Optics (physics.optics)

Abstract: As is well known that the distribution of the scattered radiation generated by an anisotropic scatterer usually lacks rotational symmetry about the direction of incidence due to the spatial anisotropy of the scatterer itself. Here we show that the rotationally symmetric distribution of the far-zone scattered momentum flow may be realized provided that the structural parameters of both the medium and the source are chosen suitably, when a polychromatic electromagnetic plane wave is scattered by an anisotropic Gaussian Schell-model medium. We derive necessary and sufficient conditions for producing such a symmetric distribution, and further elucidated the relationship between the spectral degree of polarization of the incident source and the rotationally symmetric momentum flow of the scattered field in the far zone. It is found that the realization of the rotationally symmetric scattered momentum flow is independent of the spectral degree of polarization of the source, i.e., the rotationally symmetric distribution of the far-zone scattered momentum flow is always realizable regardless of whether the incident source is fully polarized, partially polarized or completely unpolarized. Our results may find useful application in optical micromanipulation, especially when the optical force used to manipulate particles requires to be rotationally symmetric.

Abstract: Color centers in diamond, among them the negatively-charged germanium vacancy (GeV$^-$), are promising candidates for many applications of quantum optics such as a quantum network. For efficient implementation, the optical transitions need to be coupled to a single optical mode. Here, we demonstrate the transfer of a nanodiamond containing a single ingrown GeV- center with excellent optical properties to an open Fabry-P\'erot microcavity by nanomanipulation utilizing an atomic force microscope. Coupling of the GeV- defect to the cavity mode is achieved, while the optical resonator maintains a high finesse of F = 7,700 and a 48-fold spectral density enhancement is observed. This article demonstrates the integration of a GeV- defect with a Fabry-P\'erot microcavity under ambient conditions with the potential to extend the experiments to cryogenic temperatures towards an efficient spin-photon platform.

Abstract: We demonstrate the controlled spatiotemporal transfer of transverse orbital angular momentum (OAM) to electromagnetic waves: the spatiotemporal torquing of light. This is a radically different situation than OAM transfer to longitudinal, spatially-defined OAM light by stationary or slowly varying refractive index structures such as phase plates or air turbulence. We show that transverse OAM can be imparted to a short light pulse only for (1) sufficiently fast transient phase perturbations overlapped with the pulse in spacetime, or (2) energy removal from a pulse that already has transverse OAM. Our OAM theory for spatiotemporal optical vortex (STOV) pulses [Phys. Rev. Lett. 127, 193901 (2021)] correctly quantifies the light-matter interaction of this experiment, and provides a torque-based explanation for the first measurement of STOVs [Phys. Rev. X 6, 031037 (2016)].

Abstract: The intriguing properties of bound states in the continuum (BICs) have attracted a lot of attention in photonics. Besides being effective in confining light in a counter-intuitive way, the correspondence between the near-field mode pattern and the far-field radiation of BICs manifests the fascinating topological characteristics of light. Early works on photonic BICs were mainly focused on designing artificial structures to facilitate their realization, while recent advances have shifted to exploring their exceptional properties in applications. In this review, we survey important breakthroughs and recent advances in this field. We detail the unique properties of BICs, including light confinement enhancement, sharp Fano resonances, and topological characteristics. We provide insights into the unique phenomena derived from BICs and the impact of BICs on various applications. We also discuss the paradigm shift enabled or facilitated by BICs in several emerging research frontiers, such as parity-time symmetric systems, higher-order topology, exciton-photon coupling, and moir\'e superlattices.

Abstract: Nanoelectromechanical systems offer unique functionalities in photonics: The ability to elastically and reversibly deform dielectric beams with subwavelength dimensions enable electrical control of the propagation of light with a power consumption orders of magnitude below that of competing technologies, such as thermo-optic tuning. We present a study of the design, fabrication, and characterization of compact electrostatic comb-drive actuators tailored for integrated nanoelectromechanical silicon photonic circuits. Our design has a footprint of $1.2 \times 10^{3} \mu$m$^{2}$ and is found to reach displacements beyond 50 nm at 5 V with a mechanical resonance above 200 kHz, or, using different spring constants and skeletonization, a mechanical resonance above 2.5 MHz with displacements beyond 50 nm at 28 V. This is sufficient to induce very large phase shifts and other optical effects in nanoelectromechanical reconfigurable photonic circuits.

