We present a generic system of three bosonic modes coupled parametrically with a time-varying coupling modulated by a combination of two pump harmonics, and we show how this system provides the minimal platform for realizing nonreciprocal couplings that can lead to gainless photon circulation, and phase-preserving or phase-sensitive directional amplification. Explicit frequency-dependent calculations within this minimal paradigm highlight the separation of amplification and directionality bandwidths, a feature generic to such schemes. We also study the influence of counterrotating interactions that can adversely affect directionality and the associated bandwidth; we find that these effects can be mitigated by suitably designing the properties of the auxiliary mode that plays the role of an engineered reservoir to the amplification mode space.

We present a model to describe the spatiotemporal evolution of guided modes in semiconductor nanowires based on a coupled-mode formalism. Light-matter interaction is modeled based on semiconductor Bloch equations, including many-particle effects in the screened Hartree-Fock approximation. Appropriate boundary conditions are used to incorporate reflections at waveguide end facets, thus allowing for the simulation of nanowire lasing. We compute the emission characteristics and temporal dynamics of CdS and ZnO nanowire lasers and compare our results both to finite-difference time-domain simulations and to experimental data. Finally, we explore the dependence of the lasing emission on the nanowire cavity and on the material relaxation time.

We report on III-nitride waveguides with $c$-plane $\mathrm{GaN}/(\mathrm{Al},\mathrm{Ga})\mathrm{N}$ quantum wells in the strong light-matter coupling regime supporting propagating polaritons. They feature a normal-mode splitting as large as 60 meV at low temperatures thanks to the large overlap between the optical mode and the active region, a polariton decay length up to $100\mu \mathrm{m}$ for photonlike polaritons and a lifetime of 1 to 2 ps, with the latter values being essentially limited by residual absorption occurring in the waveguide. The fully lattice-matched nature of the structure allows for very low disorder and high in-plane homogeneity, which is an important asset for the realization of polaritonic integrated circuits that could support nonlinear polariton wave packets up to room temperature thanks to the large exciton binding energy of 40 meV.

Using a kinetic-equation approach, we study the dynamics of electrons and phonons in current-carrying superconducting nanostrips after the absorption of a single photon of the near-infrared or optical range. We find that the larger the $C_e/C_{\mathrm{ph}}{|}_{T_c}$ ratio (where $T_c$ is the critical temperature of a superconductor and $C_e$ and $C_{\mathrm{ph}}$ are specific heat capacities of electrons and phonons, respectively), the larger the portion of the photon’s energy goes to electrons. The electrons become more strongly heated and hence can thermalize faster during the initial stage of hot-spot formation. The thermalization time ${\tau }_{\mathrm{th}}$ can be less than 1 ps for superconductors with $C_e/C_{\mathrm{ph}}{|}_{T_c}\gg 1$ and a small diffusion coefficient of $D\simeq 0.5{\mathrm{cm}}^2/\mathrm{s}$ when thermalization occurs, mainly due to electron-phonon and phonon-electron scattering in a relatively small volume of approximately ${\xi }^{2}d$ ($\xi$ is a superconducting coherence length, while $d<\xi$ is a thickness of the strip). For longer time spans, due to diffusion of hot electrons’ effective temperature inside the hot spot decreases, the size of the hot spot increases, the superconducting state becomes unstable, and the normal domain spreads in the strip at a current larger than the so-called detection current. We find the dependence of the detection current on the photon’s energy, the location of its absorption in the strip, the width of the strip, and the magnetic field, and we compare this dependence with existing experiments. Our results demonstrate that materials with $C_e/C_{\mathrm{ph}}{|}_{T_c}\ll 1$ are bad candidates for single-photon detectors due to a small transfer of the photon’s energy to electronic system and a large ${\tau }_{\mathrm{th}}$. We also predict that even a several-micron-wide dirty superconducting bridge is able to detect a single near-infrared or optical photon if its critical current exceeds 70% of the depairing current and $C_e/C_{\mathrm{ph}}{|}_{T_c}\gtrsim 1$.

