable silicon photonics has also found new applications in quantum2 and classical 3 information processing. A key component of programmable silicon photonic integrated circuits (PICs) is the phase shifter, traditionally realized via the thermo-optic or plasma dispersion effect which are weak, volatile, and power hungry. A non-volatile phase shifter can circumvent these limitations by requiring zero power to maintain the switched phases. Previously non-volatile phase modulation was achieved via phase-change4 or ferroelectric materials5, but the switching energy remains high (pico to nano joules) and the speed is slow (micro to milli seconds). Here, we report a non-volatile III-V-on-silicon photonic phase shifter based on HfO2 memristor with sub-pJ switching energy (~400fJ), representing over an order of magnitude improvement in energy efficiency compared to the state of the art. The non-volatile phase shifter can be switched reversibly using a single 100ns pulse and exhibits an excellent endurance over 800 cycles. This technology can enable future energy-efficient programmable PICs for data centers, optical neural networks, and quantum information processing.

Authors:Valeriy I. Kondratyev, Dmitry V. Permyakov, Tatyana V. Ivanova, Ivan V. Iorsh, Dmitry N. Krizhanovskii, Maurice S. Skolnick, Vasily Kravtsov, Anton K. Samusev

Abstract: Guided 2D exciton-polaritons, resulting from the strong coupling of excitons in semiconductors with non-radiating waveguide modes, provide an attractive approach towards developing novel on-chip optical devices. These quasiparticles are characterized by long propagation distances and efficient nonlinear interaction. However, as guided exciton-polaritons are uncoupled from the free space, it is challenging to investigate them using conventional far-field spectroscopy techniques. Here we demonstrate a powerful approach for probing and manipulating guided polaritons in a Ta$_2$O$_5$ slab integrated with a WS$_2$ monolayer using evanescent coupling through a high-index solid immersion lens. Tuning the nanoscale gap between the lens and the sample, we demonstrate in-situ control over radiative losses and Rabi splitting of guided polaritons at ambient conditions. This extra degree of freedom allows for extracting all the intrinsic parameters of the strongly coupled system under study. Our results enable the future development of integrated optics employing room-temperature exciton-polaritons in 2D semiconductor-based structures.

Authors:M. A. Masharin, D. Khmelevskaia, V. I. Kondratiev, D. I. Markina, A. D. Utyushev, D. M. Dolgintsev, A. D. Dmitriev, V. A. Shahnazaryan, A. P. Pushkarev, F. Isik, I. V. Iorsh, I. A. Shelykh, H. V. Demir, A. K. Samusev, S. V. Makarov

Abstract: Deeply subwavelength lasers (or nanolasers) are highly demanded for compact on-chip bioimaging and sensing at the nanoscale. One of the main obstacles for the development of single-particle nanolasers with all three dimensions shorter than the emitting wavelength in the visible range is the high lasing thresholds and the resulting overheating. Here we exploit exciton-polariton condensation and mirror-image Mie modes in a cuboid CsPbBr$_3$ nanoparticle to achieve coherent emission at the visible wavelength of around 0.53~$\mu $m from its ultra-small ($\approx$0.007$\mu$m$^3$ or $\approx\lambda^3$/20) semiconductor nanocavity. The polaritonic nature of the emission from the nanocavity localized in all three dimensions is proven by direct comparison with corresponding one-dimensional and two-dimensional waveguiding systems with similar material parameters. Such a deeply subwavelength nanolaser is enabled not only by the high values for exciton binding energy ($\approx$35 meV), refractive index ($>$2.5 at low temperature), and luminescence quantum yield of CsPbBr$_3$, but also by the optimization of polaritons condensation on the Mie resonances. Moreover, the key parameters for optimal lasing conditions are intermode free spectral range and phonons spectrum in CsPbBr$_3$, which govern polaritons condensation path. Such chemically synthesized colloidal CsPbBr$_3$ nanolasers can be easily deposited on arbitrary surfaces, which makes them a versatile tool for integration with various on-chip systems.

Authors:Ruben Ohana, Daniel Hesslow, Daniel Brunner, Sylvain Gigan, Kilian Müller

Abstract: We introduce a novel method to perform linear optical random projections without the need for holography. Our method consists of a computationally trivial combination of multiple intensity measurements to mitigate the information loss usually associated with the absolute-square non-linearity imposed by optical intensity measurements. Both experimental and numerical findings demonstrate that the resulting matrix consists of real-valued, independent, and identically distributed (i.i.d.) Gaussian random entries. Our optical setup is simple and robust, as it does not require interference between two beams. We demonstrate the practical applicability of our method by performing dimensionality reduction on high-dimensional data, a common task in randomized numerical linear algebra with relevant applications in machine learning.

Authors:Martin Albrecht, Ondřej Hort, Michaela Kozlová, Miroslav Krůs, Jaroslav Nejdl

Abstract: Many applications of short-wavelength radiation impose strong requirements on the coherence properties of the source. However, the measurement of such properties poses a challenge, mainly due to the lack of high-quality optics and source fluctuations that often violate assumptions necessary for multi-shot or cumulative techniques. In this article, we present a new method of single-shot spatial coherence measurement adapted to the soft X-ray spectral range. Our method is based on a far-field diffraction pattern from a binary transmission mask consisting of a non-redundant array of simple apertures. Unlike all currently available methods, our technique allows measuring radiation field with an arbitrary spatial coherence function without any prior assumption on intensity distribution or the model of the degree of spatial coherence. We experimentally verified the technique by retrieving the spatial coherence functions of individual shots of laser-driven Zn plasma soft X-ray laser with one- and two-dimensional masks. The experimental results revealed nontrivial illumination pattern and strong asymmetry of the spatial coherence function, which clearly calls for abandoning the often used models that assume rotational invariance of the coherence function, such as the popular Gaussian-Schell beam model.

Authors:Javier Cuerda, Jani M. Taskinen, Nicki Källman, Leo Grabitz, Päivi Törmä

Abstract: We experimentally observe the quantum geometric tensor, namely the quantum metric and the Berry curvature, for a square lattice of radiatively coupled plasmonic nanoparticles. We observe a non-zero Berry curvature and show that it arises solely from non-Hermitian effects. The quantum metric is found to originate from a pseudospin-orbit coupling. The long-range nature of the radiative interaction renders the behavior distinct from tight-binding systems: Berry curvature and quantum metric are centered around high-symmetry lines of the Brillouin zone instead of high-symmetry points. Our results inspire new pathways in the design of topological systems by tailoring losses or gain.

Authors:Javier Cuerda, Jani M. Taskinen, Nicki Källman, Leo Grabitz, Päivi Törmä

Abstract: We theoretically predict the full quantum geometric tensor, comprising the quantum metric and the Berry curvature, for a square lattice of plasmonic nanoparticles. The gold nanoparticles act as dipole or multipole antenna radiatively coupled over long distances. The photonic-plasmonic eigenfunctions and energies of the system depend on momentum and polarization (pseudospin), and their topological properties are encoded in the quantum geometric tensor. By T-matrix numerical simulations, we identify a TE-TM band splitting at the diagonals of the first Brillouin zone, that is not predicted by the empty lattice band structure nor by the highly symmetric nature of the system. Further, we find quantum metric around these regions of the reciprocal space, and even a non-zero Berry curvature despite the trivial lattice geometry and absence of magnetic field. We show that this non-zero Berry curvature arises exclusively from non-Hermitian effects which break the time-reversal symmetry. The quantum metric, in contrast, originates from a pseudospin-orbit coupling given by the polarization and directional dependence of the radiation.

Authors:Jaeuk Kim, Salvatore Torquato

Abstract: Disordered stealthy hyperuniform dielectric composites exhibit novel electromagnetic wave transport properties in two and three dimensions. Here, we carry out the first study of the electromagnetic properties of one-dimensional (1D) disordered stealthy hyperuniform layered media. From an exact nonlocal theory, we derive an approximation formula for the effective dynamic dielectric constant tensor ${\boldsymbol \varepsilon}_e({\bf k}_q,\omega)$ of general 1D media that is valid well beyond the quasistatic regime and apply it to 1D stealthy hyperuniform systems. We consider incident waves of transverse polarization, frequency $\omega$, and wavenumber $k_q$. Our formula for ${\boldsymbol \varepsilon}_e({k}_q,\omega)$, which is given in terms of the spectral density, leads to a closed-form relation for the transmittance $T$. Our theoretical predictions are in excellent agreement with finite-difference time-domain (FDTD) simulations. Stealthy hyperuniform layered media have perfect transparency intervals up to a finite wavenumber, implying no Anderson localization, but non-stealthy hyperuniform media are not perfectly transparent. Our predictive theory provides a new path for the inverse design of the wave characteristics of disordered layered media, which are readily fabricated, by engineering their spectral densities.

Authors:Nicholas S. Yama, I-Tung Chen, Srivatsa Chakravarthi, Bingzhao Li, Christian Pederson, Bethany E. Matthews, Steven R. Spurgeon, Daniel E. Perea, Mark G. Wirth, Peter V. Sushko, Mo Li, Kai-Mei C. Fu

Abstract: The compact size, scalability, and strongly confined fields in integrated photonic devices enable new functionalities in photonic networking and information processing, both classical and quantum. Gallium phosphide (GaP) is a promising material for active integrated photonics due to its high refractive index, wide band gap, strong nonlinear properties, and large acousto-optic figure of merit. In this work we demonstrate that silicon-lattice-matched boron-doped GaP (BGaP), grown at the 12-inch wafer scale, provides similar functionalities as GaP. BGaP optical resonators exhibit intrinsic quality factors exceeding 25,000 and 200,000 at visible and telecom wavelengths respectively. We further demonstrate the electromechanical generation of low-loss acoustic waves and an integrated acousto-optic (AO) modulator. High-resolution spatial and compositional mapping, combined with ab initio calculations indicate two candidates for the excess optical loss in the visible band: the silicon-GaP interface and boron dimers. These results demonstrate the promise of the BGaP material platform for the development of scalable AO technologies at telecom and provide potential pathways toward higher performance at shorter wavelengths.

Authors:M. Zerbib, V. T. Hoang, J. C. Beugnot, K. P. Huy, B. Little, S. T. Chu, D. J. Moss, R. Morandotti, B. Wetzel, T. Sylvestre

Abstract: We report the observation of Brillouin backscattering in a 50-cm long spiral high-index doped silica chip waveguide and measured a Brillouin frequency shift of 16 GHz which is in very good agreement with theoretical predictions and numerical simulations based on the elastodynamics equation.

Authors:Hao Hong, Chen Huang, Chenjun Ma, Jiajie Qi, Can Liu, Shiwei Wu, Zhipei Sun, Enge Wang, Kaihui Liu

Abstract: Optical phase-matching involves establishing a proper phase relationship between the fundamental and generated waves to enable efficient optical parametric processes. It is typically achieved through either birefringence or periodically assigned polarization. Here, we report that twist angle in two-dimensional (2D) materials can generate a nonlinear Berry optical phase to compensate the phase mismatch in the process of nonlinear optical frequency conversion, and the vertical assembly of 2D layers with a proper twist sequence will generate a nontrivial "twist-phase-matching" (twist-PM) regime. The twist-PM model offers superior flexibility in the design of optical crystals, which works for twisted layers with either periodic or random thickness distribution. The designed crystals from twisted rhombohedra boron nitride films give rise to a second-harmonic generation conversion efficiency of ~8% within a thickness of only 3.2 um, and a facile polarization controllability that is absent in conventional crystals (from linear to left-/right-handed circular/elliptical polarizations). Our methodology establishes a platform for the rational designing and atomic manufacturing of nonlinear optical crystals based on abundant 2D materials for various functionalities.

Authors:Denis Kislov, Daniel Ofer, Andrey Machnev, Hani Barhom, Vjaceslavs Bobrovs, Alexander Shalin, Pavel Ginzburg

Abstract: The propulsion and acceleration of nanoparticles with light have both fundamental and applied significance across many disciplines. Needle-free injection of biomedical nano cargoes into living tissues is among the examples. Here we explore a new physical mechanism of laser-induced particle acceleration, based on abnormal optothermal expansion of mesoporous vaterite cargoes. Vaterite nanoparticles, a metastable form of calcium carbonate, were placed on a substrate, underneath a target phantom, and accelerated towards it with the aid of a short femtosecond laser pulse. Light absorption followed by picosecond-scale thermal expansion was shown to elevate the particles center of mass thus causing acceleration. It was shown that a 2um size vaterite particle, being illuminated with 0.5 W average power 100 fsec IR laser, is capable to overcome van der Waals attraction and acquire 15m/sec velocity. The demonstrated optothermal laser-driven needle-free injection into a phantom layer promotes the further development of light-responsive nanocapsules, which can be equipped with additional optical and biomedical functions for delivery, monitoring, and controllable biomedical dosage to name a few.

Authors:Francisco Tenopala-Carmona, Dirk Hertel, Sabina Hillebrandt, Andreas Mischok, Arko Graf, Klaus Meerholz, Malte C. Gather

Abstract: The orientation of luminescent molecules in organic light-emitting diodes (OLEDs) strongly influences device performance. However, our understanding of the factors controlling emitter orientation is limited as current measurements only provide ensemble-averaged orientation values. Here, we use single-molecule imaging to measure the transition dipole orientation of individual molecules in a state-of-the-art thermally evaporated host and thereby obtain complete orientation distributions of the hyperfluorescence-terminal emitter C545T. We achieve this by realizing ultra-low doping concentrations (10-6 wt%) of C545T and minimising background levels to reliably measure the photoluminescence of the emitter. This approach yields the orientation distributions of >1000 individual emitter molecules in a system relevant to vacuum-processed OLEDs. Analysis of solution- and vacuum-processed systems reveals that the orientation distributions strongly depend on the nanoscale environment of the emitter. This work opens the door to attaining unprecedented information on the factors that determine emitter orientation in current and future material systems for OLEDs.

Authors:Joel N. Johnson, Danielle R. Haverkamp, Yi-Hsin Ou, Khanh Kieu, Nils T. Otterstrom, Peter T. Rakich, Ryan O. Behunin

Abstract: In recent years, optical control of mechanical oscillators has emerged as a critical tool for everything from information processing to laser cooling. While traditional forms of optomechanical cooling utilize systems comprised of discrete optical and mechanical modes, it has recently been shown that cooling can be achieved in a chip-based system that possesses a continuum of modes. Through Brillouin-mediated phonon-photon interactions, cooling of a band of traveling acoustic waves can occur when anti-Stokes scattered photons exit the system more rapidly than the relaxation rate of the mechanical waves -- to a degree determined by the acousto-optic coupling. Here, we demonstrate that a continuum of traveling wave phonons can be cooled within an optical fiber, extending this physics to macroscopic length scales. Leveraging the large acousto-optic coupling permitted within a liquid-core optical fiber, heterodyne spectroscopy reveals power-dependent changes in spontaneous Brillouin scattering spectra that indicate a reduction of the thermal phonon population by 21K using 120 mW of injected laser power.

Authors:Vojtech Liska, Tereza Zemankova, Vojtech Svak, Petr Jakl, Jan Jezek, Martin Branecky, Stephen H. Simpson, Pavel Zemanek, Oto Brzobohaty

Abstract: Methods for controlling the motion of single particles, optically levitated in vacuum, have developed rapidly in recent years. The technique of cold damping makes use of feedback-controlled, electrostatic forces to increase dissipation without introducing additional thermal fluctuations. This process has been instrumental in the ground-state cooling of individual electrically charged nanoparticles. Here we show that the same method can be applied to a pair of nanoparticles, coupled by optical binding forces. These optical binding forces are about three orders of magnitude stronger than typical Coulombic inter-particle force and result in a coupled motion of both nanoparticles characterized by a pair of normal modes. We demonstrate cold damping of these normal modes, either independently or simultaneously, to sub-Kelvin temperatures at pressures of 5x10^{-3} mbar. Experimental observations are captured by a theoretical model which we use to survey the parameter space more widely and to quantify the limits imposed by measurement noise and time delays. Our work paves the way for the study of quantum interactions between meso-scale particles and the exploration of multiparticle entanglement in levitated optomechanical systems.

Authors:Christopher Oliver, Sebabrata Mukherjee, Mikael C. Rechtsman, Iacopo Carusotto, Hannah M. Price

Abstract: We extend the $t-z$ mapping formalism of time-dependent paraxial optics by identifying configurations displaying a synthetic magnetic vector potential, leading to a non-trivial band topology in propagating geometries. We consider an inhomogeneous 1D array of coupled optical waveguides beyond the standard monochromatic approximation, and show that the wave equation describing paraxial propagation of optical pulses can be recast in the form of a Schr\"{o}dinger equation, including a synthetic magnetic field whose strength can be controlled via the transverse spatial gradient of the waveguide properties across the array. We use an experimentally-motivated model of a laser-written waveguide array to demonstrate that this synthetic magnetic field can be engineered in realistic setups and can produce interesting observable effects such as cyclotron motion, a controllable Hall drift of the wavepacket displacement in space or time, and unidirectional propagation in chiral edge states. These results significantly extend the variety of physics that can be explored within propagating geometries and pave the way for exploiting this platform for higher-dimensional topological physics and strongly correlated fluids of light.

Authors:Aiping Liu, Jiawei Liu, Zhanfei Kang, Guang-Jie Chen, Xin-Biao Xu, Xifeng Ren, Guang-Can Guo, Qin Wang, Chang-Ling Zou

Abstract: A free-space-to-chip pipeline is proposed to efficiently transport single atoms from a magneto-optical trap to an on-chip evanescent field trap. Due to the reflection of the dipole laser on the chip surface, the conventional conveyor belt approach can only transport atoms close to the chip surface but with a distance of about one wavelength, which prevents efficient interaction between the atom and the on-chip waveguide devices. Here, based on a two-layer photonic chip architecture, a diffraction beam of the integrated grating with an incident angle of the Brewster angle is utilized to realize free-space-to-chip atom pipeline. Numerical simulation verified that the reflection of the dipole laser is suppressed and that the atoms can be brought to the chip surface with a distance of only 100nm. Therefore, the pipeline allows a smooth transport of atoms from free space to the evanescent field trap of waveguides and promises a reliable atom source for a hybrid photonic-atom chip.

Authors:Chunchao Wen, Jianfa Zhang, Shiqiao Qin, Zhihong Zhu, Wei Liu

Abstract: Studies into scatterings of photonic structures have been so far overwhelmingly focused on their dependencies on the spatial and spectral morphologies of the incident waves. In contrast, the evolution of scattering properties through another parameter space of incident directions (momentum space) has attracted comparably little attention, though of profound importance for various scattering-related applications. Here we investigate, from the perspective of quasi-normal modes (QNMs), the momentum-space scattering extremizations with respect to varying incident directions of plane waves. It is revealed that for effective single-QNM excitations, scatterings are maximized exactly along those directions where the QNM radiation reaches its maximum, with matched incident and radiation polarizations. For an arbitrary direction, when the incident polarization is tuned to be orthogonal to that of the mode radiation, the QNM cannot be excited and thus the scatterer becomes invisible with null scatterings. The principles we have revealed are protected by fundamental laws of reciprocity and energy conservation (optical theorem), which can be further expanded and applied for other branches of wave physics.

Authors:Ghislaine Flore Kabadiang Ngon, Conrad Bertrand Tabi, Timoléon Crépin Kofané

Abstract: The letter introduces an extended (3+1)-dimensional [(3+1)D] nonlocal cubic complex Ginzburg-Landau equation describing the dynamics of dissipative light bullets in optical fiber amplifiers under the interplay between dopants and a spatially nonlocal nonlinear response. The model equation includes the effects of fiber dispersion, linear gain, nonlinear loss, fiber nonlinearity, atomic detuning, linear and nonlinear diffractive transverse effects, and nonlocal nonlinear response. A system of coupled ordinary differential equations for the amplitude, temporal, and spatial pulse widths and position of the pulse maximum, unequal wavefront curvatures, chirp parameters, and phase shift is derived using the variational technique. A stability criterion is established, where a domain of dissipative parameters for stable steady-state solutions is found. Direct integration of the proposed nonlocal evolution equation is performed, which allows us to investigate the evolution of the Gaussian beam along a doped nonlocal optical fiber, showing stable self-organized dissipative spatiotemporal light bullets.

Authors:Conrad B. Tabi, Camus G. Latchio Tiofack, Hippolyte Tagwo, Timoléon C. Kofané

Abstract: This paper analyzes the behaviors of solitons in even higher-order dispersive media and explores the modulational instability phenomenon in optical media. The analysis considers quadratic, quartic, and sextic dispersions with weakly nonlocal Kerr nonlinearity. The results show that nonlocality enhances the MI gain and leads to rogue waves in response to different combinations of even dispersions and nonlocal nonlinearity. The study suggests that Kerr nonlocality can enhance the excitation of extreme events in even higher-order dispersive nonlinear media with potential applications in optical fibers and fiber lasers.

Authors:N. J Martin, M. Jalali Mehrabad, X. Chen, R. Dost, E. Nussbaum, D. Hallett, L. Hallacy, E. Clarke, P. K. Patil, S. Hughes, A. M Fox, M. S. Skolnick, L. R. Wilson

Abstract: Chirality in integrated quantum photonics has emerged as a promising route towards achieving scalable quantum technologies with quantum nonlinearity effects. Topological photonic waveguides, which utilize helical optical modes, have been proposed as a novel approach to harnessing chiral light-matter interactions on-chip. However, uncertainties remain regarding the nature and strength of the chiral coupling to embedded quantum emitters, hindering the scalability of these systems. In this work, we present a comprehensive investigation of chiral coupling in topological photonic waveguides using a combination of experimental, theoretical, and numerical analyses. We quantitatively characterize the position-dependence nature of the light-matter coupling on several topological photonic waveguides and benchmark their chiral coupling performance against conventional line defect waveguides for chiral quantum optical applications. Our results provide crucial insights into the degree and characteristics of chiral light-matter interactions in topological photonic quantum circuits and pave the way towards the implementation of quantitatively-predicted quantum nonlinear effects on-chip.

Authors:Chunyan Li, Antonina A. Arkhipova, Yaroslav V. Kartashov, Sergey A. Zhuravitskii, Nikolay N. Skryabin, Ivan V. Dyakonov, Alexander A. Kalinkin, Sergey P. Kulik, Victor O. Kompanets, Sergey V. Chekalin, Victor N. Zadkov

Abstract: We study both, experimentally and theoretically, propagation of light in the fs-laser written rotating square waveguide arrays and present the first experimental evidence of light localization induced by the rotation of periodic structure in the direction of light propagation. Such linear light localization occurs either in the corners of truncated square array, where it results from the interplay between the centrifugal effect and total internal reflection at the borders of truncated array, or in the center of array, where rotation creates effective attractive optical potential. The degree of localization of linear bulk and corner modes emerging due to the rotation increases with the increase of rotation frequency. Consequently, corner and bulk solitons in rotating wave-guide arrays become thresholdless for sufficiently large rotation frequencies, in contrast to solitons in non-rotating arrays that exist only above power threshold. Focusing nonlinearity enhances localization degree of corner modes, but surprising initially it leads to broadening of bulk nonlinear states, followed by their re-localization at high input powers. Our results open new prospects for control of evolution of nonlinear multidimensional excitations by dynamically varying potentials.

