Metamaterials with properly engineered linear and nonlinear optical response are of great interest for many advanced applications in nanophotonics and quantum optics. In the present work, we perform a detailed spectral investigation of the third-order nonlinear optical properties (nonlinear refractive index and nonlinear absorption coefficient) of $ϵ$-near-zero ${\mathrm{Au/Al}}_2{\mathrm{O}}_3$ multilayer metamaterials in a broad range of the visible spectrum across their $ϵ$-near-zero (ENZ) wavelength, at different incidence angles with TE- and TM-polarized light. Multilayers with different gold filling fractions (16 and 33%) are produced by magnetron sputtering to tune the spectral position of the $ϵ$-near-zero wavelength. The results demonstrate that a continuous modulation of the linear and nonlinear optical parameters of these metamaterials can be obtained as a function of the angle of incidence, with a peak of the nonlinear optical coefficients close to the ENZ wavelength. A model is proposed to describe the nonlinear optical response of the metamaterials, and an optimal agreement between experimental and simulated results is obtained in all the configurations explored. This model represents a useful tool to design multilayer metamaterials with tailored nonlinear optical properties, to be used in different experimental configurations.

Without using intrinsic chiral metamaterials with complex structures, here we use achiral metasurfaces to realize giant extrinsic chirality empowered by quasi-bound states in the continuum (quasi-BICs). The giant extrinsic chirality in the proposed achiral metasurface is mainly attributed to the breaking of the mirror symmetry of the whole system, combining the metasurface with incident waves. The transmittance of two different polarization states and the signs of circular dichroism (CD) can be arbitrarily controlled by tilting the plane of the metasurface relative to the incoming waves. Remarkably, we simultaneously select two polarization states at different bands for a certain achiral metasurface based on the symmetry-protected dual bound states in the continuum (BICs). The measured maximum CDs approach ±0.85. In addition, an application of the polarization-multiplexed-field image display assisted by chiral quasi-BICs is demonstrated. This work provides a general method to control extrinsic chirality in achiral metasurfaces and offers possibilities to design high-performance multiband spin-selective transmission, biodetection, chemical analysis, and on-chip chiral manipulation.

The conversion and interaction between quantum signals at the single-photon level are essential for scalable quantum photonic information technology. Using a fully optimized periodically poled lithium niobate microring, we demonstrate ultraefficient sum-frequency generation on a chip. The external quantum efficiency reaches $(65\pm 3)\mathrm{\%}$ with only $(104\pm 4)$-$\mu \mathrm{W}$ pump power. At peak conversion, $3\times {10}^{-5}$-noise photon is created during the cavity lifetime, which meets the requirement of quantum applications using single-photon pulses. Using a pump and signal in single-photon coherent states, we directly measure the conversion probability produced by a single pump photon to be ${10}^{-5}$, which is a significant improvement from the state of the art, and the photon-photon coupling strength to be 9.1 MHz. Our results mark steady progress toward quantum nonlinear optics at the ultimate single-photon limit, with potential applications in highly integrated photonics and quantum optical computing.

The emissivity and absorptivity of nonreciprocal thermal emitters are not constrained by the well-known Kirchhoff law of thermal radiation, which usually serves as the theoretical basis to characterize thermal properties. When thermal emitters are nondiffracting, which is the case in previous studies of nonreciprocal thermal emitters, the angular distribution of emissivity and absorptivity is mirror symmetric with respect to the normal direction, and the nonreciprocal effect is confined to a relatively narrow angular range. In this work, we consider nonreciprocal thermal emitters that can simultaneously couple to multiple diffraction channels. We show that the symmetry relation between absorptivity and emissivity can be broken in such multichannel emitters. The angular range in which the nonreciprocal effect is strong can also be significantly broadened. Such multichannel emitters can operate as one-way energy splitters. Our work significantly broadens the flexibility of thermal-radiation control in nonreciprocal thermal-emitter design.