Abstract: The Fractional Fourier Transform (FRT) corresponds to an arbitrary-angle rotation in the phase space, e.g. the time-frequency (TF) space, and generalizes the fundamentally important Fourier Transform. FRT applications range from classical signal processing (e.g. time-correlated noise optimal filtering) to emerging quantum technologies (e.g. super-resolution TF imaging) which rely on or benefit from coherent low-noise TF operations. Here a versatile low-noise single-photon-compatible implementation of the FRT is presented. Optical TF FRT can be synthesized as a series of a spectral disperser, a time-lens, and another spectral disperser. Relying on the state-of-the-art electro-optic modulators (EOM) for the time-lens, our method avoids added noise inherent to the alternatives based on non-linear interactions (such as wave-mixing, cross-phase modulation, or parametric processes). Precise control of the EOM-driving radio-frequency signal enables fast all-electronic control of the FRT angle. In the experiment, we demonstrate FRT angles of up to 1.63 rad for pairs of coherent temporally separated 11.5 ps-wide pulses in the near-infrared (800 nm). We observe a good agreement between the simulated and measured output spectra in the bright-light and single-photon-level regimes, and for a range of pulse separations (20 ps to 26.67 ps). Furthermore, a tradeoff is established between the maximal FRT angle and bandwidth, with the current setup accommodating up to 248 GHz of bandwidth. With the ongoing progress in EOM on-chip integration, we envisage excellent scalability and vast applications in all-optical TF processing both in the classical and quantum regimes

Abstract: We present a scattering formalism to analyze the spin-momentum locking in structured holey plasmonic metasurfaces. It is valid for any unit cell for arbitrary position and orientation of the holes. The spin-momentum locking emergence is found to originate from the unit cell configuration. Additionally, we find that there are several breakdown terms spoiling the perfect spin-momentum locking polarization. We prove that this breakdown also appears in systems with global symmetries of translation and rotation of the whole lattice, like the Kagome lattice. Finally, we present the excitation of surface plasmon polaritons as the paramount example of the spin-momentum locking breakdown.

Abstract: We demonstrate a remarkably effective single-stage compression technique for ultrafast pulses in the visible electromagnetic spectrum using second-harmonic pulses at 515 nmderived from a 1030 nm Yb-based femtosecond regenerative amplifier. By employing an advanced multi-plate scheme, we achieve more than fourfold compression from 180 fs to 40 fs with an extremely high spectral broadening efficiency of over 95%, and a temporal compression efficiency exceeding 75%. In addition, our method leverages a low nonlinearity medium to attain the shortest pulse durations for a single compressor while maintaining a superb spatial beam quality with 97% of the energy confined in the main lobe of the Arie disk. Moreover, our technique enhances the temporal pulse quality at 515 nm without generating substantial femtosecond-to-picosecond pulse pedestals. The resulting intense visible laser pulses with excellent spatio-temporal parameters and high repetition rate of 100 kHz to 1 MHz open up new frontiers for extreme nonlinear optics and ultrabright EUV and X-ray high-harmonic generation using short VIS wavelength.

Abstract: We demonstrate a novel technique for producing high-order harmonics with designer spectral combs in the extreme ultraviolet-soft X-ray range for resonance applications using spectrally controlled visible lasers. Our approach enables continuous tunability of the harmonic peaks while maintaining superb laser-like features such as coherence, narrow bandwidth, and brightness. The harmonics are conveniently shifted towards lower or higher energies by varying the infrared pulse parameters, second harmonic generation phase-matching conditions, and gas density inside a spectral-broadening waveguide. In the time domain, the X-rays are estimated to emerge as a train of sub-300 attosecond pulses, making this source ideal for studying dynamic processes in ferromagnetic nanostructures and other materials through resonant multidimensional coherent diffractive imaging or other X-ray absorption spectroscopy techniques. Moreover, the visible driving laser beams exhibit an ultrashort sub-10 fs pulse dues to nonlinear self-compression with a more than 30-fold enhancement in peak intensity that also extends the tunability of the linewidth of the harmonic combs.