Authors:Maximilian Buchmüller, Ivan Shutsko, Sven Oliver Schumacher, Patrick Görrn

Abstract: Short-range surface plasmon polaritons (SR-SPPs) can arise due to the hybridization of surface plasmon polaritons propagating along the two interfaces of a thin metal slab. In optics, they have gained particular interest for imaging and sensing applications, because of their short wavelengths at optical frequencies along with strong field enhancement. However, mediating the interaction of SR-SPPs with photons in planar films is difficult because of the large momentum mismatch. For efficient coupling, nanostructuring such thin films (~20nm thickness), or placing metallic nanostructures in close proximity to the planar film is technologically challenging and can strongly influence the SR-SPP properties. In this paper, harnessing SR-SPPs in planar silver films is demonstrated using disorder-engineered metasurfaces. The disorder-engineering is realized by the light-controlled growth of silver nanoparticles. The dispersion of the hybrid modes with the silver thickness is measured and compared with simulations. We anticipate these results to introduce a novel and facile method for harnessing SR-SPPs in planar optical systems and make use of their promising properties for imaging, sensing and nonlinear optics.

Authors:Yi Zhou, Dingpeng Liao, Kun Zhang, Zijie Ma, Shikai Wu, Jun Ma, Xuemei Dai, Zhengguo Shang, Zhongquan Wen, Gang Chen

Abstract: The far-field resolution of optical imaging systems is restricted by the Abbe diffraction limit, a direct result of the wave nature of light. One successful technological approach to circumventing this limit is to reduce the effective size of a point-spread-function. In the past decades, great endeavors have been made to engineer an effective point-spread-function by exploiting different mechanisms, including optical nonlinearities and structured light illumination. However, these methods are hard to be applied to objects in a far distance. Here, we propose a new way to achieve super-resolution in a far field by utilizing angular magnification. We present the first proof-of-concept demonstration of such an idea and demonstrate a new class of lenses with angular magnification for far-field super-resolution imaging. Both theoretical and experimental results demonstrate a more than two-fold enhancement beyond the angular-resolution limit in the far-field imaging. The proposed approach can be applied to super-resolution imaging of objects in far distance. It has promising potential applications in super-resolution telescopes and remote sensing.

Authors:Sergei Gladyshev, Theodosios D. Karamanos, Lina Kuhn, Dominik Beutel, Thomas Weiss, Carsten Rockstuhl, Andrey Bogdanov

Abstract: Inverse Design of All-dielectric Metasurfaces with Bound States in the Continuum

Authors:Furong Zhang, Lu He, Huizhen Zhang, Ling-Jun Kong, Xingsheng Xu, Xiangdong Zhang

Abstract: Topological photonics has been developed for more than ten years. It has been proved that the combination of topology and photons is very beneficial to the design of robust optical devices against some disturbances. However, most of the work for robust optical logic devices stays at the theoretical level. There are very few topologically-protected logic devices fabricated in experiments. Here, we report the experimental fabrication of a series of topologically-protected all-optical logic gates. Seven topologically-protected all-optical logic gates (OR, XOR, NOT, XNOR, NAND, NOR, and AND) are fabricated on silicon photonic platforms, which show strong robustness even if some disorders exist. These robust logic devices are potentially applicable in future optical signal processing and computing.

Authors:Ling-Jun Kong, Jingfeng Zhang, Furong Zhang, Xiangdong Zhang

Abstract: After more than 70 years of development, holography has become an essential tool of modern optics in many applications. In fact, for various applications of different kinds of holographic techniques, stability and antijamming ability are very important. Here, optical topological structures are introduced into holographic technology, and an entirely new concept of optical topological holography is demonstrated to solve stability and antijamming problems. Based on the optical knots and links, the topological holography is not only developed in theory, but also demonstrated experimentally. In addition, a new topological holographic coding is established by regarding each knotted/linked topological structure as an information carrier. Due to the variety of knotted and linked structures and their characteristics of topological protection, such coding can have high capacity as well as robust properties. Furthermore, with writing the hologram into the liquid crystal, robust information storage of 3D topological holography is realized.

Authors:Xurong Li, Deniz Mengu, Aydogan Ozcan, Mona Jarrahi

Abstract: Imaging systems operating in the terahertz part of the electromagnetic spectrum are in great demand because of the distinct characteristics of terahertz waves in penetrating many optically-opaque materials and providing unique spectral signatures of various chemicals. However, the use of terahertz imagers in real-world applications has been limited by the slow speed, large size, high cost, and complexity of the existing imaging systems. These limitations are mainly imposed due to the lack of terahertz focal-plane arrays (THz-FPAs) that can directly provide the frequency-resolved and/or time-resolved spatial information of the imaged objects. Here, we report the first THz-FPA that can directly provide the spatial amplitude and phase distributions, along with the ultrafast temporal and spectral information of an imaged object. It consists of a two-dimensional array of ~0.3 million plasmonic photoconductive nanoantennas optimized to rapidly detect broadband terahertz radiation with a high signal-to-noise ratio. As the first proof-of-concept, we utilized the multispectral nature of the amplitude and phase data captured by these plasmonic nanoantennas to realize pixel super-resolution imaging of objects. We successfully imaged and super-resolved etched patterns in a silicon substrate and reconstructed both the shape and depth of these structures with an effective number of pixels that exceeds 1-kilo pixels. By eliminating the need for raster scanning and spatial terahertz modulation, our THz-FPA offers more than a 1000-fold increase in the imaging speed compared to the state-of-the-art. Beyond this proof-of-concept super-resolution demonstration, the unique capabilities enabled by our plasmonic photoconductive THz-FPA offer transformative advances in a broad range of applications that use hyperspectral and three-dimensional terahertz images of objects for a wide range of applications.

Authors:A. H. Gevorgyan

Abstract: We investigated the properties of dichroic cholesteric liquid crystals (CLCs) being in external static magnetic field directed along helix axis. We have shown that in the case of the wavelength dependence of magneto-optic activity parameter, new features appear in the optics of dichroic CLCs. We have shown that in this case new Dirac points appear, moreover, at some Dirac points photonic band gaps (PBGs) appear, at others, lines of magnetically induced transparency (MIT). In this case a polarization-sensitive transmission band appears too. At certain values of the helix pitch of the CLC and of the magnitude of the external magnetic field three PBGs of different nature appear, a transmittance band, two narrow lines of MIT and one broadband MIT. This system is non-reciprocal and the nonreciprocity changes over a wide range, it is observed both for reflection and transmittance and for absorption. The soft matter nature of CLCs and their response to external influences lead to easily tunable multifunctional devices that can find a variety of applications. They can apply as tunable narrow-band or broad-band filters and mirrors, a highly tunable broad/narrow-band coherent perfect absorber, transmitter, ideal optical diode, and other devices.

Authors:Jie Xu, Panpan He, Delong Feng, Yamei Luo, Siqiang Fan, Kangle Yong, Kosmas L. Tsakmakidis

Abstract: All-optical logic gates have been studied intensively for their potential to enable broadband, low-loss, and high-speed communication. However, poor tunability has remained a key challenge in this field. In this paper, we propose a Y-shaped structure composed of Yttrium Iron Garnet (YIG) layers that can serve as tunable all-optical logic gates, including, but not limited to, OR, AND, and NOT gates, by applying external magnetic fields to magnetize the YIG layers. Our findings demonstrate that these logic gates are based on topologically protected one-way edge modes, ensuring exceptional robustness against imperfections and nonlocal effects while maintaining extremely high precision. Furthermore, the operating band of the logic gates is shown to be tunable. In addition, we introduce a straightforward and practical method for controlling and switching the logic gates between "work", "skip", and "stop" modes. These findings have important implications for the design of high-performance and precise all-optical integrated circuits.

Authors:Joseph Arnold Riley, Michal Horák, Vlastimil Křápek, Noel Healy, Victor Pacheco-Peña

Abstract: Developing methods to sense local variations in nearby materials, such as their refractive index and thickness, is important in different fields including chemistry and biomedical applications, among others. Localized surface plasmons (LSPs) excited in plasmonic nanostructures have demonstrated to be useful in this context due to the spectral location of their associated resonances being sensitive to changes near the plasmonic structures. In this manuscript, Babinet's principle is explored by exploiting LSP resonances excited in complementary metal-dielectric cylindrical plasmonic structures (plasmonic particle-dimers and aperture-dimers in our case). Both plasmonic structures are evaluated numerically and experimentally using Electron Energy Loss Spectroscopy (EELS), providing a full physical understanding of the complementary nature of the excited LSP resonances. The studied plasmonic structures are then exploited for dielectric sensing under two configurations: when a thin dielectric film is positioned atop the plasmonic structures and when the analyte surrounds/fills the plasmonic particles/apertures. The complementary sensing performance of both proposed structures is also evaluated, showing the approximate validity of the Babinet principle with sensitivities values of up to 700 nm/RIU for thin dielectric sensing.

Authors:Ross Glyn MacDonald, Alex Yakovlev, Victor Pacheco-Peña

Abstract: Electromagnetic wave-based analogue computing has become an interesting computing paradigm demonstrating the potential for high-throughput, low power, and parallel operations. In this work, we propose a technique for the calculation of derivatives of temporal signals by exploiting transmission line techniques. We consider multiple interconnected waveguides (with some of them being closed-ended stubs) forming junctions. The transmission coefficient of the proposed structure is then tailored by controlling the length and number of stubs at the junction, such that the differentiation operation is applied directly onto the envelope of an incident signal sinusoidally modulated in the time domain. The physics behind the proposed structure is explained in detail and a full theoretical description of this operation is presented, demonstrating how this technique can be used to calculate higher order or even fractional temporal derivatives. We envision that these results may enable the development of further time domain wave-based analogue processors by exploiting waveguide junctions, opening new opportunities for wave-based single operators and systems.

Authors:Mikhail Arkhipov, Anton Pakhomov, Rostislav Arkhipov, Nikolay Rosanov

Abstract: We propose a simple quantum system, namely, a nested quantum-well structure, which is able to generate a train of half-cycle pulses of a few-fs duration, when driven by a static electric field. We theoretically investigate the emission of such a structure and its dependence on the parameters of the quantum wells. It is shown that the production of a regular output pulse train with tunable properties and the pulse repetition frequencies of tens of THz is possible in certain parameter ranges. We expect the suggested structure can be used as an ultra-compact source of subcycle pulses in the optical range.

Authors:Esteban Marulanda, Edgar Rueda

Abstract: In this paper, we present an analytical study of the relationship between the statistical distribution of a physical parameter and the uncertainties in the physical quantities used to determine it through indirect measurement. We investigate two possible methods for determining the physical quantity: linear regression and inversion of the equation in the parameter. Our analysis focuses on finding the limits of "small" uncertainties to guarantee a Gaussian distribution to the indirect physical quantity. Also, we introduce the "reliability cone" concept to describe the dependence of errors on the physical parameters uncertainties. We propose a new probability distribution for significant uncertainties and define the first three moments. We apply these methods to the z-scan and f-scan techniques, presenting the most sensitive parameters for the nonlinear two-photon absorption coefficient measurement. Finally, we implement our findings on experimental data of the two-photon absorption coefficient in CdSe.

Authors:Georgia L. Jones, Qiaozhou Xiong, Xinyu Liu, Brett E. Bouma, Martin Villiger

Abstract: Intravascular polarimetry with catheter-based polarization-sensitive optical coherence tomography (PS-OCT) complements the high-resolution structural tomograms of OCT with morphological contrast available through polarimetry. Its clinical translation has been complicated by the need for modification of conventional OCT hardware to enable polarimetric measurements. Here, we present a signal processing method to reconstruct polarization properties of tissue from measurements with a single input polarization state, bypassing the need for modulation or multiplexing of input states. Our method relies on a polarization symmetry intrinsic to round-trip measurements and uses the residual spectral variation of the polarization states incident on the tissue to avoid measurement ambiguities. We demonstrate depth-resolved birefringence and optic axis orientation maps reconstructed from in-vivo data of human coronary arteries. We validate our method through comparison with conventional dual-input state measurements and find a mean cumulative retardance error of 13.2deg without observable bias. The 95% limit of agreement between depth-resolved birefringence is 2.80 x 10^(-4), which is less than the agreement between two repeat pullbacks of conventional PS-OCT (3.14 x 10^(-4)), indicating that the two methods can be used interchangeably. The hardware simplification arising from using a single input state may be decisive in realizing the potential of polarimetric measurements for assessing coronary atherosclerosis in clinical practice.

Authors:Hui Cao, Tomáš Čižmár, Sergey Turtaev, Tomáš Tyc, Stefan Rotter

Abstract: Light transport in a highly multimode fiber exhibits complex behavior in space, time, frequency and polarization, especially in the presence of mode coupling. The newly developed techniques of spatial wavefront shaping turn out to be highly suitable to harness such enormous complexity: a spatial light modulator enables precise characterization of field propagation through a multimode fiber, and by adjusting the incident wavefront it can accurately tailor the transmitted spatial pattern, temporal profile and polarization state. This unprecedented control leads to multimode fiber applications in imaging, endoscopy, optical trapping and microfabrication. Furthermore, the output speckle pattern from a multimode fiber encodes spatial, temporal, spectral and polarization properties of the input light, allowing such information to be retrieved from spatial measurements only. This article provides an overview of recent advances and breakthroughs in controlling light propagation in multimode fibers, and discusses newly emerging applications.

Authors:Bartosz Mirecki, Mikołaj Rogalski, Piotr Arcab, Piotr Rogujski, Luiza Stanaszek, Michał Józwik, Maciej Truisak

Abstract: Exposure to laser light alters cell culture examination via optical microscopic imaging techniques, also based on label-free coherent digital holography. To mitigate this detrimental feature, researchers tend to use a broader spectrum and lower intensity of illumination, which can decrease the quality of holographic imaging due to lower resolution and higher noise. We study the lensless digital holographic microscopy (LDHM) ability to operate in the low photon budget (LPB) regime to enable imaging of unimpaired live cells with minimized sample interaction. Low-cost off-the-shelf components are used, promoting the usability of such a straightforward approach. We show that recording data in the LPB regime (down to 7 uW of illumination power) does not limit the contrast nor resolution of the hologram phase and amplitude reconstruction compared to the regular illumination. The LPB generates hardware camera shot noise, however, to be effectively minimized via numerical denoising. The ability to obtain high-quality, high-resolution optical complex field reconstruction was confirmed using the USAF 1951 amplitude sample, phase resolution test target, and finally, live glial restricted progenitor cells (as a challenging strongly absorbing and scattering biomedical sample). The proposed approach based on severely limiting the photon budget in lensless holographic microscopy method can open new avenues in high-throughout (optimal resolution, large field-of-view and high signal-to-noise-ratio single-hologram reconstruction) cell culture imaging with minimized sample interaction.

Authors:Shunlin Huang, Ning Zhang, Xu Lu, Jun Liu, Jinping Yao

Abstract: Light carrying orbital angular momentum (OAM) holds unique properties and boosts myriad applications in diverse fields from micro- to macro-world. Endeavors have been made to manipulate the OAM in order to generate on-demand structured light and to explore novel properties of light. However, the generation of an ultrafast wave packet carrying numerous vortices with various OAM modes, that is vortex string, has been rarely explored and remains a significant challenge. Moreover, methods that enable parallel detection of all vortices in a vortex string are lacking. Here, we demonstrate that a vortex string with 28 spatiotemporal optical vortices (STOVs) can be successfully generated in an ultrafast wave packet. All STOVs in the string can be randomly or orderly arranged. The diffraction rules of STOV strings are also revealed theoretically and experimentally. Following these rules, the topological charges and positions of all STOVs in a vortex string can be easily recognized. The strategy for parallel generation and detection of STOV strings will open up exciting perspectives in STOV-based optical communications and also promote promising applications of the structured light in light-matter interaction, quantum information processing, etc.

Authors:Vasu Dev, Vishwa Pal

Abstract: Discrete vortex, formed by a one-dimensional (1D) ring array of lasers, contains high output power as compared to a conventional continuous vortex, therefore, has attracted considerable interest due to widespread applications in various fields. We present a method for probing the magnitude and sign of the topological charge (TC) of an unknown discrete vortex, by analyzing the interference pattern of a 1D ring array of lasers. The interference pattern of an unknown discrete vortex with TC$\neq 0$ is averaged with the interference pattern of TC= 0, which gives rise to a variation in the fringe visibility as a function of laser number (j) in a 1D ring array. The number of dips observed in the fringe visibility curve is found to be proportional to the magnitude of TC of a discrete vortex. The sign of TC is determined by averaging the interference patterns of unknown discrete vortex (TC$\neq 0$) with known TC= +1. The number of dips in the fringe visibility curve decreases by one for a positive TC, and increases by one for a negative TC. Further, we have verified our method against the phase disorder, and it is found that the phase disorder does not influence an accurate determination of TC of a discrete vortex. The working principle as well as numerical and experimental results are presented for the discrete vortices with TC from small to large values. An excellent agreement between the experimental results and numerical simulations is found. Our method can be useful in the applications of discrete vortices.

Authors:Anita Kumari, Vasu Dev, Vishwa Pal

Abstract: We numerically and experimentally study the autofocusing and self-healing of partially blocked circular Airy derivative beams (CADBs). The CADB consists of multiple rings, and partial blocking of CADB with different kinds is achieved by using symmetric and asymmetric binary amplitude masks, enabling blocking of inner/outer rings and sectorially. The CADB blocked with different types possesses the ability to autofocus, however, the required propagation distance for abrupt autofocusing vary with the amount and types of blocking. The abrupt autofocusing is quantified by a maximum k-value, and how fast it changes around the autofocusing distance ($z_{af}$). In particular, CADB blocked with inner rings (first/two/three) exhibits an abrupt autofocusing, as the k-value sharply increases [decreases] just before [after] $z_{af}$. The maximum k-value always occurs at $z_{af}$, which decreases as the number of blocked inner rings increases. For CADB blocked with outer rings, the k-value gradually changes around $z_{af}$, indicating a lack of abrupt autofocusing. The value of $z_{af}$ increases with the number of blocked outer rings. This suggests that although outer rings contain low intensities, these play an important role in autofocusing. A sectorially blocked CADB possesses an abrupt autofocusing, and maximum k-value depends on the amount of blocking. The CADB blocked with different types possesses good self-healing abilities, where blocked parts reappear as a result of redistribution of intensity. The maximum self-healing occurs at $z_{af}$, where an overlap integral approaches a maximum value. Finally, we have compared ideal CADB and partially blocked CADB having the same radii, and found that an ideal CADB possesses better abrupt autofocusing. We have found a good agreement between the numerical simulations and experimental results.

Authors:Michael I. Tribelsky

Abstract: The study's primary goal is to reveal the generic effects of the problem symmetry, its violation, and energy conservation law on the Pointing vector field singularities based on the study of resonant scattering of a linearly polarized plane electromagnetic wave by an infinite right cylinder. The polarization plane of the incident wave has an arbitrary orientation against the cylinder axis ($z$-axis), and the wave vector is antiparallel to the $x$-axis. The angle between the polarization plane and the $z$-axis plays the role of a bifurcation parameter. We show that any deviation of the incident wave from the pure TE or TM orientations makes the pattern of the Poynting vector field lines three-dimensional. Meanwhile, the translational symmetry along the $z$-axis remains. Accordingly, all singular points of the Pointing vector field, but the ones lying on the $x$-axis, become ``false'' singularities. They are singular in the projection of the field lines on the plane $xy$. However, in the three-dimensional space, these points are regular owing to the finiteness of the Pointing vector $z$-component. In contrast, the singularities belonging to the $x$-axis remain the actual singular points at any angle between the $z$-axis and the polarization plane since the $z$-component of the Poynting vector for them vanishes owing to the problem symmetry. We study the bifurcations related to the creation (annihilation) of the false and actual singular points due to their splitting (merger) because of the bifurcation parameter variations. In all inspected cases, a pitchfork bifurcation occurs: the distance between the diverging (converging) singularities as well as the corresponding roots of the characteristic equation vary as the square root of a normalized deviation of the bifurcation parameter from its critical value. We present a phenomenological theory, explaining all observed bifurcations.

Authors:Aleksandr Razumov, Holger R. Heebøll, Mario Dummont, Osama Terra, Bozhang Dong, Jasper Riebesehl, Poul Varming, Jens E. Pedersen, Francesco Da Ros, John E. Bowers, Darko Zibar

Abstract: Advanced digital signal processing techniques in combination with ultra-wideband balanced coherent detection have enabled a new generation of ultra-high speed fiber-optic communication systems, by moving most of the processing functionalities into digital domain. In this paper, we demonstrate how digital signal processing techniques, in combination with ultra-wideband balanced coherent detection can enable optical frequency comb noise characterization techniques with novel functionalities. We propose a measurement method based on subspace tracking, in combination with multi-heterodyne coherent detection, for independent phase noise sources identification, separation and measurement. Our proposed measurement technique offers several benefits. First, it enables the separation of the total phase noise associated with a particular comb-line or -lines into multiple independent phase noise terms associated with different noise sources. Second, it facilitates the determination of the scaling of each independent phase noise term with comb-line number. Our measurement technique can be used to: identify the most dominant source of phase noise; gain a better understanding of the physics behind the phase noise accumulation process; and confirm, already existing, and enable better phase noise models. In general, our measurement technique provides new insights into noise behavior of optical frequency combs.

Authors:Aun Zaidi, Noah A. Rubin, Maryna L. Meretska, Lisa Li, Ahmed H. Dorrah, Joon-Suh Park, Federico Capasso

Abstract: When light scatters off an object its polarization, in general, changes - a transformation described by the object's Mueller matrix. Mueller matrix imaging polarimetry is an important technique in science and technology to image the spatially varying polarization response of an object of interest, to reveal rich information otherwise invisible to traditional imaging. In this work, we conceptualize, implement and demonstrate a compact and minimalist Mueller matrix imaging system - composed of a metasurface to produce structured polarization illumination, and a metasurface for polarization analysis - that can, in a single shot, acquire images for all sixteen components of an object's spatially varying Mueller matrix. Our implementation, which is free of any moving parts or bulk polarization optics, should enable and empower applications in real-time medical imaging, material characterization, machine vision, target detection, and other important areas.

Authors:Wenzheng Lu, Leonarde de S. Menezes, Andreas Tittl, Haoran Ren, Stefan A. Maier

Abstract: Active metasurfaces provide unique advantages for on-demand light manipulation at a subwavelength scale for emerging applications of 3D displays, augmented/virtual reality (AR/VR) glasses, holographic projectors and light detection and ranging (LiDAR). These applications put stringent requirements on switching speed, cycling duration, controllability over intermediate states, modulation contrast, optical efficiency and operation voltages. However, previous demonstrations focus only on particular subsets of these key performance requirements for device implementation, while the other performance metrics have remained too low for any practical use. Here, we demonstrate an active Huygens' metasurface based on in-situ grown conductive polymer with holistic switching performance, including switching speed of 60 frames per second (fps), switching duration of more than 2000 switching cycles without noticeable degradation, hysteresis-free controllability over intermediate states, modulation contrast of over 1400%, optical efficiency of 28% and operation voltage range within 1 V. Our active metasurface design meets all foundational requirements for display applications and can be readily incorporated into other metasurface concepts to deliver high-reliability electrical control over its optical response, paving the way for compact and robust electro-optic metadevices.