Neural networks have proven effective for solving many difficult computational problems, yet implementing complex neural networks in software is computationally expensive. To explore the limits of information processing, it is necessary to implement new hardware platforms with large numbers of neurons, each with a large number of connections to other neurons. Here we propose a hybrid semiconductor-superconductor hardware platform for the implementation of neural networks and large-scale neuromorphic computing. The platform combines semiconducting few-photon light-emitting diodes with superconducting-nanowire single-photon detectors to behave as spiking neurons. These processing units are connected via a network of optical waveguides, and variable weights of connection can be implemented using several approaches. The use of light as a signaling mechanism overcomes fanout and parasitic constraints on electrical signals while simultaneously introducing physical degrees of freedom which can be employed for computation. The use of supercurrents achieves the low power density ($1\mathrm{mW}/{\mathrm{cm}}^2$ at 20-MHz firing rate) necessary to scale to systems with enormous entropy. Estimates comparing the proposed hardware platform to a human brain show that with the same number of neurons ($10^{11}$) and 700 independent connections per neuron, the hardware presented here may achieve an order of magnitude improvement in synaptic events per second per watt.

In this article, we devise and demonstrate experimentally a polarization-dependent diffractive bifocal vortex lens operating via the Pancharatnam-Berry phase. The interaction between the incident beam and the bifocal vortex lens establishes nonseparable spin and orbital angular-momentum photon states. The components of the nonseparable state associated with different couplings of spin and orbital angular momentum can be sorted by the bifocality of the lens. A theoretical model of the device is developed using Fresnel’s diffraction. The device is simply, efficiently, and economically realized from the optical setup using the underlying physics of Pancharatnam-Berry-phase polarization holography. The measured transmittance and diffraction efficiency of the fabricated device is high—up to 90% and 91%, respectively. Various applications of the polarization bifocal vortex lens in the field of orbital angular-momentum lasing and optical manipulation are discussed. Thus, the bifocal vortex lens can have significant impact on classical and quantum optics, as well as theoretical physics.

The modes of silicon meta-atoms are investigated, motivated by their use as building blocks of Huygens metasurfaces. A model based on these modes is presented, giving a clear physical explanation of all features in the extinction spectrum. Counterintuitively, this model can show negative contributions to extinction, which are shown to arise from the interference between nonorthogonal modes. The direct and interference contributions to extinction are determined, showing that conservation of energy is preserved. The Huygens condition of matched electric- and magnetic-dipole moments leads to strong forward scattering and suppressed backscattering. It is shown that higher-order modes with appropriate symmetry generalize this condition, leading to multiple bands of directional scattering. The presented results are obtained using a robust approach to find the modes of nanophotonic scatterers, commonly referred to as quasinormal modes. By utilizing an integral formulation of Maxwell’s equations, this work avoids the problem of normalizing diverging far fields, which other approaches require. The model and presented results are implemented in open-source code.

Here, we investigate the mechanism of a random-lasing-based sensor which shows $p\text{H}$ sensitivity exceeding by 2 orders of magnitude that of a conventional fluorescence sensor. We explain the sensing mechanism as related to gain modifications and lasing-threshold nonlinearities. A dispersive diffusive lasing theory matches the experimental results well, and it allows us to predict the optimal sensing conditions and a maximal sensitivity as large as 200 times that of an identical fluorescence-based sensor. The lack of complex alignment and the high sensitivity make this mechanism promising for future biosensing applications.

Based on the impedance-matching condition, we propose a few configurations that can greatly enhance radiation power transfer from the emitter to the photovoltaic (PV) cell for a near-field-based thermophotovoltaic system. In addition to the emitter and the PV cell, these configurations involve the use of additional materials that support resonant modes, such as a metallic material whose dielectric function can be described by a Drude model, or a dielectric material whose dielectric function can be approximated by a Lorentz oscillator model. We show that, by coating the PV cell on both the front and back sides with Lorentz materials, the transferred power can be 2.5 times larger than that without any decorations. When Drude metals are included in the configuration, the optimal transferred power can be 3 times larger than the system without additional materials. We find that the key to enhancing transmitted power is to place a thin layer of Drude or Lorentz material on the front side (facing the emitter) of the PV cell.

Inspired by moth eyes, nature’s most powerful antireflex, we present a subwavelength SiC micropyramid design, which operates in the reststrahlen band of SiC, namely, the spectral band of strong phonon-photon coupling in the SiC material. While within this band, SiC repels electromagnetic waves, we observe here a broad low-reflectivity window with unique attributes, with distinct characteristics different from typical dielectric moth-eye-like structures. To be specific, while the latter systems are entirely symmetric, the reflection response of our SiC micropyramid system can be highly asymmetric. In particular, the SiC micropyramid system can be near reflectionless for light impinging from the tip side of the micropyramids and can exhibit more than 90% reflection for light impinging from the base side of the micropyramids, over a broad wavelength range in the SiC reststrahlen band. This strongly asymmetric reflection response emanates from the cascaded coupling of vortexlike cavity modes at each of the SiC blocks comprising the micropyramids and translates into a strongly unidirectional absorber response. We discuss how, by virtue of Kirchhoff’s law, this strongly unidirectional superabsorber behavior implies a strongly unidirectional emission profile that is important for one-way infrared sources and passive-radiative-cooling systems.