We report on the impact of structural defects on mid-infrared intersubband (ISB) properties of $\mathrm{Ga}\mathrm{N}/(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ heterostructures grown by ammonia molecular beam epitaxy (${\mathrm{NH}}_3$ MBE). Twenty-period $\mathrm{Ga}\mathrm{N}/(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ multi-quantum-well (MQW) heterostructures are grown on co-loaded a-plane freestanding $\mathrm{Ga}\mathrm{N}$ substrates and heteroepitaxial a-plane $\mathrm{Ga}\mathrm{N}$ on r-plane sapphire templates (a-$\mathrm{Ga}\mathrm{N}$/r-sap) for three different quantum-well (QW) widths (3.0, 3.3, and 3.7 nm). Co-loaded structures grown on freestanding a-plane with no basal-plane stacking faults (BSFs), prismatic stacking faults (PSFs), and partial dislocations (PDs), with low threading dislocation (TD) densities of about ${10}^5{\mathrm{cm}}^{-2}$ are compared with those grown on a-$\mathrm{Ga}\mathrm{N}$ templates on $(10\overline{1}2)$ r-sapphire with BSF, PSF, PD, and TD densities of about 4 × ${10}^5$ to ${10}^6{\mathrm{cm}}^{-2}$, 5 × ${10}^3$ to 2 × ${10}^4{\mathrm{cm}}^{-2}$, about 9 × ${10}^{10}$ to 2 × ${10}^{11}{\mathrm{cm}}^{-2}$, and about ${10}^{10}{\mathrm{cm}}^{-2}$, respectively. Fourier-transform infrared absorption spectroscopy indicates ISB transition energies in the range of about 250–300 meV (wavelength range 4.1–4.8 µm) for MQWs with different QW widths. The ISB absorption spectra indicate about 5% smaller transition energies and only about 10%–20% larger spectral linewidths for structures grown on a-$\mathrm{Ga}\mathrm{N}/$r-sapphire templates compared with those on freestanding $\mathrm{Ga}\mathrm{N}$ substrates. The strong defect tolerance in the nonpolar a-plane ISB structures could be due to the nature of defects and their energy levels with respect to the conduction-band minima, which do not affect the ISB properties. Our results pave the way toward the production of low-cost scalable nonpolar III-nitride MQW heterostructures for a variety of passive and active optical materials and devices based on intersubband transitions.

Continuous-time quantum walks (CTQWs) have shown the capability to perform efficiently the spatial search of a marked site on many kinds of graphs. However, most of such graphs are hard to realize in an experimental setting. Here we study CTQW spatial search on a planar triangular lattice by means of both numerical simulations and experiments. The experiments are performed using two-dimensional waveguide arrays fabricated by femtosecond laser pulses, illuminated by coherent light. We show that the retrieval of the marked site by the quantum walker is accomplished with higher probability than the classical counterpart, in a convenient time window placed early in the evolution.

A simple and universal design principle to realize the generalized Brewster effect (GBE) is proposed. Based on the generalized Kerker effect (GKE), we can realize the GBE for arbitrary polarizations, any incident angles, and any frequencies on the traditional dielectric interface by the destructive interference of various of multipoles. An adjustable metallic meta-surface has been designed to achieve this effect experimentally in the microwave band. By adjusting the suspension height of meta-surfaces above the interface, the incident angles and frequency corresponding to the GBE can be modulated easily. For the two different polarized incident waves, the GBE can be realized at the same frequency and same incident angle, which shows the validity of the GKE. This design principle opens the door for realizing tunable GBE devices that can be applied in many scenarios.

While operating a quantum random-number generator (QRNG), it is extremely useful to have a model of the physical entropy source to guarantee that the device is delivering randomness of genuine quantum origin. In this work we consider a QRNG based on a gain-switched laser diode and we develop a model to quantify its phase noise. This model is based on the laser rate equations and the state-of-the-art techniques for the characterization of laser diodes used in lightwave systems. These tools let us achieve a faithful modeling of the phase noise and we verify its accuracy through comparisons with experimental measurements. Furthermore, the model can be used to select optimal parameters to maximize the QRNG performance and monitor the device behavior to detect malfunctioning or malicious tampering of the device.

Recently, it was shown that dissipatively coupled laser arrays simulate the classical $XY$ model. We show that phase locking of laser arrays can give rise to the spontaneous formation of vortex and antivortex phase patterns that are analogous to topological defects of the $XY$ model. These patterns are stable although their formation is less likely in comparison to the ground-state lasing mode. In addition, we show that small ratios of photon to gain lifetime destabilize vortex and antivortex phase patterns. These findings are useful for studying topological effects in optics as well as for designing laser array devices.