Authors:Ines Martin, Yoann Pertot, Olivier Albert, Thomas Pinoteau, Sébastien Coudreau, Simon Pomarède, José Villanueva, Grégory Gitzinger, Penda Leye, Ivan Jelicic, Cyril Vaneph, Daniel Kaplan, Nicolas Forget, Simone Bux

Abstract: We present a 40-MHz ultrafast optical parametric amplifier (OPA), tunable from 0.8 to 1 $\mu$m, with a relative intensity noise (RIN) matching the shot-noise floor (-160 dB/Hz) above 2 MHz. The OPA is pumped by a 20-W Kerr-lens mode-locked Ytterbium laser and seeded by a a supercontinuum generated in an all-normal-dispersion (ANDi) fiber. With an average output power >1.5 W, this compact and simple scheme is an attractive alternative to synchronously-pumped optical parametric oscillators, especially within the context of stimulated Raman scattering (SRS) imaging. To illustrate the latter, we perform chemical imaging of water droplets in oil by SRS microscopy.

Authors:Piotr Arcab, Bartosz Mirecki, Marzena Stefaniuk, Monika Pawlowska, Maciej Trusiak

Abstract: Laser-based lensless digital holographic microscopy (LDHM) is often spoiled by considerable coherent noise factor. We propose a novel LDHM method with significantly limited coherent artifacts, e.g., speckle noise and parasitic interference fringes. It is achieved by incorporating a rotating diffuser, which introduces partial spatial coherence and preserves high temporal coherence of laser light, crucial for credible in-line hologram reconstruction. We present the first implementation of the classical rotating diffuser concept in LDHM, significantly increasing the signal-to-noise ratio while preserving the straightforwardness and compactness of the LDHM imaging device. Prior to the introduction of the rotating diffusor, we performed LDHM experimental hardware optimization employing 4 light sources, 4 cameras, and 3 different optical magnifications (camera-sample distances). It was guided by the quantitative assessment of numerical amplitude/phase reconstruction of test targets, conducted upon standard deviation calculation (noise factor quantification), and resolution evaluation (information throughput quantification). Optimized rotating diffuser LDHM (RD-LDHM) method was successfully corroborated in technical test target imaging and examination of challenging biomedical sample (60um thick mouse brain tissue slice). Physical minimization of coherent noise (up to 50%) was positively verified, while preserving optimal spatial resolution of phase and amplitude imaging. Coherent noise removal, ensured by proposed RD-LDHM method, is especially important in biomedical inference, as speckles can falsely imitate valid biological features. Combining this favorable outcome with large field-of-view imaging can promote the use of reported RD-LDHM technique in high-throughput stain-free biomedical screening.

Authors:Lutz Mertenskötter, Markus Kantner

Abstract: We present a statistical inference approach to estimate the frequency noise characteristics of ultra-narrow linewidth lasers from delayed self-heterodyne beat note measurements using Bayesian inference. Particular emphasis is on estimation of the intrinsic (Lorentzian) laser linewidth. The approach is based on a statistical model of the measurement process, taking into account the effects of the interferometer as well as the detector noise. Our method therefore yields accurate results even when the intrinsic linewidth plateau is obscured by detector noise. The regression is performed on periodogram data in the frequency domain using a Markov-chain Monte Carlo method. By using explicit knowledge about the statistical distribution of the observed data, the method yields good results already from a single time series and does not rely on averaging over many realizations, since the information in the available data is evaluated very thoroughly. The approach is demonstrated for simulated time series data from a stochastic laser rate equation model with 1/f-type non-Markovian noise.

Authors:Mohammed Sabbah, Federico Belli, Christian Brahms, John C. Travers

Abstract: We experimentally and numerically investigate flat supercontinuum generation in gas-filled anti-resonant guiding hollow-core photonic crystal fiber. By comparing results obtained with either argon or nitrogen we determine the role of the rotational Raman response on the supercontinuum formation. When using argon, a supercontinuum extending from 350 nm to 2 {\mu}m is generated through modulational instability. Although argon and nitrogen exhibit similar Kerr nonlinearity and dispersion, we find that the energy density of the continuum in the normal dispersion region is significantly lower when using nitrogen. Using numerical simulations, we find that due to the closely spaced rotational lines in nitrogen, gain suppression in the fundamental mode causes part of the pump pulse to be coupled into higher-order modes which reduces the energy transfer to wavelengths shorter than the pump.

Authors:Angzhen Li, Jonathan M. Ward, Ke Tian, Jibo Yu, Shengfei She, Chaoqi Hou, Haitao Guo, Síle Nic Chormaic, Pengfei Wang

Abstract: The fabrication of whispering gallery lasers (WGL) is used to experimentally evaluate the evaporation rate (mol/$\mu$m) and ratio (mol/mol) of erbium and silica lost from a doped fiber during heating. Fixed lengths of doped silica fiber are spliced to different lengths of undoped fiber and then evaporated by feeding into the focus of a CO$_{2}$ laser. During evaporation, erbium ions are precipitated in the doped silica fiber to control the erbium concentration in the remaining SiO$_2$, which is melted into a microsphere. By increasing the length of the undoped section, a critical point is reached where effectively no ions remain in the glass microsphere. The critical point is found using the lasing spectra of the whispering gallery modes in microspheres with equal sizes. From the critical point, it is estimated that, for a given CO$_{2}$ laser power, $6.36 \times 10^{-21}$~mol of Er$^{3+}$ is lost during the evaporation process for every cubic micron of silica fiber. This is equivalent to $1.74 \times 10^{-7}$~mol of Er$^{3+}$ lost per mol of SiO$_{2}$ evaporated. This result facilitates the control of the doping concentration in WGLs and provides insight into the kinetics of laser-induced evaporation of doped silica.

Authors:Jialin Li, Lizhen Wang, Yuzhong Chen, Yujie Li, Haiming Zhu, Linjun Li, Limin Tong1

Abstract: Interface interactions in 2D vertically stacked heterostructures play an important role in optoe-lectronic applications, photodetectors based on graphene/InSe heterostructures had shown promising performance nowadays. However, nonlinear optical properties studies based on the graphene/InSe heterostructure was insufficient. Here, we fabricated graphene/InSe heterostruc-ture by mechanical exfoliation, and investigated the optically induced charge transfer between graphene/InSe heterostructures by taking photoluminescence and pump-probe measurements. The large built-in electric field at the interface is confirmed by Kelvin probe force microscopy. Furthermore, due to the efficient interfacial carrier transfer driven by built-in electric potential (~ 286 meV) and broadband nonlinear absorption, the application of graphene/InSe heterostruc-ture in mode-locked laser is realized. Our work not only provides a deeper understanding for the dipole orientation related interface interactions on the photoexcited charge transfer of gra-phene/InSe heterostructure, but also enrich the saturable absorber family for ultrafast photon-ics application.

Authors:Timur Seidov, Maxim Gorlach

Abstract: We investigate the electromagnetic fields produced by the oscillating point electric or magnetic dipole placed in a spherical volume with nonzero time-independent effective axion field. Our analytical solution shows that the fields outside the volume are a superposition of electric and magnetic dipole fields. Such multipole time-dependent generalization of the Witten effect can be realized in magneto-electrics, topological insulators or metamaterials providing a flexible probe of P- and T-symmetry breaking phenomena in different electromagnetic structures.

Authors:Francis Granger NPSC, Saransh Raj Gosain NPSC, Gilles Nogues NEEL - NPSC, Edith Bellet-Amalric NPSC, Joel Cibert NEEL - NPSC, David Ferrand NEEL - NPSC, Kuntheak Kheng NPSC

Abstract: Single-photon sources are crucial for developing secure telecommunications. However, most systems operate at cryogenic temperatures. Here, we discuss a promising solid-state system emitting single photons at room temperature in the blue-green range, allowing for quantum communications in free space and underwater. The active element is a core-shell ZnSe tapered nanowire embedding a single CdSe quantum dot grown by molecular beam epitaxy. A patterned substrate enables a thorough study of the one and same nanowire by different methods. Our source exhibits anti-bunching with g(2)(0) < 0.3 near the centre of the photoluminescence line and shows high brightness. This work paves the way for developing single-photon sources operating at non-cryogenic temperatures.

Authors:David Bronte Ciriza, Agnese Callegari, Maria Grazia Donato, Berk Çiçek, Alessandro Magazzù, Iryna Kasianiuk, Denis Kasianiuk, Falko Schmidt, Antonino Foti, Pietro G. Gucciardi, Maurizio Lanza, Luca Biancofiore, Onofrio M. Maragò

Abstract: Microengines have shown promise for a variety of applications in nanotechnology, microfluidics, and nanomedicine, including targeted drug delivery, microscale pumping, and environmental remediation. However, achieving precise control over their dynamics remains a significant challenge. In this study, we introduce a microengine that exploits both optical and thermal effects to achieve a high degree of controllability. We find that in the presence of a strongly focused light beam, a gold-silica Janus particle becomes confined at the equilibrium point between optical and thermal forces. By using circularly polarized light, we can transfer angular momentum to the particle breaking the symmetry between the two forces and resulting in a tangential force that drives directed orbital motion. We can simultaneously control the velocity and direction of rotation of the particle changing the ellipticity of the incoming light beam, while tuning the radius of the orbit with laser power. Our experimental results are validated using a geometrical optics model that considers the optical force, the absorption of optical power, and the resulting heating of the particle. The demonstrated enhanced flexibility in the control of microengines opens up new possibilities for their utilization in a wide range of applications, encompassing microscale transport, sensing, and actuation.

Authors:Alessandro Pianelli, Rakesh Dhama, Jaroslaw Judek, Rafal Mazur, Humeyra Caglayan

Abstract: All-optical ultrafast switches enabled by artificial materials are considered at the forefront of the next generation of photonic communications and data processing. During the last two decades, the photonic applications, impact, and interest have tremendously increased in the framework of epsilon-near-zero (ENZ) photonics. Here, we experimentally propose a novel multilayered metamaterial utilizing Si-compatible titanium nitride and indium-tin-oxide materials. The device exhibits two effective ENZ wavelengths in the visible and near-infrared spectrum, with switching times down to a few hundred femtoseconds at the corresponding ENZ regions. This novel approach will bring ENZ metamaterials towards new hybrid integrated CMOS photonic circuit components for ultrafast all-optical terahertz modulation.

Authors:Andrew A. Voitiv, Mark T. Lusk, Mark E. Siemens

Abstract: We present and implement a method for the experimental measurement of geometric phase of non-geodesic (small) circles on any SU(2) parameter space. This phase is measured by subtracting the dynamic phase contribution from the total phase accumulated. Our design does not require theoretical anticipation of this dynamic phase value and the methods are generally applicable to any system accessible to interferometric and projection measurements. Experimental implementations are presented for two settings: (1) the sphere of modes of orbital angular momentum, and (2) the Poincar\'e sphere of polarizations of Gaussian beams.

Authors:Daniel Friedrich, Jin Qin, Benedikt Schurr, Tommaso Tufarelli, Heiko Groß, Bert Hecht

Abstract: Room-temperature strong coupling of a single quantum emitter and a single resonant plasmonic mode is a key resource for quantum information processing and quantum sensing at ambient conditions. To beat dephasing, ultrafast energy transfer is achieved by coupling single emitters to a plasmonic nanoresonator with an extremely small mode volume and optimal spectral overlap. Typically, normal mode splittings in luminescence spectra of single-emitter strongly-coupled systems are provided as evidence for strong coupling and to obtain rough estimates of the light-matter coupling strength g. However, a complete anticrossing of a single emitter and a cavity mode as well as the characterization of the uncoupled constituents is usually hard to achieve. Here, we exploit the light-induced oxygen-dependent blue-shift of individual CdSe/ZnS semiconductor quantum dots to tune their transition energy across the resonance of a scanning plasmonic slit resonator after characterizing both single emitter and nano resonator in their uncoupled states. Our results provide clear proof of single-emitter strong light-matter coupling at ambient condition as well as a value for the Rabi splitting at zero detuning 100 meV, consistent with modeling, thereby opening the path towards plexitonic devices that exploit single-photon nonlinearities at ambient conditions.

Authors:David Tebbe 2nd Institute of Physics and JARA-FIT, RWTH Aachen University, Aachen, Germany, Marc Schütte 2nd Institute of Physics, RWTH Aachen University, Aachen, Germany, Baisali Kundu Materials Science Center, Indian Institute of Technology, Kharagpur, West Bengal, India, Bernd Beschoten 2nd Institute of Physics and JARA-FIT, RWTH Aachen University, Aachen, Germany JARA-FIT Institute for Quantum Information, Forschungszentrum Jülich GmbH and RWTH Aachen University, Aachen, Germany, Prasana K. Sahoo Materials Science Center, Indian Institute of Technology, Kharagpur, West Bengal, India, Lutz Waldecker 2nd Institute of Physics and JARA-FIT, RWTH Aachen University, Aachen, Germany

Abstract: Optical micro-spectroscopy is an invaluable tool for studying and characterizing samples ranging from classical semiconductors to low-dimensional materials and heterostructures. To date, most implementations are based on point-scanning techniques, which are flexible and reliable but slow. Here, we describe a setup for highly parallel acquisition of hyperspectral reflection and photoluminescence microscope images using a push-broom technique. Spatial as well as spectral distortions are characterized and their digital corrections are presented. We demonstrate close-to diffraction-limited spatial imaging performance and a spectral resolution limited by the spectrograph. The capabilities of the setup are demonstrated by recording a hyperspectral photoluminescence map of a CVD-grown MoSe$_2$-WSe$_2$ lateral heterostructure, from which we extract the luminescence energies, intensities and peak widths across the interface.

Authors:Jing Yang, Yuanzhen Li, Yumeng Yang, Xinrong Xie, Zijian Zhang, Han Cai, Da-Wei Wang, Fei Gao

Abstract: Flatbands play an important role in correlated quantum matter and have novel applications in photonic lattices. Synthetic magnetic fields and destructive interference in lattices are traditionally used to obtain flatbands. However, such methods can only obtain a few flatbands with most bands remaining dispersive. Here we realize all-band-flat photonic lattices of an arbitrary size by precisely controlling the coupling strengths between lattice sites to mimic those in Fock-state lattices. This allows us to go beyond the perturbative regime of strain engineering and group all eigenmodes in flatbands, which simultaneously achieves high band flatness and large usable bandwidth. We map out the distribution of each flatband in the lattices and selectively excite the eigenmodes with different chiralities. Our method paves a new way in controlling band structure and topology of photonic lattices.

Authors:Ankit Kumar Singh, Zhan-Hong Lin, Min Jiang, Thomas G. Mayerhöfer, Jer-Shing Huang

Abstract: Chiral molecules show differences in their chemical and optical properties due to different spatial arrangements of the atoms in the two enantiomers. A common way to optically differentiate them is to detect the disparity in the absorption of light by the two enantiomers, i.e. the absorption circular dichroism (CD). However, the CD of typical molecules is very small, limiting the sensitivity of chiroptical analysis based on CD. Cavity ring-down spectroscopy (CRDS) is a well-known ultrasensitive absorption spectroscopic method for low-absorbing gas-phase samples because the multiple reflections of light in the cavity greatly increase the absorption path. By inserting a prism into the cavity, the optical mode undergoes total internal reflection (TIR) at the prism surface and the evanescent wave (EW) enables the absorption detection of condensed-phase samples within a very thin layer near the prism surface, called EW-CRDS. Here, we propose an ultrasensitive chiral absorption spectroscopy platform using a dielectric metasurface-assisted EW-CRDS. We theoretically show that, upon linearly polarized and oblique incidence, the metasurface exhibits minimum scattering and absorption loss, introduces negligible polarization change, and locally converts the linearly polarized light into near fields with finite optical chirality, enabling CD detection with EW-CRDS that only works with linearly polarized light. We evaluate the ring-down time in the presence of chiral molecules and determine the sensitivity of the cavity as a function of total absorption from the molecules. The findings open the avenue for an ultrasensitive thin film detection of the chiral molecules using the CRDS techniques.

Authors:Wenxiang Cong, Ge Wang

Abstract: Fluorescence molecular tomography (FMT) is a promising modality for non-invasive imaging of internal fluorescence agents in biological tissues especially in small animal models, with applications in diagnosis, therapy, and drug design. In this paper, we present a new fluorescent reconstruction algorithm that combines time-resolved fluorescence imaging data with photon-counting micro-CT (PCMCT) images to estimate the quantum yield and lifetime of fluorescent markers in a mouse model. By incorporating PCMCT images, a permissible region of interest of fluorescence yield and lifetime can be roughly estimated as prior knowledge, reducing the number of unknown variables in the inverse problem and improving image reconstruction stability. Our numerical simulation results demonstrate the accuracy and stability of this method in the presence of data noise, with an average relative error of 18% in fluorescent yield and lifetime reconstruction.

Authors:Edgar Perez, Cori Haws, Marcelo Davanco, Jindong Song, Luca Sapienza, Kartik Srinivasan

Abstract: Single epitaxial quantum dots (QDs) are a leading technology for quantum light generation, particularly when embedded in photonic devices that enhance their emission. However, coupling this emission into a desirable optical channel, like a single mode fiber, is often challenging. Direct laser writing (DLW) enables the fabrication of three-dimensional sub-micron features out of photoresist, supporting micro- and nano- scale devices that can address this challenge. In this study, we use DLW to directly waveguide-couple epitaxially-grown InAs/GaAs QDs by fabricating 1 $\mu$m diameter polymer nanowires (PNWs) in contact with the GaAs substrate housing the QDs. The PNWs are high index contrast cylindrical waveguides perpendicular to the GaAs device layer, which couple the emission from an underlying QD to the HE$_{11}$ mode of the PNW, enhancing the collection efficiency to a single-mode fiber. PNW fabrication does not alter the QD device layer (e.g., via etching), making PNWs well suited for augmenting existing photonic geometries that enhance QD emission. We study PNWs as standalone devices and in conjunction with metallic nanorings -- an already-established geometry for increasing vertical extraction of light from embedded QDs. Since PNWs are fabricated on substrates that abosorb and reflect at the DLW exposure wavelength, we report methods to mitigate standing wave reflections and heat, which otherwise prevent PNW fabrication. We observe a factor of ($3.0 \pm 0.7)\times$ improvement in a nanoring system with a PNW compared to the same system without a PNW, in line with numerical results, highlighting a PNW's ability to waveguide QD emission and increase collection efficiency simultaneously. These results demonstrate a new approach in which DLW can provide additional functionality for quantum emitter photonics, in a manner compatible with existing top-down fabrication approaches.

Authors:Byung Gyu Chae

Abstract: This study presents the theoretical foundation for optimizing the enhanced-NA Fresnel hologram to recover the low space-bandwidth. Optical kernel functions in real and Fourier spaces act as a basis function in digital hologram synthesis. The higher spectrum components of the optical kernel functions beyond the bandwidth exists in the form of their replications. The expansion of angular spectrum of the digital hologram by its repetition during optimization procedure increases the image resolution, resulting in a viewing angle that is dependent on the hologram numerical aperture. We numerically and experimentally verify our strategy to implement a wide viewing-angle holographic display without shrinking the image size.

Authors:Kostas Kanellopulos, Robert G. West, Silvan Schmid

Abstract: Understanding light-matter interaction at the nanoscale requires probing the optical properties of matter at the individual nano-absorber level. To this end, we have developed a nanomechanical photothermal sensing platform that can be used as a full spectromicroscopy tool for single molecule and single particle analysis. As a demonstration, the absorption cross-section of individual gold nanorods is resolved from the spectroscopic and polarization standpoint. By exploiting the capabilities of nanomechanical photothermal spectromicroscopy, the longitudinal localized surface plasmon resonance (LSPR) in the NIR range is unravelled and quantitatively characterized. The polarization features of the transversal surface plasmon resonance (TSPR) in the VIS range are also analyzed. The measurements are compared with the finite element method (FEM), elucidating the role played by electron-surface and bulk scattering in these plasmonic nanostructures, as well as the interaction between the nano-absorber and the nanoresonator, ultimately resulting in an absorption strength modulation.

Authors:Saeed Farajollahi, Zhiwei Fang, Jintian Lin, Shahin Honari, Ya Cheng, Tao Lu

Abstract: Polygon and star modes enable unidirectional emission and single-frequency lasing in whispering gallery microcavities. To understand their properties and facilitate design, we have adopted both two-dimensional and three-dimensional full-wave perturbation methods to simulate these modes. Our simulation demonstrates that a tapered optical fiber can be used as a weak perturbation to coherently combine multiple whispering gallery modes into a polygon or star mode. Additionally, our simulation predicts an optical quality factor as high as $10^7$ for the polygon modes, which is in good agreement with the experiment results.

Authors:Mark J. Ablowitz, Justin T. Cole, S. D. Nixon

Abstract: A honeycomb Floquet lattice with helically rotating waveguides and an interface separating two counter-propagating subdomains is analyzed. Two topologically protected localized waves propagate unidirectionally along the interface. Switching can occur when these interface modes reach the edge of the lattice and the light splits into waves traveling in two opposite directions. The incoming mode, traveling along the interface, can be routed entirely or partially along either lattice edge with the switching direction based on a suitable mixing of the interface modes.

Authors:Iridanos Loulas, Grigorios P. Zouros, Evangelos Almpanis, Kosmas L. Tsakmakidis

Abstract: In this study we investigate the directional scattering of terahertz radiation by dielectric cylinders, focusing on the enhancement of directionality using incident radiation of complex-frequency. We explore the optimization of the second Kerker condition, which corresponds to backward scattering. At first, by carefully tailoring the electric and magnetic polarizabilities of the cylinders, we successfully achieve significant backward scattering, and then manage to even further improve it by deploying a decaying incoming wave (\textit{complex}-frequency). Additionally, we present preliminary results on the directional scattering of THz radiation by a magneto-optical cylinder, demonstrating the potential of this approach for advanced control over the propagation of THz waves. Our findings contribute to a deeper understanding of THz directional scattering and pave the way for the development of novel THz devices and applications, such as high-resolution imaging, sensing, and communication systems.

Authors:Ashwin K. Boddeti, Yi Wang, Xitlali G. Juarez, Alexandra Boltasseva, Teri W. Odom, Vladimir Shalaev, Hadiseh Alaeian, Zubin Jacob

Abstract: Dimensionality plays a crucial role in long-range dipole-dipole interactions (DDIs). We demonstrate that a resonant nanophotonic structure modifies the apparent dimensionality in an interacting ensemble of emitters, as revealed by population decay dynamics. Our measurements on a dense ensemble of interacting quantum emitters in a resonant nanophotonic structure with long-range DDIs reveal an effective dimensionality reduction to $\bar{d} = 2.20 (12)$, despite the emitters being distributed in 3D. This contrasts the homogeneous environment, where the apparent dimension is $\bar{d} = 3.00$. Our work presents a promising avenue to manipulate dimensionality in an ensemble of interacting emitters.

