Optics (physics.optics)

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.