Optical antennas made of low-loss dielectrics have several advantages over plasmonic antennas, including high radiative quantum efficiency, negligible heating, and excellent photostability. However, due to weak spatial confinement, conventional dielectric antennas fail to offer light-matter interaction strengths on par with those of plasmonic antennas. We propose here an all-dielectric antenna configuration that can support strongly confined modes ($V\sim {10}^{-4}{\lambda }_0^3$) while maintaining unity antenna quantum efficiency. This configuration consists of a high-index pillar structure with a transverse gap that is filled with a low-index material, where the contrast of indices induces a strong enhancement of the electric field perpendicular to the gap. We provide a detailed explanation of the operational principle of such photonic gap antennas (PGAs) based on the dispersion relation of symmetric and asymmetric horizontal slot waveguides. To discuss the properties of PGAs, we consider silicon pillars with air or the polymer CYTOP as the gap material. We show by full-wave simulations that PGAs with an emitter embedded in the gap can enhance the spontaneous emission rate by a factor of approximately $1000$ for air gaps and approximately $400$ for CYTOP gaps over a spectral bandwidth of $\mathrm{\Delta }\lambda \approx 300\mathrm{nm}$ at $\lambda =1.25\mu \mathrm{m}$. Furthermore, the PGAs can be designed to provide unidirectional out-of-plane radiation across a substantial portion of their spectral bandwidth. This is achieved by setting the position of the gap at an optimized off-centered position of the pillar so as to properly break the vertical symmetry of the structure. We also demonstrate that, when acting as receivers, PGAs can lead to a near-field intensity enhancement by a factor of approximately $3000$ for air gaps and approximately $1200$ for CYTOP gaps.

We demonstrate size-selective optical trapping and transport for nanoparticles near an optical nanofiber taper. Using a two-wavelength counterpropagating mode configuration, we show that 100-nm-diameter and 150-nm-diameter gold nanospheres (GNSs) are trapped by the evanescent field in the taper region at different optical powers. Conversely, when one nanoparticle species is trapped the other may be transported, leading to a sievelike effect. Our results show that sophisticated optical manipulation can be achieved in a passive configuration by taking advantage of mode behavior in nanophotonic devices.

A transducer of single photons between microwave and optical frequencies can be used to realize quantum communication over optical fiber links between distant superconducting quantum computers. A promising scalable approach to constructing such a transducer is to use ensembles of quantum emitters interacting simultaneously with electromagnetic fields at optical and microwave frequencies. However, inhomogeneous broadening in the transition frequencies of the emitters can be detrimental to this collective action. In this paper, we utilize a gradient-based optimization strategy to design the temporal shape of the laser field driving the transduction system to mitigate the effects of inhomogeneous broadening. We study the improvement of transduction efficiencies as a function of inhomogeneous broadening in different single-emitter cooperativity regimes and correlate it with a restoration of super-radiance effects in the emitter ensembles. Furthermore, to assess the optimality of our pulse designs, we provide certifiable bounds on the design problem and compare them to the achieved performance.

Stimulated parametric down-conversion is a nonlinear optical process in which the orbital angular momentum of light is conserved under most practical conditions. However, there are instances in which the topological charge is not compatible with the remaining modal structure of the stimulated idler beam. As a result, the stimulated down-converted light diffracts and gives rise to Laguerre-Gaussian-like beams with nonzero radial indices. We present a theoretical and experimental analysis demonstrating this fundamental aspect of nonlinear processes with light beams possessing orbital angular momentum.

We present an electro-optic modulation technique that enables a single laser diode to realize a cold-atom source and a quantum inertial sensor based on matter-wave interferometry. Using carrier-suppressed dual-single-sideband modulation, an in-phase and quadrature modulator generates two optical sidebands from separate radio-frequency (rf) signals. These sidebands are controlled independently in frequency, phase, and power using standard rf components. Our laser source exhibits improved rejection of parasitic sidebands compared to those based on phase modulators, which generate large systematic shifts in atom interferometers. We measure the influence of residual laser lines on an atom-interferometric gravimeter and show agreement with a theoretical model. We estimate a reduction of the systematic shift by 2 orders of magnitude compared to previous architectures and reach a long-term sensitivity of $15\mathrm{n}g$ on the gravitational acceleration with an interrogation time of only $T=20\mathrm{ms}$. Finally, we characterize the performance of our integrated laser system and show that it is suitable for mobile sensing applications, including gravity surveys and inertial navigation.