Authors:Jan Wingenbach, Stefan Schumacher, Xuekai Ma

Abstract: Exceptional points (EPs) with their intriguing spectral topology have attracted considerable attention in a broad range of physical systems, with potential sensing applications driving much of the present research in this field. Here we theoretically demonstrate the realization of EPs in a system with significant nonlinearity, a non-equilibrium exciton-polariton condensate. With the possibility to control loss and gain and nonlinearity by optical means, this system allows for a comprehensive analysis of the interplay of nonlinearities (Kerr-type and saturable gain) and non-Hermiticity. Not only do we find that EPs can be intentionally shifted in parameter space by the saturable gain, we also observe intriguing rotations and intersections of Riemann surfaces, and find nonlinearity-enhanced sensing capabilities. Our results are quite general in nature and illustrate the potential of tailoring spectral topology and related phenomena in non-Hermitian systems by nonlinearity.

Authors:Tomer Bucher, Harel Nahari, Hanan Herzig Sheinfux, Ron Ruimy, Arthur Niedermayr, Raphael Dahan, Qinghui Yan, Yuval Adiv, Michael Yannai, Jialin Chen, Yaniv Kurman, Sang Tae Park, Daniel J. Masiel, Eli Janzen, James H. Edgar, Fabrizio Carbone, Guy Bartal, Shai Tsesses, Frank H. L. Koppens, Giovanni Maria Vanacore, Ido Kaminer

Abstract: Accessing the low-energy non-equilibrium dynamics of materials with simultaneous spatial and temporal resolutions has been a bold frontier of electron microscopy in recent years. One of the main challenges is the ability to retrieve extremely weak signals while simultaneously disentangling amplitude and phase information. Here, we present an algorithm-based microscopy approach that uses light-induced electron modulation to demonstrate the coherent amplification effect in electron imaging of optical near-fields. We provide a simultaneous time-, space-, and phase-resolved measurement in a micro-drum made from a hexagonal boron nitride membrane, visualizing the sub-cycle spatio-temporal dynamics of 2D polariton wavepackets therein. The phase-resolved measurement reveals vortex-anti-vortex singularities on the polariton wavefronts, together with an intriguing phenomenon of a traveling wave mimicking the amplitude profile of a standing wave. Our experiments show a 20-fold coherent amplification of the near-field signal compared to conventional electron near-field imaging, resolving peak field intensities of ~W/cm2 (field amplitude of few kV/m). As a result, our work opens a path toward spatio-temporal electron microscopy of biological specimens and quantum materials - exciting yet sensitive samples, which are currently difficult to investigate.

Authors:Midya Parto, Christian Leefmans, James Williams, Alireza Marandi

Abstract: Sensors are indispensable tools of modern life that are ubiquitously used in diverse settings ranging from smartphones and autonomous vehicles to the healthcare industry and space technology. By interfacing multiple sensors that collectively interact with the signal to be measured, one can go beyond the signal-to-noise ratios (SNR) than those attainable by the individual constituting elements. Such distributed sensing techniques have also been implemented in the quantum regime, where a linear increase in the SNR has been achieved via using entangled states. Along similar lines, coupled non- Hermitian systems have provided yet additional degrees of freedom to obtain better sensors via higher-order exceptional points. Quite recently, a new class of non-Hermitian systems, known as non-Hermitian topological sensors (NTOS) has been theoretically proposed. Remarkably, the synergistic interplay between non-Hermiticity and topology is expected to bestow such sensors with an enhanced sensitivity that grows exponentially with the size of the sensor network. Here, we experimentally demonstrate NTOS using a network of photonic time-multiplexed resonators in the synthetic dimension represented by optical pulses. By judiciously programming the delay lines in such a network, we realize the archetypical Hatano-Nelson model for our non-Hermitian topological sensing scheme. Our experimentally measured sensitivities for different lattice sizes confirm the characteristic exponential enhancement of NTOS. We show that this peculiar response arises due to the combined synergy between non-Hermiticity and topology, something that is absent in Hermitian topological lattices. Our demonstration of NTOS paves the way for realizing sensors with unprecedented sensitivities.

Authors:Michael Horodynski, Tobias Reiter, Matthias Kühmayer, Stefan Rotter

Abstract: Moving objects with optical or acoustical waves is a topic both of fundamental interest and of importance for a range of practical applications. One particularly intriguing example is the tractor beam, which pulls an object toward the wave's source, in opposition to the wave's momentum. In this study, we introduce a protocol that enables the identification of wave states that produce the optimal tractor force for arbitrary objects. Our method relies solely on the solution of a simple eigenvalue problem involving the system's measurable scattering matrix. Using numerical simulations, we demonstrate the efficacy of this wavefront shaping protocol for a representative set of different targets. Moreover, we show that the diffractive nature of waves enables the possibility of a tractor beam, that works even for targets where a geometric optics approach fails to explain the pulling forces.

Authors:Zheng Ge, Zhao-Qi-Zhi Han, Zhi-Yuan Zhou, Xiao-Hua Wang, Yin-Hai Li, Li Chen, Wu-Zhen Li, Su-Jian Niu, Yi-Yang Liu, Bao-Sen Shi

Abstract: Converting the medium infrared field to the visible band is an effective image detection method. We propose a comprehensive theory of image up-conversion under continuous optical pumping, and discuss the relationship between the experimental parameters and imaging field of view, resolution, quantum efficiency, and conversion bandwidth. Theoretical predictions of upconversion imaging results are given based on numerical simulations, which show good agreement with experimental results. In particular, coherent and incoherent light illumination are studied separately and the advantages and disadvantages of their imaging performance are compared and analysed. This work provides a study of the upconversion image detection performance of the system, which is of great value in guiding the design of the detection system and bringing it to practical applications.

Authors:Vì Kronberg, Martijn Anthonissen, Jan ten Thije Boonkkamp, Wilbert IJzerman

Abstract: We introduce a novel approach to calculating three-dimensional freeform reflectors with a scattering surface. Our method is based on optimal transport and utilizes a Fredholm integral equation to express scattering. By solving this integral equation through a process similar to deconvolution, which we call `unfolding,' we can recover a typical specular design problem. Consequently, we consider freeform reflector design with a scattering surface as a two-step process wherein the target distribution is first altered to account for scattering, and then the resulting specular problem is solved. We verify our approach using a custom raytracer that implements the surface scattering model we used to derive the Fredholm integral.

Authors:Sylvain Guerber, Daivid Fowler, Ismael Charlet, Philippe Grosse, Kim Abdoul-Carime, Jonathan Faugier-Tovar, Bertrand Szelag

Abstract: Over the last decade, Optical Phased Arrays (OPA) have been extensively studied, targeting applications such as Light Detection And Ranging (LiDAR) systems, holographic displays, atmospheric monitoring and free space communications. Leveraging the maturity of the silicon photonics platform, the usual mechanical based beam steering system could be replaced by an integrated OPA, significantly reducing the cost and size of the LiDAR while improving its performance (scanning speed, power efficiency, resolution) thanks to solid state beam steering. However, the realization of an OPA that meets the specifications of a LiDAR system (low divergence and single output beam) is not trivial. Targeting the realization of a complete LiDAR system, the technical challenges inherent to the development of high performance OPAs have been studied at CEA LETI. In particular, efficient genetic algorithms have been developed for the calibration of high channel count OPAs as well as an advanced measurement setup compatible with wafer-scale OPA characterization.

Authors:Alekhya Ghosh, Lewis Hill, Gian-Luca Oppo, Pascal Del'Haye

Abstract: Symmetry and symmetry breaking of light states play an important role in photonic integrated circuits and have recently attracted lots of research interest that is relevant to the manipulation of light polarisation, telecommunications, all optical computing, and more. We consider four-field symmetry breaking within two different configurations of photonic dimer systems, both comprised of two identical Kerr ring resonators. In each configuration we observe multiple degrees and levels of spontaneous symmetry breaking between circulating photon numbers and further, a wide range of oscillatory dynamics, such as chaos and multiple variations of periodic switching. These dynamics are of interest for optical data processing, optical memories, telecommunication systems and integrated photonic sensors.

Authors:Mauro David, Davide Disnan, Elena Arigliani, Anna Lardschneider, Georg Marschick, Hanh T. Hoang, Hermann Detz, Bernhard Lendl, Ulrich Schmid, Gottfried Strasser, Borislav Hinkov

Abstract: Long-wave infrared (LWIR, 8-14 um) photonics is a rapidly growing research field within the mid-IR with applications in molecular spectroscopy and optical free-space communication. LWIR-applications are often addressed using rather bulky tabletop-sized free-space optical systems, preventing advanced photonic applications such as rapid-time-scale experiments. Here, device miniaturization into photonic integrated circuits (PICs) with maintained optical capabilities is key to revolutionize mid-IR photonics. Sub-wavelength mode confinement in plasmonic structures enabled such miniaturization approaches in the visible-to-near-IR spectral range. However, adopting plasmonics for the LWIR needs suitable low-loss and -dispersion materials with compatible integration strategies to existing mid-IR technology. In this work we further unlock the field of LWIR/mid-IR PICs, by combining photolithographic patterning of organic polymers with dielectric-loaded surface plasmon polariton (DLSPP) waveguides. In particular, polyethylene shows favorable optical properties, including low refractive index and broad transparency between ~2-200 um. We investigate the whole value chain, including design, fabrication, and characterization of polyethylene-based DLSPP waveguides and demonstrate their first-time plasmonic operation and mode guiding capabilities along s-bend structures. Low bending losses of ~1.3 dB and straight-section propagation lengths of ~1 mm, pave the way for unprecedented, complex on-chip mid-IR photonic devices. Moreover, DLSPPs allow full control of the mode parameters (propagation length and guiding capabilities) for precisely addressing advanced sensing and telecommunication applications with chip-scale devices.

Authors:Farshid Ashtiani, Mohamad Hossein Idjadi, Ting-Chen Hu, Stefano Grillanda, David Neilson, Mark Earnshaw, Mark Cappuzzo, Rose Kopf, Alaric Tate, Andrea Blanco-Redondo

Abstract: Optical neural networks (ONNs) enable high speed parallel and energy efficient processing compared to conventional digital electronic counterparts. However, realizing large scale systems is an open problem. Among various integrated and non-integrated ONNs, free-space diffractive ONNs benefit from a large number of pixels of spatial light modulators to realize millions of neurons. However, a significant fraction of computation time and energy is consumed by the nonlinear activation function that is typically implemented using a camera sensor. Here, we propose a novel surface-normal photodetector (SNPD) with a nonlinear response to replace the camera sensor that enables about three orders of magnitude faster (5.7 us response time) and more energy efficient (less than 10 nW/pixel) response. Direct efficient vertical optical coupling, polarization insensitivity, inherent nonlinearity with no control electronics, low optical power requirements, and the possibility of implementing large scale arrays make the SNPD a promising nonlinear activation function for diffractive ONNs. To show the applicability, successful classification simulation of MNIST and Fashion MNIST datasets using the measured response of SNPD with accuracy comparable to that of an ideal ReLU function are demonstrated.

Authors:Nathan Roberts, Guido Baardink, Anton Souslov, Peter J. Mosley

Abstract: Topological design enables physicists to engineer robustness into a system. When connected to a topological invariant, the propagation of light remains unchanged in the presence of disorder. However, a general challenge remains to directly characterise the topological properties of systems by experiment. In this work, we demonstrate a novel technique for directly observing a photonic winding number using a single measurement. By propagating light with a sufficiently broad spectrum along a topological photonic crystal fibre, we calculate the winding number invariant from the output intensity pattern. We quantify the limitations of this single-shot method, which works even for surprisingly narrow and asymmetric spectral distributions. Furthermore, we dynamically evaluate the effectiveness of our method by uncovering the loss of the bulk invariant as we twist the fibre. The characterisation method that we present is highly accessible and transferable across topological photonic platforms.

Authors:Ting Yu Teo, Nanxi Li, Landobasa Y. M. Tobing, Amy S. K. Tong, Doris K. T. Ng, Zhihao Ren, Chengkuo Lee, Lennon Y. T. Lee, Robert Edward Simpson

Abstract: Capping layers are essential for protecting phase change materials (PCMs) used in non-volatile photonics technologies. This work demonstrates how $(ZnS)_{0.8}-(SiO_2)_{0.2}$ caps radically influence the performance of $Sb_{2}S_{3}$ and Ag-doped $Sb_{2}S_{3}$ integrated photonic devices. We found that at least 30 nm of capping material is necessary to protect the material from Sulfur loss. However, adding this cap affects the crystallization temperatures of the two PCMs in different ways. The crystallization temperature of $Sb_{2}S_{3}$ and Ag-doped $Sb_{2}S_{3}$ increased and decreased respectively, which is attributed to interfacial energy differences. Capped and uncapped Ag-doped $Sb_{2}S_{3}$ microring resonator (MRR) devices were fabricated and measured to understand how the cap affects the device performance. Surprisingly, the resonant frequency of the MRR exhibited a larger red-shift upon crystallization for the capped PCMs. This effect was due to the cap increasing the modal overlap with the PCM layer. Caps can, therefore, be used to provide a greater optical phase shift per unit length, thus reducing the overall footprint of these programmable devices. Overall, we conclude that caps on PCMs are not just useful for stabilizing the PCM layer, but can also be used to tune the PCM crystallization temperature and reduce device footprint. Moreover, the capping layer can be exploited to enhance light-matter interactions with the PCM element.

Authors:Yang Liu, Zheru Qiu, Xinru Ji, Andrea Bancora, Grigory Lihachev, Johann Riemensberger, Rui Ning Wang, Andrey Voloshin, Tobias J. Kippenberg

Abstract: Erbium-doped fiber lasers exhibit high coherence and low noise as required for applications in fiber optic sensing, gyroscopes, LiDAR, and optical frequency metrology. Endowing Erbium-based gain in photonic integrated circuits can provide a basis for miniaturizing low-noise fiber lasers to chip-scale form factor, and enable large-volume applications. Yet, while major progress has been made in the last decade on integrated lasers based on silicon photonics with III-V gain media, the integration of Erbium lasers on chip has been compounded by large laser linewidth. Recent advances in photonic integrated circuit-based high-power Erbium-doped amplifiers, make a new class of rare-earth-ion-based lasers possible. Here, we demonstrate a fully integrated chip-scale Erbium laser that achieves high power, narrow linewidth, frequency agility, and the integration of a III-V pump laser. The laser circuit is based on an Erbium-implanted ultralow-loss silicon nitride Si$_3$N$4$ photonic integrated circuit. This device achieves single-mode lasing with a free-running intrinsic linewidth of 50 Hz, a relative intensity noise of $<$-150 dBc/Hz at $>$10 MHz offset, and output power up to 17 mW, approaching the performance of fiber lasers and state-of-the-art semiconductor extended cavity lasers. An intra-cavity microring-based Vernier filter enables wavelength tunability of $>$ 40 nm within the C- and L-bands while attaining side mode suppression ratio (SMSR) of $>$ 70 dB, surpassing legacy fiber lasers in tuning and SMRS performance. This new class of low-noise, tuneable Erbium waveguide laser could find applications in LiDAR, microwave photonics, optical frequency synthesis, and free-space communications. Our approach is extendable to other wavelengths, and more broadly, constitutes a novel way to photonic integrated circuit-based rare-earth-ion-doped lasers.

Authors:C. Silva, R. Cabrita, V. N. Solovov, P. Brás, A. Lindote, G. Pereira, M. I. Lopes

Abstract: We present a novel method, based on the Saunderson corrections, to predict the reflectance between a liquid interface and a dielectric diffuser. In this method, the diffuse properties of the dielectric are characterized using a single parameter, the multiple-scattering albedo, which is the same irrespective of being in contact with air or liquid. We tested this method using an apparatus based on a total integrating sphere capable of measuring reflectance in both liquid and gas interfaces across various wavelengths of light. We observed that the difference in the value of the multiple-scattering albedo between the sphere full of liquid and empty was less than 0.9$\times 10^{-3}$, with the average difference normalized to the respective uncertainty of only 0.7. These results confirm the reliability of our method and its potential for use in a wide range of practical applications.

Authors:Shuai Wan, Pi-Yu Wang, Rui Ma, Zheng-Yu Wang, Rui Niu, De-Yong He, Guang-Can Guo, Fang Bo, Junqiu Liu, Chun-Hua Dong

Abstract: Generated in high-Q optical microresonators, dissipative Kerr soliton microcombs constitute broadband optical frequency combs with chip sizes and repetition rates in the microwave to millimeter-wave range. For frequency metrology applications such as spectroscopy, optical atomic clocks and frequency synthesizers, octave-spanning soliton microcombs generated in dispersion optimized microresonator are required, which allow self-referencing for full frequency stabilization. In addition, field-deployable applications require the generation of such soliton microcombs simple, deterministic, and reproducible. Here, we demonstrate a novel scheme to generate self-emerging solitons in integrated lithium niobate microresonators. The single soliton features a broadband spectral bandwidth with dual dispersive waves, allowing 2f-3f self-referencing. Via harnessing the photorefractive effect of lithium niobate to significantly extend the soliton existence range, we observe a spontaneous yet deterministic single-soliton formation. The soliton is immune to external perturbation and can operate continuously over 13 hours without active feedback control. Finally, via integration with a pre-programed DFB laser, we demonstrate turnkey soliton generation. With further improvement of microresonator Q and hybrid integration with chip-scale laser chips, compact soliton microcomb devices with electronic actuation can be created, which can become central elements for future LiDAR, microwave photonics and optical telecommunications.

Authors:Camilo Florian, Xiaohan Du, Craig B. Arnold

Abstract: Pushing the limits of precision and reproducibility in ultrafast laser-based nanostructuring requires detailed control over the properties of the illumination. Most traditional methods of laser-based manufacturing rely on the simplicity of Gaussian beams for their well-understood propagation behavior and ease of generation. However, a variety of benefits can be obtained by moving beyond Gaussian beams to single or multiple tailored beams working toward optimal spatial and temporal control over the beam profiles. In this chapter, we center our attention on methods to generate and manipulate complex light beams and the resulting material interactions that occur in response to irradiations with these non-traditional sources. We begin with a discussion on the main differences between Gaussian and more complex light profiles, describing the mechanisms of phase and spatial control before narrowing the discussion to approaches for spatial structuring associated with materials processing with ultrashort laser pulses. Such structuring can occur in both far-field propagating architectures, considering rapidly varying spatial profiles generated mechanically or optically, as well as near-field, non-propagating beams associated with plasmonic and dielectric systems. This chapter emphasizes some of the unique abilities of complex light to shape materials at the nanoscale from a fundamental perspective while referencing potential applications of such methods.

Authors:Tomer Bucher, Ron Ruimy, Shai Tsesses, Raphael Dahan, Guy Bartal, Giovanni Maria Vanacore, Ido Kaminer

Abstract: The complex range of interactions between electrons and electromagnetic fields gave rise to countless scientific and technological advances. A prime example is photon-induced nearfield electron microscopy (PINEM), enabling the detection of confined electric fields in illuminated nanostructures with unprecedented spatial resolution. However, PINEM is limited by its dependence on strong fields, making it unsuitable for sensitive samples, and its inability to resolve complex phasor information. Here, we leverage the nonlinear, over-constrained nature of PINEM to present an algorithmic microscopy approach, achieving far superior nearfield imaging capabilities. Our algorithm relies on free-electron Ramsey-type interferometry to produce orders-of-magnitude improvement in sensitivity and ambiguity-immune nearfield phase reconstruction, both of which are optimal when the electron exhibits a fully quantum behavior. Our results demonstrate the potential of combining algorithmic approaches with novel modalities in electron microscopy, and may lead to various applications from imaging sensitive biological samples to performing full-field tomography of confined light.

Authors:Gregory Moille, Jordan Stone, Michal Chojnacky, Curtis Menyuk, Kartik Srinivasan

Abstract: The phase-coherent frequency division of a stabilized optical reference laser to the microwave domain is made possible by optical frequency combs (OFCs). Fundamentally, OFC-based clockworks rely on the ability to lock one comb tooth to this reference laser, which probes a stable atomic transition. The active feedback process associated with locking the comb tooth to the reference laser introduces complexity, bandwidth, and power requirements that, in the context of chip-scale technologies, complicate the push to fully integrate OFC photonics and electronics for fieldable clock applications. Here, we demonstrate passive, electronics-free synchronization of a microresonator-based dissipative Kerr soliton (DKS) OFC to a reference laser. We show that the Kerr nonlinearity within the same resonator in which the DKS is generated enables phase locking of the DKS to the externally injected reference. We present a theoretical model to explain this Kerr-induced synchronization (KIS), and find that its predictions for the conditions under which synchronization occur closely match experiments based on a chip-integrated, silicon nitride microring resonator. Once synchronized, the reference laser is effectively an OFC tooth, which we show, theoretically and experimentally, enables through its frequency tuning the direct external control of the OFC repetition rate. Finally, we examine the short- and long-term stability of the DKS repetition rate and show that the repetition rate stability is consistent with the frequency division of the expected optical clockwork system.

Authors:Carolin P. Bauer, Justinas Pupeikis, Benjamin Willenberg, Ursula Keller, Christopher R. Phillips

Abstract: Coherent dual-comb spectroscopy (DCS) is a form of Fourier transform spectroscopy (FTS) benefiting from advantageous properties of optical frequency combs. Unlike traditional FTS, DCS enables high-resolution measurements at high speeds because it does not face the trade-off between resolution and update rate inherent to mechanical scanning of the optical delay. However, high complexity of the optical system and limited sensitivity of the measurements remain major challenges for deploying broadband DCS in the short-wave infrared (SWIR, 1.4-3 {\mu}m) and mid-infrared (mid-IR, >3 {\mu}m) regions where there are strong ro-vibrational absorption bands of many molecules. We address these challenges via a wavelength-tunable dual-comb optical parametric oscillator (OPO) where both OPO pump beams are generated in a single laser cavity, while both signal and idler beams are generated in a single OPO cavity. These linear cavities are based on spatial multiplexing and operated in free-running mode. The near-common-path of the beams in each cavity leads to low uncorrelated noise, enabling comb-line-resolved measurements at a moderate optical comb line spacing of 250 MHz. The source is operated at an instantaneous bandwidth below 1 THz, resulting in high power per comb line of up to 70 {\mu}W (signal) and 150 {\mu}W (idler). Combined with repetition rate differences of up to 20 kHz, aliasing-free measurements are enabled. The accessible spectrum spans 1290 nm to 1670 nm (signal) and 2700 nm to 5160 nm (idler). In a proof-of-principle SWIR DCS experiment, we achieve a signal-to-noise ratio (SNR) of 34 dB for an integration time of 2 s.