Abstract: Diffractive nonlocal metasurfaces have recently opened a broad range of exciting developments in nanophotonics research and applications, leveraging spatially extended (yet locally patterned) resonant modes to control light with new degrees of freedom. While conventional grating responses are elegantly captured by temporal coupled mode theory (TCMT), TCMT is not well equipped to capture the more sophisticated responses observed in the nascent field of nonlocal metasurfaces. Here, we introduce spatio-temporal coupled mode theory (STCMT), capable of elegantly capturing the key features of the resonant response of wavefront-shaping nonlocal metasurfaces. This framework can quantitatively guide nonlocal metasurface design, and is compatible with local metasurface frameworks, making it a powerful tool to rationally design and optimize a broad class of ultrathin optical components. We validate this STCMT framework against full-wave simulations of various nonlocal metasurfaces, demonstrating that this tool offers a powerful semi-analytical framework to understand and model the physics and functionality of these devices, without the need for computationally intense full-wave simulations. We also discuss how this model may shed physical insights into nonlocal phenomena in photonics and into the functionality of the resulting devices. As a relevant example, we showcase STCMT's flexibility by applying it to study and rapidly prototype nonlocal metasurfaces that spatially shape thermal emission.

Abstract: The ability to tailor with a high accuracy the inter-site connectivity in a lattice is a crucial tool for realizing novel topological phases of matter. Here, we report the experimental realization of photonic dimer chains with long-range hopping terms of arbitrary strength and phase, providing a rich generalization of the celebrated Su-Schrieffer-Heeger model. Our experiment is based on a synthetic dimension scheme involving the frequency modes of an optical fiber loop platform. This setup provides direct access to both the band dispersion and the geometry of the Bloch wavefunctions throughout the entire Brillouin zone allowing us to extract the winding number for any possible configuration. Finally, we highlight a topological phase transition solely driven by a time-reversal-breaking synthetic gauge field associated with the phase of the long-range hopping, providing a route for engineering topological bands in photonic lattices belonging to the AIII symmetry class.

Abstract: Optical second harmonic generation (SHG) is a nonlinear optical effect widely used for nonlinear optical microscopy and laser frequency conversion. Closed-form analytical solution of the nonlinear optical responses is essential for evaluating the optical responses of new materials whose optical properties are unknown a priori. A recent open-source code, SHAARP(si), can provide such closed form solutions for crystals with arbitrary symmetries, orientations, and anisotropic properties at a single interface. However, optical components are often in the form of slabs, thin films on substrates, and multilayer heterostructures with multiple reflections of both the fundamental and up to ten different SHG waves at each interface, adding significant complexity. Many approximations have therefore been employed in the existing analytical approaches, such as slowly varying approximation, weak reflection of the nonlinear polarization, transparent medium, high crystallographic symmetry, Kleinman symmetry, easy crystal orientation along a high-symmetry direction, phase matching conditions and negligible interference among nonlinear waves, which may lead to large errors in the reported material properties. To avoid these approximations, we have developed an open-source package named Second Harmonic Analysis of Anisotropic Rotational Polarimetry in Multilayers (SHAARP(ml)). The reliability and accuracy are established by experimentally benchmarking with both the SHG polarimetry and Maker fringes predicted from the package using standard materials.

Abstract: We demonstrate and analyse a novel approach to enhance the threshold power of stimulated Brillouin scattering (SBS) in optical fibers, using a longitudinal compressive strain gradient. We derive analytical expressions for the power spectral density of the backscattered Stokes wave in the general case of passive and amplifying optical fibers, by considering the strain and optical power distributions. Our method provides an accurate prediction of the SBS gain spectrum, that we illustrate with a quantitative comparison between measurements and calculations of the SBS Stokes spectra, before and after applying the compression gradient. Our experimental results demonstrate the successful enhancement of the SBS threshold power by a factor of about 3 for the passive fiber and 2 for the amplifying fiber. The enhancement that we manage to calculate in the case of the passive fiber is in perfect agreement with the experimental result.