The kinetic inductance (KI) of superconducting devices can be exploited for reducing the footprint of linear elements as well as for introducing nonlinearity to the circuit. We characterize the linear and nonlinear properties of a multimode resonator fabricated from amorphous tungsten silicide ($\mathrm{WSi}$), with a fundamental frequency of $f_1=172$ MHz. We show how the multimode structure of the device can be used to extract the different quality factors and to aid the nonlinear characterization. In the linear regime, the footprint is reduced by a factor of approximately $2.9$ with standard lateral dimensions, with no significant degradation of the internal quality factor compared to a similar $\mathrm{Al}$ device. In the nonlinear regime, we observe self-positive frequency shifts at low powers, which can be attributed to saturation of tunneling two-level systems. The cross-mode nonlinearities are described well by a Kerr model with a self-Kerr coefficient on the order of $|K_{11}|/2\pi \approx 1.5\times {10}^{-7}$ Hz per photon. These properties, together with a reproducible fabrication process, make $\mathrm{WSi}$ a promising candidate for creating linear and nonlinear circuit quantum electrodynamics elements.

An optical microcavity provides a prominent platform for single-nanoparticle detection with ultrahigh sensitivity. Recently, microcavity sensors working at the exceptional point have caught great attention due to their ability to enhance sensitivity, but suffer from simultaneously amplified noise. Here, we propose employing a nonlinear microcavity to enhance the sensitivity of single-nanoparticle detection based on spontaneous chiral symmetry breaking. It is found that sensors operating at the symmetry-breaking threshold experience a gigantic enhancement in sensitivity, which is caused by the square-root response to perturbation in a lossless microcavity. Through the analysis for a realistic microcavity, a 30-fold sensitivity enhancement is demonstrated by working at the threshold, and the enhancement is also confirmed by numerical simulation. Furthermore, the noise performance is analyzed to be superior in thermorefractive noise and quantum noise performance in comparison to a conventional microcavity-sensing scheme. Merging the spontaneous chiral symmetry-breaking effect with practical sensing applications, the results pave a universal way for high-performance microcavity sensing with enhanced sensitivity and reduced noise.

Chiral, topologically protected, photonic surface states can be found at the boundary between gyrotropic photonic crystals where a changing magnetic field induces different topology across the interface. Typically, photonic crystals with either a suitable band structure on both sides of the interface to provide a band gap and evanescent decay of the surface states away from the interface, or an outer layer with engineered material properties is required. In this paper, we show the emergence of topological, unidirectional surface states at the termination of finite gyrotropic photonic crystals with a simple square lattice and ${\mathcal{C}}_4$ rotational symmetry bounded by a vacuum, eliminating the need for an outside layer to enable chiral surface modes. We start from an infinite, time-reversal-symmetry-breaking photonic crystal with a band gap associated with bands with nonzero Chern numbers, different from all-zero Chern numbers in air. We then modify the photonic crystal to move this band gap below the light line, while maintaining the Chern-number discontinuities. Band-structure calculations for a supercell approximating a photonic crystal finite in the direction normal to the surface demonstrate the existence, dispersion, and chirality of the surface mode. Extensive direct scattering calculations for a point source and spatial Fourier analysis further reveal a unidirectional free-space topological surface state, which propagates counterclockwise around the surface of a finite photonic crystal, providing a nearly foolproof way to cross-check the surface-mode band structure unaffected by backscattering from local defects. Additionally, scattering simulations allow an independent characterization of the state dispersion and unveil the robustness of the topological plasmonic mode propagation around the ${90}^{\circ }$ bends of the structure, being due to only radiation leakage. In contrast to buried topological surface states, the observed surface modes at the photonic crystal–air interface have the advantage of being accessible to the outside world, allowing one to take advantage of the defect-tolerant backscattering-free surface modes to engineer emission from photonic crystal surfaces into arbitrary free-space beam shapes and directions.

Topological notions in physics have become a powerful perspective that leads to the discoveries of interface states. In this work, we present a scheme to steer waves by leveraging the properties of valley interface states (VISs) in a valley photonic crystal (VPC). Due to the chiral characteristics, the VISs deterministically possess either positive or negative dispersion relation, which can cause an obliquely incident wave to undergo a lateral shift. By stacking multiple VPC interfaces, the VIS-induced lateral shifts can relay in the transmitted wave towards the far side of incidence. By selectively engaging one type of VISs, the resultant outgoing wave appears to have undergone negative refraction. While this refraction can be flipped to positive by selectively exciting the other type of VISs with positive dispersion. This finding is verified in microwave experiments. Our scheme opens application scenarios for topological systems in wave manipulations.