Authors:John H. Gaida, Hugo Lourenço-Martins, Murat Sivis, Thomas Rittmann, Armin Feist, F. Javier García de Abajo, Claus Ropers

Abstract: Time-resolved electron microscopy aims at tracking nanoscale excitations and dynamic states of matter with a temporal resolution ultimately reaching the attosecond regime. Periodically time-varying fields in an illuminated specimen cause free-electron inelastic scattering, which enables the spectroscopic imaging of near-field intensities. However, access to the evolution of nanoscale fields and structures within the light cycle requires a sensitivity to the optical phase. Here, we introduce Free-Electron Homodyne Detection (FREHD) as a universally applicable approach to electron microscopy of phase-resolved optical responses at high spatiotemporal resolution. In this scheme, a phase-controlled reference interaction serves as the local oscillator to extract arbitrary sample-induced modulations of a free-electron wave function. We demonstrate this principle through the phase-resolved imaging of plasmonic fields with few-nanometer spatial and sub-cycle temporal resolutions. Due to its sensitivity to both phase- and amplitude-modulated electron beams, FREHD measurements will be able to detect and amplify weak signals stemming from a wide variety of microscopic origins, including linear and nonlinear optical polarizations, atomic and molecular resonances and attosecond-modulated structure factors.

Authors:Dandan Hui, Husain Alqattan, Mohamed Sennary, Nikolay V. Golubev, Mohammed Th. Hassan

Abstract: The electron motion in atoms and molecules is at the heart of all phenomena in nature that occur outside the nucleus. Recently, ultrafast electron and X-ray imaging tools have been developed to image the ultrafast dynamics of matter in real time and space. The cutting-edge temporal resolution of these imaging tools is on the order of a few tens to a hundred femtoseconds, limiting imaging to atomic dynamics. Hence electron motion imaging remains beyond the reach. Here, we achieved attosecond electron imaging temporal resolution in a transmission electron microscope, orders of magnitude faster than the highest reported imaging resolution, to demonstrate, which we coin it as (attomicroscopy) to image the field-induced electron dynamics in neutral multilayer graphene. Our results show that the electron motion between the carbon atoms in graphene is due to the field-driven electron dynamics in the conduction band and depends on the field waveform, strength, and polarization direction. This attomicroscopy imaging provides more insights into the electron motion of neutral matter in real time and space and would have long-anticipated real-life attosecond science applications in quantum physics, chemistry, and biology.

Authors:Shuxian Quan, Ling Chen, Siyao Wu, Baocheng Zhang

Abstract: Vectorial polarized fields of light has been applied to detect the rotational velocity by the rotational Doppler effect, but the measurement was made for the rotation of a single-particle system. When the rotational surface is rough, the scattered vectorial Doppler signal spectrum is complex. In this paper, we make the complex spectrum analyses using orbital angular momentum modal expansion method. It is found that the highest peak in the Fourier form of the complex spectrum is obtained at the frequency shift 2l{\Omega} related to the topological charge (l) of the incident vortex light and the rotational velocity ({\Omega}) of the rough surface. Based on the complex spectrum analysis, we construct a method to measure the magnitude and direction of the rotational velocity simultaneously for a general object, which has the practical application in remote sensing and astronomy.

Authors:Tobias Heinrich, Hung-Tzu Chang, Sergey Zayko, Murat Sivis, Claus Ropers

Abstract: We introduce a machine-learning-based approach to enhance the sensitivity of optical-extreme ultraviolet (XUV) transient absorption spectroscopy. A reference spectrum is used as input to a three-layer feed-forward neural network, allowing for an efficient elimination of source noise from measurement data. In pump-probe experiments using high-harmonic radiation, we show a more than tenfold improvement in noise suppression in XUV transient absorption spectra compared to conventional referencing. Utilizing strong spectral correlations in the source fluctuations, the network facilitates a pixel-wise noise reduction without the need for wavelength calibration of the reference spectrum. The presented method can be adapted to a wide range of beam lines and enables the investigation of subtle electron and lattice dynamics in the weak excitation regime, relevant for the study of photovoltaics and photoinduced phase transitions of strongly correlated materials.

Authors:Sashank Kaushik Sridhar, Sayan Ghosh, Avik Dutt

Abstract: Topological effects manifest in a wide range of physical systems, such as solid crystals, acoustic waves, photonic materials and cold atoms. These effects are characterized by `topological invariants' which are typically integer-valued, and lead to robust quantized channels of transport in space, time, and other degrees of freedom. The temporal channel, in particular, allows one to achieve higher-dimensional topological effects, by driving the system with multiple incommensurate frequencies. However, dissipation is generally detrimental to such topological effects, particularly when the systems consist of quantum spins or qubits. Here we introduce a photonic molecule subjected to multiple RF/optical drives and dissipation as a promising candidate system to observe quantized transport along Floquet synthetic dimensions. Topological energy pumping in the incommensurately modulated photonic molecule is enhanced by the driven-dissipative nature of our platform. Furthermore, we provide a path to realizing Weyl points and measuring the Berry curvature emanating from these reciprocal-space ($k$-space) magnetic monopoles, illustrating the capabilities for higher-dimensional topological Hamiltonian simulation in this platform. Our approach enables direct $k$-space engineering of a wide variety of Hamiltonians using modulation bandwidths that are well below the free-spectral range (FSR) of integrated photonic cavities.

Authors:Mario G. Silveirinha

Abstract: Over the past decade, there has been a great interest in topological effects, with concepts originally developed in the context of electron transport in condensed matter platforms now being extended to optical systems. While topological properties in electronic systems are often linked to the quantization of electric conductivity observed in the integer quantum Hall effect, a direct analogue in optics remains elusive. In this study, we bridge this gap by demonstrating that the response of the Poynting vector (which may be regarded as a "photon current") to the mechanical acceleration of a medium provides a precise photonic analogue of the electric conductivity. In particular, it is shown that the photonic conductivity determines the irreversible energy transferred from a periodic mechanical driving of the medium to the electromagnetic field. Furthermore, it is demonstrated that for nonreciprocal systems enclosed in a cavity, the constant acceleration of the system induces a flow of photons along a direction perpendicular to the acceleration, analogous to the Hall effect but for light. The spectral density of the photonic conductivity is quantized in the band gaps of the bulk region with the conductivity quantum determined by the gap Chern number.

Authors:Felix Paries, Nicolas Tiercelin, Geoffrey Lezier, Mathias Vanwolleghem, Felix Selz, Maria-Andromachi Syskaki, Fabian Kammerbauer, Gerhard Jakob, Martin Jourdan, Mathias KlÄui, Zdenek Kaspar, Tobias Kampfrath, Tom S. Seifert, Georg Von Freymann, Daniel Molter

Abstract: Spintronic terahertz emitters promise terahertz sources with an unmatched broad frequency bandwidth that are easy to fabricate and operate, and therefore easy to scale at low cost. However, current experiments and proofs of concept rely on free-space ultrafast pump lasers and rather complex benchtop setups. This contrasts with the requirements of widespread industrial applications, where robust, compact, and safe designs are needed. To meet these requirements, we present a novel fiber-tip spintronic terahertz emitter solution that allows spintronic terahertz systems to be fully fiber-coupled. Using single-mode fiber waveguiding, the newly developed solution naturally leads to a simple and straightforward terahertz near-field imaging system with a 90%-10% knife-edge-response spatial resolution of 30 ${\mu}m$.

Authors:Ying Gao, Xuekai Ma, Xiaokun Zhai, Chunzi Xing, Meini Gao, Haitao Dai, Hao Wu, Tong Liu, Yuan Ren, Xiao Wang, Anlian Pan, Wei Hu, Stefan Schumacher, Tingge Gao

Abstract: In planar microcavities, the transverse-electric and transverse-magnetic (TE-TM) mode splitting of cavity photons arises due to their different penetration into the Bragg mirrors and can result in optical spin-orbit coupling (SOC). In this work, we find that in a liquid crystal (LC) microcavity filled with perovskite microplates, the pronounced TE-TM splitting gives rise to a strong SOC that leads to the spatial instability of microcavity polariton condensates under single-shot excitation. Spatially varying hole burning and mode competition occurs between polarization components leading to different condensate profiles from shot to shot. The single-shot polariton condensates become stable when the SOC vanishes as the TE and TM modes are spectrally well separated from each other, which can be achieved by application of an electric field to our LC microcavity with electrically tunable anisotropy. Our findings are well reproduced and traced back to their physical origin by our detailed numerical simulations. With the electrical manipulation our work reveals how the shot-to-shot spatial instability of spatial polariton profiles can be engineered in anisotropic microcavities at room temperature, which will benefit the development of stable polariton-based optoeletronic and light-emitting devices.

Authors:Mohd Saif Shaikh, Shuyu Wen, Mircea-Traian Catuneanu, Mao Wang, Artur Erbe, Slawomir Prucnal, Lars Rebohle, Shengqiang Zhou, Kambiz Jamshidi, Manfred Helm, Yonder Berencén

Abstract: Photonic integrated circuits require photodetectors that operate at room temperature with sensitivity at telecom wavelengths and are suitable for integration with planar complementary-metal-oxide-semiconductor (CMOS) technology. Silicon hyperdoped with deep-level impurities is a promising material for silicon infrared detectors because of its strong room-temperature photoresponse in the short-wavelength infrared region caused by the creation of an impurity band within the silicon band gap. In this work, we present the first experimental demonstration of lateral Te-hyperdoped Si PIN photodetectors operating at room temperature in the optical telecom bands. We provide a detailed description of the fabrication process, working principle, and performance of the photodiodes, including their key figure of merits. Our results are promising for the integration of active and passive photonic elements on a single Si chip, leveraging the advantages of planar CMOS technology.

Authors:Urban Senica, Alexander Dikopoltsev, Andres Forrer, Sara Cibella, Guido Torrioli, Mattias Beck, Jérôme Faist, Giacomo Scalari

Abstract: Frequency-modulated (FM) combs feature flat intensity spectra with a linear frequency chirp, useful for metrology and sensing applications. Generating FM combs in semiconductor lasers generally requires a fast saturable gain, usually limited by the intrinsic gain medium properties. Here, we show how a spatial modulation of the laser gain medium can enhance the gain saturation dynamics and nonlinearities to generate self-starting FM combs. We demonstrate this with tapered planarized THz quantum cascade lasers (QCLs). While simple ridge THz QCLs typically generate combs which are a mixture of amplitude and frequency modulation, the on-chip field enhancement resulting from extreme spatial confinement leads to an ultrafast saturable gain regime, generating a pure FM comb with a flatter intensity spectrum, a clear linear frequency chirp and very intense beatnotes up to -30 dBm. The observed linear frequency chirp is reproduced using a spatially inhomogeneous mean-field theory model which confirms the crucial role of field enhancement. In addition, the modified spatial temperature distribution within the waveguide results in an improved hightemperature comb operation, up to a heat sink temperature of 115 K, with comb bandwidths of 600 GHz at 90 K. The spatial inhomogeneity also leads to dynamic switching between various harmonic states in the same device.

Authors:Kenichi N. Komagata Laboratoire Temps-Fréquence, Institut de Physique, Université de Neuchâtel, Neuchâtel, Switzerland, Alexandre Parriaux Laboratoire Temps-Fréquence, Institut de Physique, Université de Neuchâtel, Neuchâtel, Switzerland, Mathieu Bertrand Institute for Quantum Electronics, ETH Zurich, Zurich, Switzerland, Johannes Hillbrand Institute for Quantum Electronics, ETH Zurich, Zurich, Switzerland, Valentin J. Wittwer Laboratoire Temps-Fréquence, Institut de Physique, Université de Neuchâtel, Neuchâtel, Switzerland, Jérôme Faist Institute for Quantum Electronics, ETH Zurich, Zurich, Switzerland, Thomas Südmeyer Laboratoire Temps-Fréquence, Institut de Physique, Université de Neuchâtel, Neuchâtel, Switzerland

Abstract: We demonstrate the use of a low power near-infrared laser illuminating the front facet of a quantum cascade laser (QCL) as an optical actuator for the coherent control of a mid-infrared frequency comb. We show that by appropriate current control of the QCL comb and intensity modulation of the near-infrared laser, a tight phase lock of a comb line to a distributed feedback laser is possible with 2 MHz of locking bandwidth and 200 mrad of residual phase noise. A characterization of the whole scheme is provided showing the limits of the electrical actuation which we bypassed using the optical actuation. Both comb degrees of freedom can be locked by performing electrical injection locking of the repetition rate in parallel. However, we show that the QCL acts as a fast near-infrared light detector such that injection locking can also be achieved through modulation of the near-infrared light. These results on the coherent control of a quantum cascade laser frequency comb are particularly interesting for coherent averaging in dual-comb spectroscopy and for mid-infrared frequency comb applications requiring high spectral purity.

Authors:M. H. Shachar, J. E. Garay

Abstract: Scattering problems are important in describing light propagation in wide ranging media such as the atmosphere, colloidal solutions, metamaterials, glass ceramic composites, transparent polycrystalline ceramics, and surfaces. The Rayleigh Gans Debye (RGD) approximation has enjoyed great success in describing a wide range of scattering phenomena. We derive a generalized RGD formulation from the perturbation of Maxwell equations. In contrast to most treatments of RGD scattering, our formulation can model any soft scattering phenomena in linear media, including scattering by stochastic process, lossy media, and by anisotropic inhomogeneities occurring at multiple length scales. Our first-principles derivation makes explicit underlying assumptions and provides jumping off points for more general treatments. The derivation also facilitates a deeper understanding of soft scattering. It is demonstrated that sources of scattering are not interfaces as is often presumed, but excess accelerating charges emitting uncompensated radiation. Approximations to the equations are also presented and discussed. For example, the scattering coefficient in the large size RGD limit is shown to be proportional to the correlation length and the variance of a projected phase shift.

Authors:Anastasiia Sheveleva, Saïd Hamdi, Aurélien Coillet, Christophe Finot, Pierre Colman

Abstract: We demonstrate that the Dynamic Mode Decomposition technique can effectively reduce the amount of noise in Dispersive Fourier Transform dataset; and allow for finer quantitative analysis of the experimental data. We therefore were able to demonstrate that the oscillation pattern of a soliton molecule actually results from the interplay of several elementary vibration modes.

Authors:Srinvasa Reddy Tamalampudi, Juan Esteban Villegas, Ghada Dushaq, Raman Sankar, Bruna Paredes, Mahmoud Rasras

Abstract: On-chip integration of two-dimensional (2D) materials offers great potential for the realization of novel optoelectronic devices in different photonic platforms. In particular, indium selenide (InSe) is a very promising 2D material due to its ultra-high carrier mobility and outstanding photo-responsivity. Here, we report a high-speed photodetector based on a multilayer 90 nm thick InSe integrated on a silicon nitride (SiN) waveguide. The device exhibits a low dark current of 10 nA at 1V bias, a remarkable photoresponsivity of 0.38 AW-1, and high external quantum efficiency of 48.4% measured at 5 V bias. This performance is tested at near-infrared (NIR) 976 nm wavelength under ambient conditions. Furthermore, using numerical and experimental investigations, the estimated absorption coefficient per unit length is 0.11dB/um. To determine the dynamic response of the photodetector, its small and large signal frequency response are also evaluated. A 3-dB radiofrequency (RF) bandwidth of 85 MHz is measured with an open-eye diagram observed at 1 Gbit/s data transmission. Given these outstanding optoelectronic merits, active photonic devices based on integrated multilayer InSe can be realized for a variety of applications including short-reach optical interconnects, LiDAR imaging, and biosensing.

Authors:Matěj Hejda, Eli A. Doris, Simon Bilodeau, Joshua Robertson, Dafydd Owen-Newns, Bhavin J. Shastri, Paul R. Prucnal, Antonio Hurtado

Abstract: Spiking neurons and neural networks constitute a fundamental building block for brain-inspired computing, which is posed to benefit significantly from photonic hardware implementations. In this work, we experimentally investigate an interconnected system based on an ultrafast spiking VCSEL-neuron and a silicon photonics (SiPh) integrated micro-ring resonator (MRR) weight bank, and demonstrate two different functional arrangements of these devices. First, we show that MRR weightbanks can be used in conjuction with the spiking VCSEL-neurons to perform amplitude weighting of sub-ns optical spiking signals. Second, we show that a continuous firing VCSEL-neuron can be directly modulated using a locking signal propagated through a single weighting micro-ring, and we utilize this functionality to perform optical spike firing rate-coding via thermal tuning of the micro-ring resonator. Given the significant track record of both integrated weight banks and photonic VCSEL-neurons, we believe these results demonstrate the viability of combining these two classes of devices for use in functional neuromorphic photonic systems.

Authors:Shiqi Xia, Sihong Lei, Daohong Song, Luigi Di Lauro, Imtiaz Alamgir, Liqin Tang, Jingjun Xu, Roberto Morandotti, Hrvoje Buljan, Zhigang Chen

Abstract: Synthetic dimensions (SDs) opened the door for exploring previously inaccessible phenomena in high-dimensional synthetic space. However, construction of synthetic lattices with desired coupling properties is a challenging and unintuitive task, largely limiting the exploration and current application of SD dynamics. Here, we overcome this challenge by using deep learning artificial neural networks (ANNs) to validly design the dynamics in SDs. We use ANNs to construct a lattice in real space that has a predesigned spectrum of mode eigenvalues. By employing judiciously chosen perturbations (wiggling of waveguides), we show experimentally and theoretically resonant mode coupling and tailored dynamics in SDs, which leads to effective transport or confinement of a complex beam profile. As an enlightening example, we demonstrate morphing of light into a topologically protected edge mode in ANN-designed Su-Schrieffer-Heeger photonic lattices. Such ANN-assisted construction of SDs advances towards utopian networks, opening new avenues in fundamental research beyond geometric limitations. Our findings may offer a flexible and efficient solution for mode lasing, optical switching, and communication technologies.

Authors:D. Dorer, M. H. Frosz, S. Haze, M. Deiß, W. Schoch, J. Hecker Denschlag

Abstract: We report on an anti-resonant hollow-core fiber (AR-HCF) designed for stable transmission of laser light in a broad wavelength range of 250 nm to 450 nm. We tested for wavelengths of 300 nm and 320 nm. The characterized fiber shows a low transmission power attenuation of 0.13 dB/m and an excellent single-mode profile. The fiber maintains stable transmission after an exposure of tens of hours with up to 60 mW CW-laser light and shows no indication of solarization effects. We further tested its performance under bending and observed a small critical bending radius of about 6 cm. These characteristics make the presented fiber a useful tool for many applications, especially in quantum optics labs where it may be instrumental to improve on stability and compactness.

Authors:Samira Mansourzadeh, Tim Vogel, Mostafa Shalaby, Clara J. Saraceno

Abstract: As high-average power ultrafast lasers become increasingly available for nonlinear conversion, the temperature dependence of the material properties of nonlinear crystals becomes increasingly relevant. Here, we present temperature-dependent THz complex refractive index measurements of the organic crystal BNA over a wide range of temperatures from 300 K down to 80 K for THz frequencies up to 4 THz for the first time. Our measurements show that whereas the temperature-dependent refractive index has only minor deviation from room temperature values, the temperature-dependent absorption coefficient decreases at low temperature. We additionally compare these measurements with conversion efficiency and spectra observed during THz generation experiments in the same temperature range and using the same crystal, using an ultrafast Yb-laser for excitation. Surprisingly, the damage threshold of the material does not improve significantly upon cooling, pointing to a nonlinear absorption mechanism being responsible for damage. However, we observe a significant increase in THz yield at lower temperatures, which is most likely due to the reduced THz absorption. These findings will be useful for future designs of high average power pumped organic-crystal based THz-TDS systems.

Authors:Xiaozhou Zan, Xiangdong Guo, Aolin Deng, Zhiheng Huang, Le Liu, Fanfan Wu, Yalong Yuan, Jiaojiao Zhao, Yalin Peng, Lu Li, Yangkun Zhang, Xiuzhen Li, Jundong Zhu, Jingwei Dong, Dongxia Shi, Wei Yang, Xiaoxia Yang, Zhiwen Shi, Luojun Du, Qing Dai, Guangyu Zhang

Abstract: Stacking order plays a crucial role in determining the crystal symmetry and has significant impacts on electronic, optical, magnetic, and topological properties. Electron-phonon coupling, which is central to a wide range of intriguing quantum phenomena, is expected to be intricately connected with stacking order. Understanding the stacking order-dependent electron-phonon coupling is essential for understanding peculiar physical phenomena associated with electron-phonon coupling, such as superconductivity and charge density waves. In this study, we investigate the effect of stacking order on electron-infrared phonon coupling in graphene trilayers. By using gate-tunable Raman spectroscopy and excitation frequency-dependent near-field infrared nanoscopy, we show that rhombohedral ABC-stacked trilayer graphene has a significantly stronger electron-infrared phonon coupling strength than the Bernal ABA-stacked trilayer graphene. Our findings provide novel insights into the superconductivity and other fundamental physical properties of rhombohedral ABC-stacked trilayer graphene, and can enable nondestructive and high-throughput imaging of trilayer graphene stacking order using Raman scattering.

Authors:Jean-Philippe Banon, Ingve Simonsen, Rémi Carminati

Abstract: Using a single-scattering theory, we derive the expression of the degree of polarization of the light scattered from a layer exhibiting both surface and volume scattering. The expression puts forward the intimate connection between the degree of polarization and the statistical correlation between surface and volume disorders. It also permits a quantitative analysis of depolarization for uncorrelated, partially correlated and perfectly correlated disorders. We show that measuring the degree of polarization could allow one to assess the surface-volume correlation function, and that, reciprocally, the degree of polarization could be engineered by an appropriate design of the correlation function.

Authors:M. Asadolah Salmanpour, M. Mosleh, S. M. Hamidi

Abstract: Use of hot atomic vapor as a new tool for tracing the complex nature of light has become a knowledge-based topic in recent years. In this paper, we examine the polarization ellipse of the Bloch surface wave (BSW) through the effect of a magnetic field on the coupling of these surface waves in BSW-hot atomic vapor cell. For this purpose, we fabricate a one-dimensional photonic crystal-based Bloch wave atom cell, where under different configurations of magnetic field, polarization ellipse of Bloch surface waves has been recorded experimentally. Our results indicate that by applying the magnetic field in different directions, Faraday and Voigt, the characteristics of electromagnetically induced transparency (EIT-like) of hybrid system change. We have used these changes to redefine the geometry of Voigt and Faraday for evanescent waves, as well as to measure the ratio of the components of the elliptical polarized electric field. These characterizations can open new insight into the miniaturized atomic field in high quality and low volumetric areas.

Authors:Jungho Moon, Sungsam Kang, Jin Hee Hong, Seokchan Yoon, Wonshik Choi

Abstract: Second-harmonic generation (SHG) microscopy provides label-free imaging of biological tissues with unique contrast mechanisms, but its resolution is limited by the diffraction limit. Here, we present the first experimental demonstration of super-resolution quantitative phase imaging of the SHG field based on synthetic aperture Fourier holographic microscopy. We discuss the mathematical model of synthetic-aperture imaging of SHG fields, as well as the computational adaptive optics technique for correcting sample-induced aberration. We demonstrate proof-of-concept experiments where SHG targets are embedded within a thick scattering medium to validate the performance of the proposed imaging technique. It is shown to be able to overcome the conventional Abbe diffraction limit even in the complex aberrations and strong multiple scattering. We also demonstrate SHG-based super-resolution deep-tissue phase imaging of ex-vivo zebrafish muscle tissue.

Authors:Dehui Zhang, Dong Xu, Yuhang Li, Yi Luo, Jingtian Hu, Jingxuan Zhou, Yucheng Zhang, Boxuan Zhou, Peiqi Wang, Xurong Li, Bijie Bai, Huaying Ren, Laiyuan Wang, Mona Jarrahi, Yu Huang, Aydogan Ozcan, Xiangfeng Duan

Abstract: Nonlinear optical processing of ambient natural light is highly desired in computational imaging and sensing applications. A strong optical nonlinear response that can work under weak broadband incoherent light is essential for this purpose. Here we introduce an optoelectronic nonlinear filter array that can address this emerging need. By merging 2D transparent phototransistors (TPTs) with liquid crystal (LC) modulators, we create an optoelectronic neuron array that allows self-amplitude modulation of spatially incoherent light, achieving a large nonlinear contrast over a broad spectrum at orders-of-magnitude lower intensity than what is achievable in most optical nonlinear materials. For a proof-of-concept demonstration, we fabricated a 10,000-pixel array of optoelectronic neurons, each serving as a nonlinear filter, and experimentally demonstrated an intelligent imaging system that uses the nonlinear response to instantly reduce input glares while retaining the weaker-intensity objects within the field of view of a cellphone camera. This intelligent glare-reduction capability is important for various imaging applications, including autonomous driving, machine vision, and security cameras. Beyond imaging and sensing, this optoelectronic neuron array, with its rapid nonlinear modulation for processing incoherent broadband light, might also find applications in optical computing, where nonlinear activation functions that can work under ambient light conditions are highly sought.

Authors:Sandeep Singh, Vimlesh Kumar, Varun Sharma, Daniele Faccio, G. K. Samanta

Abstract: Hong-Ou-Mandel (HOM) interference, bunching of two indistinguishable photons on a balanced beam-splitter, has emerged as a promising tool for quantum sensing. The interference dip-width, thus the spectral-bandwidth of interfering pair-photons, highly influences the resolution of HOM-based sensors. Typically, the pair-photons bandwidth, generated through parametric down-conversion, is increased using bulky and expensive ultrafast lasers, limiting their use outside the lab. Here we show the generation of pair-photons with flexible spectral-bandwidth even using single-frequency, continuous-wave diode laser enabling high-precision, real-time sensing. Using 1-mm-long periodically-poled KTP crystal, we produced degenerate, high-brightness, paired-photons with spectral-bandwidth of 163.42$\pm$1.68 nm resulting in a HOM-dip width of 4.01$\pm$0.04 $\mu$m to measure a displacement of 60 nm, and vibration amplitude of $205\pm0.75$ nm with increment (resolution) of $\sim$80 nm, and frequency of 8 Hz. Deployment of Fisher-information and maximum likelihood estimator enables optical delay measurement as small as 4.97 nm with precision (Cram\'er-Rao bound) and accuracy of 0.89 and 0.54 nm, respectively. The 17$\times$ enhancement of Fisher-information for the use of 1 mm crystal over 30 mm empowers the HOM-based sensor achieving any arbitrary precision (say $\sim$5 nm) in small number of iterations ($\sim$3300) and time (19 minutes); establishing it's capability for real-time, precision-augmented, in-field quantum sensing applications.

Authors:Rupak Bag, Dibyendu Roy

Abstract: We explore special features of quantum light-matter interactions inside structured waveguides due to their finite bandwidth, band edges, and non-trivial topological properties. We model the waveguides as either a tight-binding (TB) chain or a Su-Schrieffer-Heeger (SSH) chain. For unstructured waveguides with infinite bandwidth, the transmission and reflection amplitude of a side-coupled two-level emitter (2LE) are the same as the reflection and transmission amplitude of a direct-coupled 2LE. We show that this analogy breaks down for structured waveguides with finite bandwidth due to the appearance of Lamb shift only for the direct-coupled 2LE. We further predict a robust light-emitter coupling at zero collective decay width of a single giant 2LE (with two couplings at different points) near the band edges of the structured waveguides where topological features can be beneficial. Finally, we study single-photon dynamics in a heterojunction of a long TB and short SSH waveguide connected to a 2LE at the SSH end. We show the propagation of a photon from the excited emitter to the TB waveguide only when the SSH waveguide is in the topological phase. Thus, the heterojunction acts as a quantum switch or conditional propagation channel.

Authors:Ruyi Zhang, Ting Lin, Shaoqin Peng, Jiachang Bi, Shunda Zhang, Guanhua Su, Jie Sun, Junhua Gao, Hongtao Cao, Qinghua Zhang, Lin Gu, Yanwei Cao

Abstract: The fabrication of flexible single-crystalline plasmonic or photonic components in a scalable way is fundamentally important to flexible electronic and photonic devices with high speed, high energy efficiency, and high reliability. However, it remains to be a big challenge so far. Here, we have successfully synthesized flexible single-crystalline optical hyperbolic metamaterials by directly depositing refractory nitride superlattices on flexible fluoro phlogopite-mica substrates with magnetron sputtering. Interestingly, these flexible hyperbolic metamaterials show dual-band hyperbolic dispersion of dielectric constants with low dielectric losses and high figure-of-merit in the visible to near-infrared ranges. More importantly, the optical properties of these nitride-based flexible hyperbolic metamaterials show remarkable stability under either heating or bending. Therefore, the strategy developed in this work offers an easy and scalable route to fabricate flexible, high-performance, and refractory plasmonic or photonic components, which can significantly expand the applications of current electronic and photonic devices.

Authors:Ronghui Lin, Vytautas Valuckas, Thi Thu Ha Do, Arash Nemati, Arseniy I. Kuznetsov, Jinghua Teng, Son Tung Ha

Abstract: Freeform nanostructures have the potential to support complex resonances and their interactions, which are crucial for achieving desired spectral responses. However, the design optimization of such structures is nontrivial and computationally intensive. Furthermore, the current "black box" design approaches for freeform nanostructures often neglect the underlying physics. Here, we present a hybrid data-efficient neural optimizer for resonant nanostructures by combining a reinforcement learning algorithm and Powell's local optimization technique. As a case study, we design and experimentally demonstrate silicon nanostructures with a highly-saturated red color. Specifically, we achieved CIE color coordinates of (0.677, 0.304)-close to the ideal Schrodinger's red, with polarization independence, high reflectance (>85%), and a large viewing angle (i.e., up to ~ 25deg). The remarkable performance is attributed to underlying generalized multipolar interferences within each nanostructure rather than the collective array effects. Based on that, we were able to demonstrate pixel size down to ~400 nm, corresponding to a printing resolution of 65,000 pixels per inch. Moreover, the proposed design model requires only ~300 iterations to effectively search a 13-dimensional design space - an order of magnitude more efficient than the previously reported approaches. Our work significantly extends the free-form optical design toolbox for high-performance flat-optical components and metadevices.

Authors:Ze-Hao Wang CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, 230026, China CAS Center For Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, 230026, China, Long-Kun Shan CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, 230026, China CAS Center For Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, 230026, China, Tong-Tian Weng CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, 230026, China CAS Center For Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, 230026, China, Tian-Long Chen University of Texas at Austin, Austin, TX 78705, USA, Qi-Yu Wang CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, 230026, China CAS Center For Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, 230026, China, Xiang-Dong Chen CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, 230026, China CAS Center For Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, 230026, China Hefei National Laboratory, University of Science and Technology of China, Hefei 230088, China, Zhang-Yang Wang University of Texas at Austin, Austin, TX 78705, USA, Guang-Can Guo CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, 230026, China CAS Center For Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, 230026, China Hefei National Laboratory, University of Science and Technology of China, Hefei 230088, China, Fang-Wen Sun CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, 230026, China CAS Center For Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, 230026, China Hefei National Laboratory, University of Science and Technology of China, Hefei 230088, China

Abstract: Optical microscopy image plays an important role in scientific research through the direct visualization of the nanoworld, where the imaging mechanism is described as the convolution of the point spread function (PSF) and emitters. Based on a priori knowledge of the PSF or equivalent PSF, it is possible to achieve more precise exploration of the nanoworld. However, it is an outstanding challenge to directly extract the PSF from microscopy images. Here, with the help of self-supervised learning, we propose a physics-informed masked autoencoder (PiMAE) that enables a learnable estimation of the PSF and emitters directly from the raw microscopy images. We demonstrate our method in synthetic data and real-world experiments with significant accuracy and noise robustness. PiMAE outperforms DeepSTORM and the Richardson-Lucy algorithm in synthetic data tasks with an average improvement of 19.6\% and 50.7\% (35 tasks), respectively, as measured by the normalized root mean square error (NRMSE) metric. This is achieved without prior knowledge of the PSF, in contrast to the supervised approach used by DeepSTORM and the known PSF assumption in the Richardson-Lucy algorithm. Our method, PiMAE, provides a feasible scheme for achieving the hidden imaging mechanism in optical microscopy and has the potential to learn hidden mechanisms in many more systems.

Authors:Sai Yan, Jingnan Yang, Shushu Shi, Zhanchun Zuo, Can Wang, Xiulai Xu

Abstract: We propose a new design on integrated optical devices on-chip with an extra width degree of freedom by using a photonic crystal waveguide with Dirac points between two photonic crystals with opposite valley Chern numbers. With such an extra waveguide, we demonstrate numerically that the topologically protected photonic waveguide keeps properties of valley-locking and immunity to defects. Due to the design flexibility of the width-tunable topologically protected photonic waveguide, many unique on-chip integrated devices have been proposed, such as energy concentrators with a concentration efficiency improvement by more than one order of magnitude, topological photonic power splitter with arbitrary power splitting ratio. The topologically protected photonic waveguide with the width degree of freedom could be beneficial for scaling up photonic devices, which provides a new flexible platform to implement integrated photonic networks on chip.

Authors:Naser Omrani, Fardin Ghorbani, Sina Beyraghi, Homayoon Oraizi, Hossein Soleimani

Abstract: In this paper, we present a novel approach for the design of leaky-wave holographic antennas that generates OAM-carrying electromagnetic waves by combining Flat Optics (FO) and machine learning (ML) techniques. To improve the performance of our system, we use a machine learning technique to discover a mathematical function that can effectively control the entire radiation pattern, i.e., decrease the side lobe level (SLL) while simultaneously increasing the central null depth of the radiation pattern. Precise tuning of the parameters of the impedance equation based on holographic theory is necessary to achieve optimal results in a variety of scenarios. In this research, we applied machine learning to determine the approximate values of the parameters. We can determine the optimal values for each parameter, resulting in the desired radiation pattern, using a total of 77,000 generated datasets. Furthermore, the use of ML not only saves time, but also yields more precise and accurate results than manual parameter tuning and conventional optimization methods.

Authors:Quentin Gresil, Antony Lee, Olivier Leveque, Karen Caicedo, Blanca Martin Munoz, Caroline Kulcsar, Francois Goudail, Pierre Bon, Laurent Cognet

Abstract: Important applications of single-particle tracking (SPT) aim at deciphering the diffusion properties of single fluorescent nanoparticles immersed in heterogeneous environments, such as multi-cellular biological tissues. To maximize the particle localization precision in such complex environments, high numerical aperture objectives are often required, which intrinsically restrict depth-of-focus (DOF) to less than a micrometer and impedes recording long trajectories when particles escape the plane of focus. In this work, we show that a simple binary phase mask can work with the spherical aberration inevitably induced by thick sample inhomogeneities, to extend the DOF of a single-molecule fluorescence microscope over more than 4 {\mu}m. The effect of point-spread-function (PSF) engineering over spherical aberration regularizes inhomogeneities of the PSF along the optical axis by restricting it to a narrow distribution. This allows the use of a single fitting function (i.e. Gaussian function) to localize single emitters over the whole extended DOF. Application of this simple approach on diffusing nanoparticles demonstrate that SPT trajectories can be recorded on significantly longer times.

Authors:Yingfang Zhang, Zhihao Lan, Liyazhou Hu, Yiqing Shu, Xun Yuan, Penglai Guo, Xiaoling Peng, Weicheng Chen, Jianqing Li

Abstract: Electromagnetic topological edge states typically are created in photonic systems with crystalline symmetry and these states emerge because of the topological feature of bulk Bloch bands in momentum space according to the bulk-edge correspondence principle. In this work, we demonstrate the existence of chiral topological electromagnetic edge states in Penrose-tiled photonic quasicrystals made of magneto-optical materials, without relying on the concept of bulk Bloch bands in momentum space. Despite the absence of bulk Bloch bands, which naturally defiles the conventional definition of topological invariants in momentum space characterizing these states, such as the Chern number, we show that some bandgaps in these photonic quasicrystals still could host unidirectional topological electromagnetic edge states immune to backscattering in both cylinders-in-air and holes-in-slab configurations. Employing a real-space topological invariant based on the Bott index, our calculations reveal that the bandgaps hosting these chiral topological edge states possess a nontrivial Bott index of $\pm 1$, depending on the direction of the external magnetic field. Our work opens the door to the study of topological states in photonic quasicrystals.

Authors:Unė G. Būtaitė, Christina Sharp, Michael Horodynski, Graham M. Gibson, Miles J. Padgett, Stefan Rotter, Jonathan M. Taylor, David B. Phillips

Abstract: Optical tweezers enable non-contact trapping of micro-scale objects using light. Despite their widespread use, it is currently not known how tightly it is possible to three-dimensionally trap micro-particles with a given photon budget. Reaching this elusive limit would enable maximally-stiff particle trapping for precision measurements on the nanoscale, and photon-efficient tweezing of light-sensitive objects. Here we solve this problem by customising a trapping light field to suit a specific particle, with the aim of simultaneously optimising trap stiffness in all three dimensions. Initially taking a theoretical approach, we develop an efficient multi-parameter optimisation routine to design bespoke optical traps for a wide range of micro-particles. We show that the confinement volume of micro-spheres held in these sculpted traps can be reduced by one-to-two orders-of-magnitude in comparison to a conventional optical tweezer of the same power. We go on to conduct proof-of-principle experiments, and use a wavefront shaping inspired strategy to suppress the Brownian fluctuations of optically trapped micro-spheres in every direction concurrently, thus demonstrating order-of-magnitude reductions in their confinement volumes. Our work paves the way towards the fundamental limits of optical control over the mesoscopic realm.

Authors:Rebecca Cheng, Mengjie Yu, Amirhassan Shams-Ansari, Yaowen Hu, Christian Reimer, Mian Zhang, Marko Lončar

Abstract: Optical parametric oscillation (OPO) has widely been utilized as a means of generating light with wide spectral coverage from a single pump laser. These oscillators can be driven using either continuous-wave (CW) light, which only requires lining up of the pump frequency with OPO resonance, or pulsed light, which also mandates that the repetition rate of the pulse and free spectral range of the OPO cavity are carefully tuned to match each other. Advancements in nanophotonics have ignited interest in chip-scale OPOs, which enable low-footprint and high-efficiency solutions to broadband light generation. CW-pumped integrated OPO has been demonstrated using both $\chi^{(2)}$ and $\chi^{(3)}$ parametric oscillation. However, realizing pulse-driven on-chip OPO remains challenging, as microresonator cavities have limited tuning range in the FSR and resonance frequency compared to traditional bulk cavities. Here, we overcome this limitation and demonstrate a $\chi^{(3)}$ pulse-driven OPO by using a tunable on-chip femtosecond pulse generator to synchronously pump the oscillator. The output frequency comb generated by our OPO has 30-GHz repetition rate, spans 2/5 of an octave and consists of over 1400 comb lines with a pump-to-comb conversion efficiency of 10%.

Authors:Alberto Nardi, Alisa Davydova, Nikolai Kuznetsov, Miles H. Anderson, Charles Möhl, Johann Riemensberger, Tobias J. Kippenberg, Paul Seidler

Abstract: Photonic crystals, material structures in which the dielectric function varies periodically in one, two, or three dimensions, can provide exquisite control over the propagation and confinement of light. By tailoring their band structure, exceptional optical effects can be achieved, such as slow light propagation or, through the creation of photonic bandgaps, optical cavities with both a high quality factor and a small mode volume. Photonic crystal cavities have been used to realize compact nano-lasers and achieve strong coupling to quantum emitters, such as semiconductor quantum dots, color centers, or cold atoms. A useful attribute of photonic crystals is the ability to create chirped mirrors. Chirping has underpinned advances in ultra-fast lasers based on bulk mirrors, but has yet to be fully exploited in integrated photonics, where it could provide a means to engineer otherwise unattainable dispersion profiles for a range of nonlinear optical applications, including soliton frequency comb generation. The vast majority of integrated resonators for frequency combs make use of microring geometries, where only waveguide width and height are varied to engineer dispersion. Generation of frequency combs has been demonstrated with one-dimensional photonic crystal cavities made of silicon nitride, but the low index contrast prevents formation of broad soliton combs. We overcome these challenges by using a photonic-crystal Fabry-P\'erot resonator made of gallium phosphide, a material with a high refractive index and a Kerr nonlinearity 200 times larger than that of silicon nitride. We employ chirped photonic crystal mirrors to provide anomalous dispersion. With subharmonic pulsed pumping at an average power of 23.6 mW, we are able to access stable dissipative Kerr frequency combs. We demonstrate soliton formation with a 3-dB bandwidth of 3.0 THz, corresponding to a pulse duration of 60 fs.

Authors:Marek Kozoň, Rutger Schrijver, Matthias Schlottbom, Jaap J. W. van der Vegt, Willem L. Vos

Abstract: We employ unsupervised machine learning to enhance the accuracy of our recently presented scaling method for wave confinement analysis [1]. %The accuracy of the scaling method decreases for systems of small size, which are however the most interesting ones both experimentally and computationally. We employ the standard k-means++ algorithm as well as our own model-based algorithm. We investigate cluster validity indices as a means to find the correct number of confinement dimensionalities to be used as an input to the clustering algorithms. Subsequently, we analyze the performance of the two clustering algorithms when compared to the direct application of the scaling method without clustering. We find that the clustering approach provides more physically meaningful results, but may struggle with identifying the correct set of confinement dimensionalities. We conclude that the most accurate outcome is obtained by first applying the direct scaling to find the correct set of confinement dimensionalities and subsequently employing clustering to refine the results. Moreover, our model-based algorithm outperforms the standard k-means++ clustering.

Authors:Boquan Ren, A. A. Arkhipova, Yiqi Zhang, Y. V. Kartashov, Hongguang Wang, S. A. Zhuravitskii, N. N. Skryabin, I. V. Dyakonov, A. A. Kalinkin, S. P. Kulik, V. O. Kompanets, S. V. Chekalin, V. N. Zadkov

Abstract: Introduction of controllable deformations into periodic materials that lead to disclinations in their structure opens novel routes for construction of higher-order topological insulators hosting topological states at disclinations. Appearance of these topological states is consistent with the bulk-disclination correspondence principle, and is due to the filling anomaly that results in fractional charges to the boundary unit cells. So far, topological disclination states were observed only in the linear regime, while the interplay between nonlinearity and topology in the systems with disclinations has been never studied experimentally. We report here bon the experimental observation of the nonlinear photonic disclination states in waveguide arrays with pentagonal or heptagonal disclination cores inscribed in transparent optical medium using the fs-laser writing technique. The transition between nontopological and topological phases in such structures is controlled by the Kekul\'e distortion coefficient $r$ with topological phase hosting simultaneously disclination states at the inner disclination core and spatially separated from them corner, zero-energy, and extended edge states at the outer edge of the structure. We show that the robust nonlinear disclination states bifurcate from their linear counterparts and that location of their propagation constants in the gap and, hence, their spatial localization can be controlled by their power. Nonlinear disclination states can be efficiently excited by Gaussian input beams, but only if they are focused into the waveguides belonging to the disclination core, where such topological states reside.

Authors:Mario Ferraro, Fabio Mangini, Mario Zitelli, Stefan Wabnitz

Abstract: The input power-induced transformation of the transverse intensity profile at the output of graded-index multimode optical fibers from speckles into a bell-shaped beam sitting on a low intensity background is known as spatial beam self-cleaning. Its remarkable properties are the output beam brightness improvement and robustness to fiber bending and squeezing. These properties permit to overcome the limitations of multimode fibers in terms of low output beam quality, which is very promising for a host of technological applications. In this review, we outline recent progress in the understanding of spatial beam self-cleaning, which can be seen as a state of thermal equilibrium in the complex process of modal four-wave mixing. In other words, the associated nonlinear redistribution of the mode powers which ultimately favors the fundamental mode of the fiber can be described in the framework of statistical mechanics applied to the gas of photons populating the fiber modes. On the one hand, this description has been corroborated by a series of experiments by different groups. On the other hand, some open issues still remain, and we offer a perspective for future studies in this emerging and controversial field of research.

Authors:L. Mauro, J. Fregoni, J. Feist, R. Avriller

Abstract: We provide a theoretical framework based on classical electromagnetism, to describe optical properties of Fabry-P\'erot cavities, filled with multilayered and linear chiral materials. We find a formal link between transfer-matrix, scattering-matrix and Green-function approaches to compute the polarization-dependent optical transmission, and cavity-modified circular dichroism signals. We show how general symmetries like Lorentz reciprocity and time-reversal symmetry constrain the modelling of such cavities. We apply this approach to investigate numerically and analytically the properties of various Fabry-P\'erot cavities, made of either metallic or helicity-preserving dielectric photonic crystal mirrors. In the latter case, we analyze the onset of chiral cavity-polaritons in terms of partial helicity-preservation of electromagnetic waves reflected at the mirrors interfaces. Our approach is relevant for designing innovative Fabry-P\'erot cavities for chiral-sensing, and for probing cavity-modified stereochemistry.

Authors:Altai Perry, Xiaojing Weng, Erfan Nozari, Luat Vuong

Abstract: Spectral computational methods leverage modal or nonlocal representations of data, and a physically realized approach to spectral computation pertains to encoded diffraction. Encoded diffraction offers a hybrid approach that pairs analog wave propagation with digital back-end electronics, however the intermediate sensor patterns are correlations rather than linear signal weights, which limits the development of robust and efficient downstream analyses. Here, with vortex encoders, we show that the solution for the signal field from sensor intensity adopts the form of polynomial regression, which is subsequently solved with a learned, linear transformation. This result establishes an analytic rationale for a spectral-methods paradigm in physically realized machine learning systems. To demonstrate this paradigm, we quantify the learning that is transferred with an image basis using speckle parameters, Singular-Value Decomposition Entropy ($H_{SVD}$) and Speckle-Analogue Density (SAD). We show that $H_{SVD}$, a proxy for image complexity, indicates the rate at which a model converges. Similarly, SAD, an averaged spatial frequency, marks a threshold for structurally similar reconstruction. With a vortex encoder, this approach with parameterized training may be extended to distill features. In fact, with images reconstructed with our models, we achieve classification accuracies that rival decade-old, state-of-the-art computer algorithms. This means that the process of learning compressed spectral correlations distills features to aid image classification, even when the goal images are feature-agnostic speckles. Our work highlights opportunities for analytic and axiom-driven machine-learning designs appropriate for real-time applications.

Authors:Romain Demur, Luc Leviandier, Elsa Turpin, Jerome Bourderionnet, Eric Lallier

Abstract: Coherent beam combination is one promising way to overcome the power limit of one single laser. In this paper, we use a Multi-Plane Light Converter to combine 12 fibers at 1.03 micron with a phase locking setup. The overall loss measurement gives a combination efficiency in the fundamental Hermite-Gaussian mode as high as 70%. This setup can generate the fundamental and higher-order Hermite-Gaussian modes and has beam steering capabilities.

Authors:Eileen Otte Geballe Laboratory for Advance Materials, Stanford University, Stanford, CA, USA, Alexander D. White E. L. Ginzton Laboratory, Stanford University, Stanford, CA, USA, Nicholas A. Güsken Geballe Laboratory for Advance Materials, Stanford University, Stanford, CA, USA, Jelena Vučković E. L. Ginzton Laboratory, Stanford University, Stanford, CA, USA, Mark L. Brongersma Geballe Laboratory for Advance Materials, Stanford University, Stanford, CA, USA

Abstract: Spatial modes of light have become highly attractive to increase the dimension and, thereby, security and information capacity in quantum key distribution (QKD). So far, only transverse electric field components have been considered, while longitudinal polarization components have remained neglected. Here, we present an approach to include all three spatial dimensions of electric field oscillation in QKD by implementing our tunable, on-a-chip vector beam decoder (VBD). This inversely designed device pioneers the "preparation" and "measurement" of three-dimensionally polarized mutually unbiased basis states for high-dimensional (HD) QKD and paves the way for the integration of HD QKD with spatial modes in multifunctional on-a-chip photonics platforms.

Authors:Nikola Opačak, Dmitry Kazakov, Lorenzo L. Columbo, Maximilian Beiser, Theodore P. Letsou, Florian Pilat, Massimo Brambilla, Franco Prati, Marco Piccardo, Federico Capasso, Benedikt Schwarz

Abstract: Recent years witnessed rapid progress of chip-scale integrated optical frequency comb sources. Among them, two classes are particularly significant -- semiconductor Fabry-Per\'{o}t lasers and passive ring Kerr microresonators. Here, we merge the two technologies in a ring semiconductor laser and demonstrate a new paradigm for free-running soliton formation, called Nozaki-Bekki soliton. These dissipative waveforms emerge in a family of traveling localized dark pulses, known within the famed complex Ginzburg-Landau equation. We show that Nozaki-Bekki solitons are structurally-stable in a ring laser and form spontaneously with tuning of the laser bias -- eliminating the need for an external optical pump. By combining conclusive experimental findings and a complementary elaborate theoretical model, we reveal the salient characteristics of these solitons and provide a guideline for their generation. Beyond the fundamental soliton circulating inside the ring laser, we demonstrate multisoliton states as well, verifying their localized nature and offering an insight into formation of soliton crystals. Our results consolidate a monolithic electrically-driven platform for direct soliton generation and open a door for a new research field at the junction of laser multimode dynamics and Kerr parametric processes.

Authors:Takuma Nakamura, Kazuki Hashimoto, Takuro Ideguchi

Abstract: Raman scattering spectroscopy is widely used as an analytical technique in various fields, but its measurement process tends to be slow due to the low scattering cross-section. In the last decade, various broadband coherent Raman scattering spectroscopy techniques have been developed to address this limitation, achieving a measurement rate of about 100 kSpectra/s. Here, we present a significantly increased measurement rate of 50 MSpectra/s, which is 500 times higher than the previous state-of-the-art, by developing time-stretch coherent Raman scattering spectroscopy. Our newly-developed system, based on a mode-locked Yb fiber laser, enables highly-efficient broadband excitation of molecular vibrations via impulsive stimulated Raman scattering with an ultrashort femtosecond pulse and sensitive time-stretch detection with a picosecond probe pulse at a high repetition rate of the laser. As a proof-of-concept demonstration, we measure broadband coherent Stokes Raman scattering spectra of organic compounds covering the molecular fingerprint region from 200 to 1,200 cm-1. This high-speed broadband vibrational spectroscopy technique holds promise for unprecedented measurements of sub-microsecond dynamics of irreversible phenomena and extremely high-throughput measurements.

Authors:Keir N Murphy, Mira Naftaly, Alison Nordon, Daniel Markl

Abstract: Fundamental knowledge of scattering in granular compacts is essential to ensure accuracy of spectroscopic measurements and determine material characteristics such as size and shape of scattering objects. Terahertz time-domain spectroscopy (THz-TDS) was employed to investigate the effect of particle size and concentration on scattering in specially fabricated compacts consisting of borosilicate microspheres in a polytetrafluoroethylene (PTFE) matrix. As expected, increasing particle size leads to an increase in overall scattering contribution. At low concentrations, the scattering contribution increases linearly with concentration. Scattering increases linearly at low concentrations, saturates at higher concentrations with a maximum level depending on particle size, and that the onset of saturation is independent of particle size. The effective refractive index becomes sublinear at high particle concentrations and exceeds the linear model at maximum density, which can cause errors in calculations based on it, such as porosity. The observed phenomena are attributed to the change in the fraction of photons propagating ballistically versus being scattered. At low concentrations, photons travel predominately ballistically through the PTFE matrix. At high concentrations, the photons again propagate ballistically through adjacent glass microspheres. In the intermediate regime, photons are predominately scattered.

Authors:Matteo Cherchi, Fei Sun, Markku Kapulainen, Tapani Vehmas, Mikko Harjanne, Timo Aalto

Abstract: We show theoretically and experimentally how a flat-top second-order response can be achieved with a self-coupled single add-drop ring resonator based on two couplers with different splitting ratios. The resulting device is a 1x1 filter, reflecting light back in the input waveguide at resonating wavelengths in the passbands, and transmitting light in the output waveguide at all other non-resonating wavelengths. Different implementations of the filter have been designed and fabricated on a micron-scale silicon photonics platform. They are based on compact Euler bends - either U-bends or L-bends - and Multi-Mode Interferometers as splitters for the ring resonators. Different finesse values have been achieved by using either 50:50 MMIs in conjunction with 85:15 MMIs or 85:15 MMIs in conjunction with 95:05 double MMIs. Unlike ordinary lowest order directional couplers, the MMIs couple most of the power in the cross-port which make them particularly suitable for the topology of the self-coupled ring, which would otherwise require a waveguide crossing. Experimental results are presented, showing good agreement with simulations. The proposed devices can find applications as wavelength-selective reflectors for relatively broad-band lasers or used as 2x2 add-drop filters when two exact replicas of the device are placed on the arms of a Mach-Zehnder interferometer.

Authors:Chen Qian, Shicheng Jiang, Tong Wu, Hongming Weng, Chao Yu, Ruifeng Lu

Abstract: The solid-state harmonic generation (SHG) derives from photocurrent coherence. The crystal symmetry, including spatial-inversion, mirror, rotational symmetries and time-reversal symmetry, constrains the amplitude and phase of the photocurrent, thus manipulates the coherent processes in SHG. We revisit the expression of photocurrent under the electric dipole approximation and give a picture of non-equilibrium dynamical process of photoelectron on laser-dressed effective bands. We reveal the indispensable role of shift vector and transition dipole phase in the photocurrent coherence in addition to the dynamical phase. Microscopic mechanism of the selection rule, orientation dependence, polarization characteristics, time-frequency analysis and ellipticity dependence of harmonics governed by crystal symmetry is clarified analytically and numerically. The theory in this paper integrates non-equilibrium electronic dynamics of condensed matter in strong laser fields, and paves a way to explore more nonlinear optical phenomena induced by the crystal symmetry.

Authors:Kenta Otsubo, Takaki Kiyozumi, Kohei Noda, Kentaro Nakamura, Heeyoung Lee, Yosuke Mizuno

Abstract: We show that the systematic error unique to Brillouin optical correlation-domain reflectometry (BOCDR) can be effectively suppressed by use of low-coherence light, and demonstrate distributed strain measurement with ~3 cm spatial resolution.

Authors:Ruiqing Sun, Delong Yang, Yao Hu, Qun Hao, Xin Li, Shaohui Zhang

Abstract: Fourier Ptychographic Microscopy (FPM) is a computational technique that achieves a large space-bandwidth product imaging. It addresses the challenge of balancing a large field of view and high resolution by fusing information from multiple images taken with varying illumination angles. Nevertheless, conventional FPM framework always suffers from long acquisition time and a heavy computational burden. In this paper, we propose a novel physical neural network that generates an adaptive illumination mode by incorporating temporally-encoded illumination modes as a distinct layer, aiming to improve the acquisition and calculation efficiency. Both simulations and experiments have been conducted to validate the feasibility and effectiveness of the proposed method. It is worth mentioning that, unlike previous works that obtain the intensity of a multiplexed illumination by post-combination of each sequentially illuminated and obtained low-resolution images, our experimental data is captured directly by turning on multiple LEDs with a coded illumination pattern. Our method has exhibited state-of-the-art performance in terms of both detail fidelity and imaging velocity when assessed through a multitude of evaluative aspects.

Authors:Florian Le Roux, Andreas Mischok, Francisco Tenopala-Carmona, Malte C. Gather

Abstract: Non-linearities in organic exciton-polariton microcavities represent an attractive platform for second-generation quantum devices. However, progress in this area hinges on the development of material platforms for high-performance polariton lasing, scalable and sustainable fabrication, and ultimately strategies for electrical pumping. Here, we show how introducing Schlieren textures in a liquid crystalline conjugated polymer and the associated microdomains of distinct chain orientation enable in-plane Anderson localization of polaritons. In high-Q distributed Bragg reflector microcavities, this strong localization facilitated polariton lasing at unprecedented thresholds of 136 fJ per pulse, thus providing a pathway to the study of fundamental effects at low polariton numbers. Anderson localization further permitted polariton lasing in more lossy metallic microcavities while maintaining a competitive lasing threshold. The facile fabrication of these cavities will drastically reduce the complexity of integrating polariton laser with other structures and the high conductivity of metallic mirrors provides a route to electrical pumping.

Authors:Taigao Ma, Haozhu Wang, L. Jay Guo

Abstract: Foundation models are large machine learning models that can tackle various downstream tasks once trained on diverse and large-scale data, leading research trends in natural language processing, computer vision, and reinforcement learning. However, no foundation model exists for optical multilayer thin film structure inverse design. Current inverse design algorithms either fail to explore the global design space or suffer from low computational efficiency. To bridge this gap, we propose the Opto Generative Pretrained Transformer (OptoGPT). OptoGPT is a decoder-only transformer that auto-regressively generates designs based on specific spectrum targets. Trained on a large dataset of 10 million designs, our model demonstrates remarkable capabilities: 1) autonomous global design exploration by determining the number of layers (up to 20) while selecting the material (up to 18 distinct types) and thickness at each layer, 2) efficient designs for structural color, absorbers, filters, distributed brag reflectors, and Fabry-Perot resonators within 0.1 seconds (comparable to simulation speeds), 3) the ability to output diverse designs, and 4) seamless integration of user-defined constraints. By overcoming design barriers regarding optical targets, material selections, and design constraints, OptoGPT can serve as a foundation model for optical multilayer thin film structure inverse design.

Authors:Xin Mu, Fu-Der Chen, Ka My Dang, Michael G. K. Brunk, Jianfeng Li, Hannes Wahn, Andrei Stalmashonak, Peisheng Ding, Xianshu Luo, Hongyao Chua, Guo-Qiang Lo, Joyce K. S. Poon, Wesley D. Sacher

Abstract: Advances in chip-scale photonic-electronic integration are enabling a new generation of foundry manufacturable implantable silicon neural probes incorporating nanophotonic waveguides and microelecctrodes for optogenetic stimulation and electrophysiological recording in neuroscience research. Further extending neural probe functionalities with integrated microfluidics is a direct approach to achieve neurochemical injection and sampling capabilities. In this work, we use two-photon polymerization to integrate microfluidic channels onto photonic neural probes with 3D printing. The photonic neural probes include silicon nitride nanophotonic waveguides and grating emitters. The customizability of 3D printing enables a unique geometry of microfluidics that conforms to the shape of each neural probe, enabling integration of microfluidics with a variety of existing neural probes while avoiding the complexities of monolithic microfluidics integration. We demonstrate the photonic and fluidic functionalities of the neural probes via fluorescein injection in agarose gel and photoloysis of caged fluorescein in solution and in flxed brain tissue.

Authors:Manuel Monterrosas-Romero, Seyed K. Alavi, Ester M. Koistinen, Sungkun Hong

Abstract: Optical trapping has proven to be a valuable experimental technique for precisely controlling small dielectric objects. However, due to their very nature, conventional optical traps are diffraction limited and require high intensities to confine the dielectric objects. In this work, we propose a novel optical trap based on dielectric photonic crystal nanobeam cavities, which overcomes the limitations of conventional optical traps by significant factors. This is achieved by exploiting an optomechanically induced backaction mechanism between a dielectric nanoparticle and the cavities. We perform numerical simulations to show that our trap can fully levitate a submicron-scale dielectric particle with a trap width as narrow as 56 nm. It allows for achieving a high trap stiffness, therefore, a high Q-frequency product for the particle's motion while reducing the optical absorption by a factor of 43 compared to the cases for conventional optical tweezers. Moreover, we show that multiple laser tones can be used further to create a complex, dynamic potential landscape with feature sizes well below the diffraction limit. The presented optical trapping system offers new opportunities for precision sensing and fundamental quantum experiments based on levitated particles.

Authors:Bereket Dalga Dana, Ji Boyu, Jingquan Lin, Longnan Li, Alemayehu Nana Koya, Wei Li

Abstract: Compared to single nanoparticles, strongly coupled plasmonic nanoparticles provide attractive advantages owing to their ability to exhibit multiple resonances with unique spectral features and higher local field intensity. These enhanced plasmonic properties of coupled metal nanoparticles have been used for various applications including realization of strong light-matter interaction, photocatalysis, and sensing applications. In this article, we review the basic physics of hybrid plasmonic modes in coupled metallic nanodimers and assess their potentials for refractive index sensing. In particular, we overview various modes of hybrid plasmons including bonding and antibonding modes in symmetric nanodimers, Fano resonances in asymmetric nanodimers, charge transfer plasmons in linked nanoparticle dimers, hybrid plasmon modes in nanoshells, and gap modes in particle-on-mirror configurations. Beyond the dimeric nanosystems, we also showcase the potentials of hybrid plasmonic modes in periodic nanoparticle arrays for sensing applications. Finally, based on the critical assessment of the recent researches on coupled plasmonic modes, the outlook on the future prospects of hybrid plasmon based refractometric sensing are discussed We believe that, given their tunable resonances and ultranarrow spectral signatures, coupled metal nanoparticles are expected to play key roles in developing precise plasmonic nanodevices with extreme sensitivity.

Authors:Snehashis Sadhukhan, Somnath Ghosh

Abstract: Photonic Time Crystals (PTCs) provide a completely new platform exhibiting light wave amplification owing to periodically varying electromagnetic properties. The need to control this amplification is becoming increasingly important, especially with the emergence of meta surface based practical realization of PTCs. The work introduces isolated temporal defect in PTCs to establish a new degree of control over the amplification. We find that in presence of the defect, the transmittance and reflectance become close to unity for a specific value of momentum (k_d) within the bandgaps accompanied by a significant impact on the amount of amplification. We show the impact of the temporal defect on the exponential growth of intensity with PTC periods. The effect primarily depends on the Floquet frequency of the PTC that becomes real at k_d giving rise to four pulses instead of two as an outcome of gap propagation. We further demonstrate that by manipulating the temporal and dielectric properties of the defect, the defect state in momentum can be tuned to serve the design interest for specialty applications.

Authors:Stephen M. Barnett, Fiona C. Speirits, Joerg B. Goette

Abstract: We show that Skyrmion field lines, constructed from the local Stokes parameters, trace out lines of constant optical polarisation.

Authors:Andrei G. Vladimirov

Abstract: Weak interaction of temporal cavity solitons due to gain saturation and recovery in a delay differential model of a long cavity semiconductor laser is studied numerically and analytically using an asymptotic approach. It is shown that in addition to the usual soliton repulsion leading to a harmonic mode-locking regimes a soliton attraction is also possible in a laser with nonzero linewidth enhancement factor. It is shown numerically that this attraction can lead either to a pulse merging or to a pulse bound state formation.

Authors:Nicolas Berti, Massimiliano Guasoni, Julien Fatome

Abstract: We report on an extension of the concept of nonlinear self-repolarization process by means of two different architectures based on dual-Omnipolarizers. More specifically, we compare the performance in terms of polarization attraction capabilities provided by two novel arrangements: The first configuration relies on two cascaded Omnipolarizers, whilst the second architecture integrates an additional device directly into the feedback loop. Our study reveals that for a constant power budget, the cascading of two subsequent Omnipolarizers enables to improve the efficiency of the attraction process, yielding an output Degree-of-Polarization close to unity, but at the cost of twofold equipments.

Authors:Jernej Frank, Alexander Duplinskiy, Kaden Bearne, A. I. Lvovsky

Abstract: We investigate Hermite Gaussian Imaging (HGI) -- a novel passive super-resolution technique -- for complex 2D incoherent objects in the sub-Rayleigh regime. The method consists of measuring the field's spatial mode components in the image plane in the overcomplete basis of Hermite-Gaussian modes and their superpositions and subsequently using a deep neural network to reconstruct the object from these measurements. We show a three-fold resolution improvement over direct imaging. Our HGI reconstruction retains its superiority even if the same neural network is applied to improve the resolution of direct imaging. This superiority is also preserved in the presence of shot noise. Our findings are the first step towards passive super-resolution imaging protocols in fluorescent microscopy and astronomy.

Authors:Ogulcan E. Orsel, Jiho Noh, Gaurav Bahl

Abstract: Undesirable light scattering is an important fundamental cause for photon loss in nanophotonics. Rayleigh backscattering can be particularly difficult to avoid in wave-guiding systems and arises from both material defects and geometric defects at the subwavelength scale. It has been previously shown that systems with broken time-reversal symmetry (TRS) can naturally suppress detrimental Rayleigh backscattering, but these approaches have never been demonstrated in integrated photonics or through practical TRS-breaking techniques. In this work, we show that it is possible to suppress disorder-induced Rayleigh backscattering in integrated photonics via electrical excitation, even when defects are clearly present. Our experiment is performed in a lithium niobate on insulator (LNOI) integrated ring resonator at telecom wavelength, in which TRS is strongly broken through an acousto-optic interaction that is induced via radiofrequency input. We present evidence that Rayleigh backscattering in the resonator is almost completely suppressed by measuring both the optical density of states and through direct measurements of the back-scattered light. We additionally provide an intuitive argument to show that, in an appropriate frame of reference, the suppression of backscattering can be readily understood as a form of topological protection.

Authors:Giuseppina Simone

Abstract: Despite being extremely old concepts, plasmonics and surface plasmon resonance-based biosensors have been increasingly popular in the recent two decades due to the growing interest in nanooptics and are now of relevant significance in regards to applications associated with human health. Plasmonics integration into point-of-care devices for health surveillance has enabled significant levels of sensitivity and limit of detection to be achieved and has encouraged the expansion of the fields of study and market niches devoted to the creation of quick and incredibly sensitive label-free detection. The trend reflects in wearable plasmonic sensor development as well as point-of-care applications for widespread applications, demonstrating the potential impact of the new generation of plasmonic biosensors on human well-being through the concepts of personalized medicine and global health. In this context, the aim here is to discuss the potential, limitations, and opportunities for improvement that have arisen as a result of the integration of plasmonics into microsystems and lab-on-chip over the past five years. Recent applications of plasmonic biosensors in microsystems and sensor performance are analyzed. The final analysis focuses on the integration of microfluidics and lab-on-a-chip with quantum plasmonics technology prospecting it as a promising solution for chemical and biological sensing. Here it is underlined how the research in the field of quantum plasmonic sensing for biological applications has flourished over the past decade with the aim to overcome the limits given by quantum fluctuations and noise. The significant advances in nanophotonics, plasmonics and microsystems used to create increasingly effective biosensors would continue to benefit this field if harnessed properly.

Authors:Hanying Deng, Zhihao Qu, Yingji He, Changming Huang, Nicolae C. Panoiu, Fangwei Ye

Abstract: We study the nonlinear optical properties of heterojunctions made of graphene nanoribbons (GNRs) consisting of two segments with either the same or different topological properties. By utilizing a quantum mechanical approach that incorporates distant-neighbor interactions, we demonstrate that the presence of topological interface states significantly enhances the second- and third-order nonlinear optical response of GNR heterojunctions that are created by merging two topologically inequivalent GNRs. Specifically, GNR heterojunctions with topological interface states display third-order harmonic hyperpolarizabilities that are more than two orders of magnitude larger than those of their similarly sized counterparts without topological interface states, whereas the secondorder harmonic hyperpolarizabilities exhibit a more than ten-fold contrast between heterojunctions with and without topological interface states. Additionally, we find that the topological state at the interface between two topologically distinct GNRs can induce a noticeable red-shift of the quantum plasmon frequency of the heterojunctions. Our results reveal a general and profound connection between the existence of topological states and an enhanced nonlinear optical response of graphene nanostructures and possible other photonic systems.

Authors:Johannes Bütow, Jörg S. Eismann, Varun Sharma, Dorian Brandmüller, Peter Banzer

Abstract: Structured light is a key component of many modern applications, ranging from superresolution microscopy to imaging, sensing, and quantum information processing. As the utilization of these powerful tools continues to spread, the demand for technologies that enable the spatial manipulation of fundamental properties of light, such as amplitude, phase, and polarization grows further. In this respect, technologies based on liquid-crystal cells, e.g., spatial light modulators, became very popular in the last decade. However, the rapidly advancing field of integrated photonics allows entirely new routes towards beam shaping that not only outperform liquid-crystal devices in terms of speed, but also have substantial potential with respect to robustness and conversion efficiencies. In this study, we demonstrate how a programmable integrated photonic processor can generate and control higher-order free-space structured light beams at the click of a button. Our system offers lossless and reconfigurable control of the spatial distribution of light's amplitude and phase, with switching times in the microsecond domain. The showcased on-chip generation of spatially tailored light enables an even more diverse set of methods, applications, and devices that utilize structured light by providing a pathway towards combining the strengths of programmable integrated photonics and free-space structured light.

Authors:Tianjian Lv, Bing Han, Ming Yan, Zhaoyang Wen, Kun Huang, Kangwen Yang, Heping Zeng

Abstract: Coherent anti-Stokes Raman scattering (CARS) spectroscopy with time-delayed ultrashort pulses and a single-pixel photodetector has shown great potential for spectroscopic imaging and transient studies in chemistry and biological research. However, those systems rely on mechanical delay lines or two asynchronous optical combs with inflexible repetition frequencies, technically limiting their acquisition speeds. Here, we demonstrate a hybrid dual-comb CARS system involving a broadband fiber laser and a highly-flexible, frequency-modulated electro-optic comb. We achieve multiplex CARS spectra (2800-3200 cm-1), with a moderate resolution (22 cm-1), at a maximum refresh rate of 1 MHz, limited by the radio-frequency synthesizer we use. Fast spectroscopic CARS imaging is demonstrated for liquid mixtures. Our system enables spectral measurements in the high-wavenumber C-H stretching region at a record speed that is an order of magnitude higher than state-of-the-art systems, which may open up new opportunities for fast chemical sensing and imaging.

Authors:Matias Koivurova, Charles W. Robson, Marco Ornigotti

Abstract: We study the implications of time-varying wave mechanics, and show how the standard wave equation is modified if the speed of a wave is not constant in time. In particular, waves which experience longitudinal acceleration are shown to have clear relativistic properties when a constant reference speed exists. Moreover, the accelerating wave equation admits only solutions propagating forward in time, which are continuous across material interfaces. We then consider the special case of electromagnetic waves, finding that the Abraham-Minkowski controversy is caused by relativistic effects, and the momentum of light is in fact conserved between different media. Furthermore, we show that the accelerating waves conserve energy when the wave is moving along a geodesic and demonstrate two example solutions. We conclude with some remarks on the role of the accelerating wave equation in the context of the arrow of time.

Authors:Jason Lynch, Evan Smith, Adam Alfieri, Baokun Song, Cindy Yueli Chen, Chavez Lawrence, Cherie Kagan, Honggang Gu, Shiyuan Liu, Lian-Mao Peng, Shivashankar Vangala, Joshua R. Hendrickson, Deep Jariwala

Abstract: Telecommunications and polarimetry both require the active control of the polarization of light, Currently, this is done by combining intrinsically anisotropic materials with tunable isotropic materials into heterostructures using complicated fabrication techniques due to the lack of scalable materials that possess both properties. Tunable birefringent and dichromic materials are scarce and rarely available in high-quality thin films over wafer scales. In this paper, we report semiconducting, highly aligned, single-walled carbon nanotubes (SWCNTs) over 4" wafers with normalized birefringence and dichroism values 0.09 and 0.58, respectively. The real and imaginary parts of the refractive index of the SWCNT films are tuned by up to 5.9% and 14.3% in the infrared at 2200 nm and 1660 nm, respectively, using electrostatic doping. Our results suggest that aligned SWCNTs are among the most anisotropic and tunable optical materials known and opens new avenues for their application in integrated photonics and telecommunications.

Authors:Nuo Chen, Kangping Lou, Yalong Yu, Xuanjian He, Tao Chu

Abstract: Thin-film lithium niobate is a promising platform owing to its large electro-optic coefficients and low propagation loss. However, the large footprints of devices limit their application in large-scale integrated optical systems. A crucial challenge is how to maintain the performance advantage given the design space restrictions in this situation. This article proposes and demonstrates a high-efficiency lithium niobate electro-optic (EO) modulator with high-permittivity cladding to improve the electric field strength in waveguides and its overlap with optical fields while maintaining low optical loss and broad bandwidth. The proposed modulator exhibits considerable improvement, featuring a low half-wave voltage-length product of 1.41 Vcm, a low excess loss of 0.5 dB, and a broad 3 dB EO bandwidth of more than 40 GHz. This modulation efficiency is the highest reported for a broadband lithium niobate modulator so far. The design scheme of using high-permittivity cladding may provide a promising solution for improving the integration of photonic devices on the thin-film lithium niobate platform and these devices may serve as fundamental components in large-scale photonic integrated circuits in the future.

Authors:Joshua T. Y. Tse, Shunsuke Murai, Katsuhisa Tanaka

Abstract: Surface lattice resonance supported on nanoparticle arrays is a promising candidate in enhancing fluorescent effects in both absorption and emission. The optical enhancement provided by surface lattice resonance is primarily through the light confinement beyond the diffraction limit, where the nanoparticle arrays can enhance light-matter interaction for increased absorption as well as providing more local density of states for enhanced spontaneous emission. In this work, we optimize the in-coupling efficiency to the fluorescent molecules by finding the conditions to maximize the absorption, also known as the critical coupling condition. We studied the transmission characteristics and the fluorescent emission of a $TiO_2$ nanoparticle array embedded in an index-matching layer with fluorescent dye at various concentrations. A modified coupled-mode theory that describes the nanoparticle array was then derived and verified by numerical simulations. With the analytical model, we analyzed the experimental measurements and discovered the condition to critically couple light into the fluorescent dye, which is demonstrated as the strongest emission. This study presents a useful guide for designing efficient energy transfer from excitation beam to the emitters, which maximizes the external conversion efficiency.

Authors:Lukas Seitner TUM School of Computation, Information and Technology, Technical University of Munich, Garching, Germany, Johannes Popp TUM School of Computation, Information and Technology, Technical University of Munich, Garching, Germany, Ina Heckelmann Institute for Quantum Electronics, Eidgenössische Technische Hochschule Zürich, Zurich, Switzerland, Réka-Eszter Vass Institute for Quantum Electronics, Eidgenössische Technische Hochschule Zürich, Zurich, Switzerland Physik-Institut, Universität Zürich, Zurich, Switzerland, Bo Meng Institute for Quantum Electronics, Eidgenössische Technische Hochschule Zürich, Zurich, Switzerland State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun, People's Republic of China, Michael Haider TUM School of Computation, Information and Technology, Technical University of Munich, Garching, Germany, Jérôme Faist Institute for Quantum Electronics, Eidgenössische Technische Hochschule Zürich, Zurich, Switzerland, Christian Jirauschek TUM School of Computation, Information and Technology, Technical University of Munich, Garching, Germany TUM Center for Quantum Engineering

Abstract: Ring quantum cascade lasers have recently gained considerable attention, showing ultrastable frequency comb and soliton operation, and thus opening a way to integrated spectrometers in the mid-infrared and terahertz fingerprint regions. Thanks to a self-consistent Maxwell-Bloch model, we demonstrate, in excellent agreement with the experimental data, that a small but finite coupling between the counter-propagating waves arising from distributed backscattering is essential to stabilize the soliton solution.

Authors:Owen D. Miller

Abstract: Near-field optics is an exciting frontier of photonics and plasmonics. The tandem of strongly localized fields and enhanced emission rates offers significant opportunities for wide-ranging applications, while also creating basic questions: How large can such enhancements be? To what extent do material losses inhibit optimal response? Over what bandwidths can these effects be sustained? This chapter surveys theoretical techniques for answering these questions. We start with physical intuition and mathematical definitions of the response functions of interest (LDOS, CDOS, SERS, NFRHT, etc.), after which we describe the general theoretical techniques for bounding such functions. Finally, we apply those techniques specifically to near-field optics, for which we describe known bounds, optimal designs, and open questions.

Authors:Fan Feng, Chen Liang, Dongdong Chen, Ke Du, Runjia Yang, Chang Lu, Shumin Chen, Wenting He, Pingyong Xu, Liangyi Chen, Louis Tao, Heng Mao

Abstract: Based on image moment theory, an approach for space-variant Shack-Hartmann wavefront reconstruction is presented in this article. The relation between the moment of a pair of subimages and the local transformation coefficients is derived. The square guide 'star' is used to obtain a special solution from this relation. The moment-based wavefront reconstruction has a reduced computational complexity compared to the iteration-based algorithm. Image restorations are executed by the tiling strategy with 5 $\times$ 5 PSFs as well as the conventional strategy with a global average PSF. Visual and quantitative evaluations support our approach.

Authors:Yijie Shen, Chao He, Zipei Song, Binguo Chen, Honghui He, Yifei Ma, Julian A. J. Fells, Steve J. Elston, Stephen M. Morris, Martin J. Booth, Andrew Forbes

Abstract: Skyrmions are topologically protected quasiparticles, originally studied in condensed-matter systems and recently in photonics, with great potential in ultra-high-capacity information storage. Despite the recent attention, most optical solutions require complex and expensive systems yet produce limited topologies. Here we demonstrate an extended family of quasiparticles beyond normal skyrmions, which are controlled in confined photonic gradient-index media, extending to higher-order members such as multiskyrmions and multimerons, with increasingly complex topologies. We introduce new topological numbers to describe these complex photonic quasiparticles and propose how this new zoology of particles could be used in future high-capacity information transfer. Our compact creation system lends integrated and programmable solutions of complex particle textures, with potential impacts on both photonic and condensed-matter systems for revolutionizing topological informatics and logic devices.

Authors:Xin Liu, Sergey A. Ponomarenko, Fei Wang, Yangjian Cai, Chunhao Liang

Abstract: In the age of information explosion, the conventional optical communication protocols are rapidly reaching the limits of their capacity, as almost all available degrees of freedom (e.g., wavelength, polarization) for division multiplexing have been explored to date. Recent advances in coherent mode division multiplexing have greatly facilitated high-speed optical communications and secure, high-capacity information storage and transfer. However, coherent mode division multiplexing is quite vulnerable to even minute environmental disturbances which can cause significant information loss. Here, we propose and experimentally demonstrate a paradigm shift to incoherent mode division multiplexing for high-security optical information encryption by harnessing the degree of coherence of structured random light beams. In contrast to the conventional techniques, our approach does not require mode orthogonality to circumnavigate unwanted mode crosstalk. In addition, our protocol has, in principle, no upper bound on its capacity. Thanks to the extreme robustness of structured random light to external perturbations, we are able to achieve highly accurate information encryption and decryption in the adverse environment. The proposed protocol opens new horizons in an array of fields, such as optical communications and cryptography, and it can be relevant for information processing with acoustical, matter as well as other types of waves.

Authors:Siying Liu, Chuanjian Zheng, Qun Hao, Xin Li, Shaohui Zhang

Abstract: We propose a single-shot quantitative differential phase contrast (DPC) method with polarization multiplexing illumination. In the illumination module of our system, the programmable LED array is divided into four quadrants and covered with polarizing films of four different polarization angles. We use a polarization camera with polarizers before the pixels in the imaging module. By matching the polarization angle between the polarizing films over the custom LED array and the polarizers in the camera, two sets of asymmetric illumination acquisition images can be calculated from a single-shot acquisition image. Combined with the phase transfer function, we can calculate the quantitative phase of the sample. We present the design, implementation, and experimental image data demonstrating the ability of our method to obtain quantitative phase images of the phase resolution target, as well as Hela cells.

Authors:Yannick Schrödel, Claas Hartmann, Tino Lang, Jiaan Zheng, Max Steudel, Matthias Rutsch, Sarper H. Salman, Martin Kellert, Mikhail Pergament, Thomas Hahn-Jose, Sven Suppelt, Jan Helge Dörsam, Anne Harth, Wim P. Leemans, Franz X. Kärtner, Ingmar Hartl, Mario Kupnik, Christoph M. Heyl

Abstract: Control over intensity, shape, direction and phase of coherent light is a cornerstone of 20 photonics. Modern laser optics, however, frequently demands parameter regimes where either the wavelength or the optical power restricts control e.g. due to absorption or damage. Limitations are imposed by the properties of solid media, upon which most photonic control schemes rely. We propose to circumvent these limitations using gas media tailored by high-intensity ultrasound waves. We demonstrate a first implementation of this approach by modulating ultrashort laser 25 pulses using ultrasound waves in ambient air, entirely omitting transmissive solid media. At peak powers of 20 GW exceeding the limits of solid-based acousto-optical modulation by about three orders of magnitude, we reach a diffraction efficiency greater than 50% while preserving excellent beam quality. Our results open a route towards versatile gas-phase Sono-Photonic methods, i.e. gas-based photonic systems controlled by sonic waves.

Authors:Yuhang Li, Jingxi Li, Yifan Zhao, Tianyi Gan, Jingtian Hu, Mona Jarrahi, Aydogan Ozcan

Abstract: We demonstrate universal polarization transformers based on an engineered diffractive volume, which can synthesize a large set of arbitrarily-selected, complex-valued polarization scattering matrices between the polarization states at different positions within its input and output field-of-views (FOVs). This framework comprises 2D arrays of linear polarizers with diverse angles, which are positioned between isotropic diffractive layers, each containing tens of thousands of diffractive features with optimizable transmission coefficients. We demonstrate that, after its deep learning-based training, this diffractive polarization transformer could successfully implement N_i x N_o = 10,000 different spatially-encoded polarization scattering matrices with negligible error within a single diffractive volume, where N_i and N_o represent the number of pixels in the input and output FOVs, respectively. We experimentally validated this universal polarization transformation framework in the terahertz part of the spectrum by fabricating wire-grid polarizers and integrating them with 3D-printed diffractive layers to form a physical polarization transformer operating at 0.75 mm wavelength. Through this set-up, we demonstrated an all-optical polarization permutation operation of spatially-varying polarization fields, and simultaneously implemented distinct spatially-encoded polarization scattering matrices between the input and output FOVs of a compact diffractive processor that axially spans 200 wavelengths. This framework opens up new avenues for developing novel optical devices for universal polarization control, and may find various applications in, e.g., remote sensing, medical imaging, security, material inspection and machine vision.

Authors:Mengjiao Ren Hunan Normal University, Chengpeng Ji Hunan Normal University, Xueyan Tang Hunan Normal University, Haishan Tian Hunan Normal University, Leyong Jiang Hunan Normal University, Xiaoyu Dai Hunan University, Xinghua Wu Jiujiang University

Abstract: In this paper, we study the sensitivity-tunable Terahertz (THz) liquid/gas biosensor in a coupling prism-three-dimensional Dirac semimetal (3D DSM) multilayer structure. The high sensitivity of the biosensor originates from the sharp reflected peak caused by surface plasmon resonance (SPR) mode. This structure achieves the tunability of sensitivity due to that the reflectance could be modulated by the Fermi energy of 3D DSM. Besides, it is found that the sensitivity curve depends heavily on the structural parameters of 3D DSM. After parameter optimization, we obtained sensitivity over 100{\deg}/RIU for liquid biosensor. We believe this simple structure provides a reference idea for realizing high sensitivity and tunable biosensor device.

Authors:Yibo Peng School of Information Science and Technology, ShanghaiTech University, Shanghai, China Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai, China University of Chinese Academy of Sciences, Beijing, China, Siting Liu School of Information Science and Technology, ShanghaiTech University, Shanghai, China Shanghai Engineering Research Center of Energy Efficient and Custom AI IC, ShanghaiTech University, Shanghai, China, Vassilios Kovanis Bradley Department of Electrical and Computer Engineering, Virginia Tech Research Center, Arlington, Virginia, USA, Cheng Wang School of Information Science and Technology, ShanghaiTech University, Shanghai, China Shanghai Engineering Research Center of Energy Efficient and Custom AI IC, ShanghaiTech University, Shanghai, China

Abstract: It has been found that noise-induced excitability in quantum well and quantum dot semiconductor laser systems usually produces spike patterns of non-uniform amplitude. In this letter, we experimentally record that an inter-subband quantum cascade laser injected with a monochromatic laser exhibits a series of highly-uniform spike trains in the time domain. Theoretical analysis demonstrates that such high uniformity has its origin in the ultrashort carrier lifetime of the quantum cascade laser gain medium that is typically close to one picosecond.

Authors:Radoslaw Kolkowski, Andriy Shevchenko

Abstract: Optical bound states in the continuum (BICs) have enabled suppression of radiation loss in various photonic structures. However, their quality ($Q$) factors can be limited by the intrinsic absorption loss, if present. We propose a type of BICs in which both the radiation and absorption losses are suppressed. The underlying mechanism is based on mutual coupling between three optical modes without explicitly restricting their symmetry. We numerically demonstrate such BICs in periodic metasurfaces made of resonant gold nanoparticles on a slab waveguide, showing $Q$ factors approaching $10^7$ in the visible spectral range. Our findings suggest that the $Q$ factors have no upper limit even in lossy plasmonic systems.

Authors:Sangmin Ji, Satoshi Iwamoto

Abstract: Improving the photon-spin conversion efficiency without polarization dependence is a major challenge in realizing quantum interfaces gate-defined quantum dots (QDs) for polarization-encoded photonic quantum network systems. Previously, we reported the design of an air-bridge bull's-eye cavity that enhances the photon absorption efficiency of an embedded gate-defined QD regardless of the photon polarization. Here, we numerically demonstrate that a further 1.6 times improvement in efficiency is possible by simply adjusting the distance of the substrate from the semiconductor slab where the bull's-eye structure is formed. Our analysis clarifies that the upward-preferred coupling and narrow far-field emission pattern realized by substrate-induced asymmetry enable the improvement.

Authors:Michael Zwilich, Florian Schepers, Carsten Fallnich

Abstract: A longitudinal mode-locked state can be converted to a transverse mode-locked state by exploiting the spectral and spatial filtering of an empty optical resonator. Carrier and amplitude modulation sidebands were simultaneously transmitted by the conversion resonator, yielding phase-locked superpositions of up to five transverse modes. Equivalently, an amplitude-modulated beam was converted into a beam that periodically moved across the transverse plane. Precise control over the spatial beam shape during oscillation was gained by independently altering the set of transverse modes and their respective powers, which demonstrated an increased level of control in the generation of transverse mode-locked states.

Authors:Mitchell R. Whittam, Aristeidis G. Lamprianidis, Yannick Augenstein, Carsten Rockstuhl

Abstract: The far-field back-scattering amplitude of an electric field from a relativistically-moving sphere is analyzed. Contrary to prior research, we do so by expressing the fields in the helicity basis, and we highlight here its advantages when compared to the commonly-considered parity basis. With the purpose of exploring specific scattering phenomena considering relativistic effects, we identify conditions that minimize the back-scattered field, leading to a relativistic formulation of the first Kerker condition. The requirements to be satisfied by the sphere are expressed in terms of Mie angles, which constitute an effective parametrization of any possible optical response a sphere might have. We are able to identify multiple combinations of Mie angles up to octupolar order via gradient-based optimization that satisfy our newly formulated relativistic Kerker condition, yielding minima for the back-scattered energy as low as 0.016% of the average scattered energy. Our results can be extended to involve multiple particles forming a metasurface, potentially having direct implications on the design of light sails as considered by the Breakthrough Starshot Initiative.

Authors:Songyan Hou, Hao Hu, Paokang Chen, Ian Briggs, Weichuan Xing, Zhihong Liu, Linran Fan

Abstract: Electro-optic modulators lie at the heart of complex integration and high density electro-optic systems. One of the representative electro-optic modulators is thin film lithium niobate based microring modulator which has demonstrated advantages of compact footprint, low optical loss and high modulation efficiency. However, the linewidth and extinction ratio of ring modulators are fundamentally limited by the ring losses and coupling, respectively. To this end, we propose a novel type of electro-optic modulators with ring-pair structure on thin film lithium niobate platform, which brings substantially improvement of linewidth and extinction ratio. The ring-pair modulator exhibits a larger linewidth up to 22 GHz, 1.74-time larger than that of single ring resonator with same design parameters. Moreover, the experimental results also reveal that the added-up extinction ratio of ring-pair resonator goes beyond 30 dB, much larger than that in an individual ring resonator. These advantages of ring-pair modulator pave a new way for the application of compact ring-based modulators with large working wavelength window and high extinction ratio, to be exploited in quantum optics, programmable nanophotonics and optical sensors, etc.

Authors:Qin Tang, Dandan Zhang, Tingting Liu, Wenxing Liu, Qinghua Liao, Jizhou He, Shuyuan Xiao, Tianbao Yu

Abstract: The magneto-optical Faraday and Kerr effects are widely used in modern optical devices. In this letter, we propose an all-dielectric metasurface composed of perforated magneto-optical thin films, which can support the highly confined toroidal dipole resonance and provide full overlap between the localized electromagnetic field and the thin film, and consequently enhance the magneto-optical effects to an unprecedented degree. The numerical results based on finite element method show that the Faraday and Kerr rotations can reach -13.59${\deg}$ and 8.19${\deg}$ in the vicinity of toroidal dipole resonance, which are 21.2 and 32.8 times stronger than those in the equivalent thickness of thin films, respectively. In addition, we design an environment refractive index sensor based on the resonantly enhanced Faraday and Kerr rotations, with sensitivities of 62.96 nm/RIU and 73.16 nm/RIU, and the corresponding maximum figures of merit 132.22${\deg}$/RIU and 429.45${\deg}$/RIU, respectively. This work provides a new strategy for enhancing the magneto-optical effects at nanoscale, and paves the way for the research and development of magneto-optical metadevices such as sensors, memories, and circuits.

Authors:Saeed Pahlavan

Abstract: In this paper, the asymmetric propagation of electromagnetic waves inside a two-dimensional air slit cut in an InSb slab is studied. It has been shown that due to the anisotropic magnetic properties of InSb under a DC magnetic bias, forward and backward waves show different field patterns inside the air slit. The proposed waveguide can have potential asymmetric applications in mmWave and sub-THz regions.

Authors:Huai-Yi Xie

Abstract: We construct the dyadic Greens functions (DGFs) for a topological insulator (TI) stratified sphere within the framework of axion electrodynamics. For these DGFs, the additional expansion coefficients are included to account for the axion coupling effect. With the application of these DGFs, we derive the formulation of light scattering from a dipole near a TI stratified sphere. In our numerical studies, we give three types of configurations (a metal-coated TI sphere, a metal-TI-metal-coated TI sphere and an alternating metal-TI stratified sphere) to investigate how the topological magneto-electric (TME) response of the TI sphere (shells) influences on the multipolar plasmonic resonance of the metal shells. For these types, the results show that the TME effect causes some modifications of the decay rate spectrum for an emitting dipole near a TI stratified sphere. For the multipolar resonances of the metal shells, it is observed that the TME-induced red-shifts for the bonding and lower order antibonding modes are found but those for the higher order antibonding modes are insignificant. In addition, for a metal-coated TI sphere, we take into account the effects of losses in the TI core of which the dielectric function is chosen to be the form of the bulk or five quintuple layers (5QL) slab and then the some modifications of the TME-induced decay rate spectrum are obviously suppressed. These phenomenological characteristics provide useful guidance to probing the TME effect via molecular fluorescence experiments.

Authors:Rakesh Arul, Kishan Menghrajani, Marie S. Rider, Rohit Chikkaraddy, William L. Barnes, Jeremy J. Baumberg

Abstract: Strong coupling of molecular vibrations with light creates polariton states, enabling control over many optical and chemical properties. However, the near-field signatures of strong coupling are difficult to map as most cavities are closed systems. Surface-enhanced Raman microscopy of open metallic gratings under vibrational strong coupling enables the observation of spatial polariton localization in the grating near-field, without the need for scanning probe microscopies. The lower polariton is localized at the grating slots, displays a strongly asymmetric lineshape, and gives greater plasmon-vibration coupling strength than measured in the far-field. Within these slots, the local field strength pushes the system into the ultrastrong coupling regime. Models of strong coupling which explicitly include the spatial distribution of emitters can account for these effects. Such gratings form a new system for exploring the rich physics of polaritons and the interplay between their near- and far-field properties through polariton-enhanced Raman scattering (PERS).