Single-photon sources are an integral part of various quantum technologies, and solid-state quantum emitters at room temperature appear to be a promising implementation. We couple the fluorescence of individual silicon-vacancy centers in nanodiamonds to a tunable optical microcavity to demonstrate a single-photon source with high efficiency, increased emission rate, and improved spectral purity compared to the intrinsic emitter properties. We use a fiber-based microcavity with a mode volume as small as $3.4{\lambda }^3$ and a quality factor of $1.9\times 10^4$ and observe an effective Purcell factor of up to 9.2. Furthermore, we study modifications of the internal rate dynamics and propose a rate model that closely agrees with the measurements. We observe lifetime changes of up to 31%, limited by the finite quantum efficiency of the emitters studied here. With improved materials, our achieved parameters predict single-photon rates beyond 1 GHz.

The magnetic tunnel junction (MTJ) is one of the most important building blocks of spintronic logic and memory components for beyond-CMOS computation and communication. Although switching of MTJs without magnetic field has been achieved by charge and spin current injection, the operation speed is limited fundamentally by the spin-precession time to many picoseconds. We report the demonstration of ultrafast all-optical switching of an MTJ using single subpicosecond infrared laser pulses. This optically switchable MTJ uses ferrimagnetic Gd(Fe,Co) as the free layer and its switching is read out by measuring its tunneling magnetoresistance with a $\mathrm{\Delta }R/R$ ratio of 0.6%. A switching repetition rate at MHz has been demonstrated, but the fundamental upper limit should be higher than tens of GHz rate. This result represents an important step toward integrated optospintronic devices that combines spintronics and photonics technologies to enable ultrafast conversion between fundamental information carriers of electron spins and photons.

We present a model study of an alternative implementation of a two-port Hall-effect microwave gyrator. Our setup involves three electrodes, one of which acts as a common ground for the others. Based on the capacitive-coupling model of Viola and DiVincenzo, we analyze the performance of the device and we predict that ideal gyration can be achieved at specific frequencies. Interestingly, the impedance of the three-terminal gyrator can be made arbitrarily small for certain coupling strengths, so that no auxiliary impedance matching is required. Although the bandwidth of the device shrinks as the impedance decreases, it can be improved by reducing the magnetic field; it can be realistically increased up to 150 MHz at $50\mathrm{\Omega }$ by working at the filling factor $\nu =10$. We also examine the effects of the parasitic capacitive coupling between electrodes and we find that, although, in general, they strongly influence the response of device, their effect is negligible at low impedance. Finally, we analyze an interferometric implementation of a circulator, which incorporates the gyrator in a Mach-Zender–like construction. Perfect circulation in both directions can be achieved, depending on frequency and on the details of the interferometer.

Whispering-gallery mode (WGM) resonators are an important platform for linear, nonlinear, and quantum optical experiments. In such experiments, independent control of in-coupling and out-coupling rates to different modes can lead to higher conversion efficiencies and greater flexibility in the generation of nonclassical states based on parametric down-conversion. In this work, we introduce a scheme that enables selective out-coupling of WGMs belonging to a specific polarization family, while the orthogonally polarized modes remain largely unperturbed. Our technique utilizes material birefringence in both the resonator and the coupler such that a negative (positive) birefringence allows for polarization-selective coupling to TE (TM) WGMs. We formulate a refined coupling condition suitable for describing the case where the refractive indices of the resonator and the coupler are almost the same, from which we derive a criterion for polarization-selective coupling. Finally, we experimentally demonstrate our proposed method using a lithium niobate disk resonator coupled to a lithium niobate prism, where we show a 22-dB suppression of coupling to TM modes relative to TE modes.

We report on the design and implementation of a field-programmable Josephson amplifier (FPJA)—a compact and lossless superconducting circuit that can be programmed in situ by a set of microwave drives to perform reciprocal and nonreciprocal frequency conversion and amplification. In this work, we demonstrate four modes of operation: frequency conversion (transmission of $-0.5$ dB, reflection of $-30$ dB), circulation (transmission of $-0.5$ dB, reflection of $-30$ dB, isolation of 30 dB), phase-preserving amplification (gain $>20\mathrm{dB}$, one photon of added noise) and directional phase-preserving amplification (reflection of $-10$ dB, forward gain of 18 dB, reverse isolation of 8 dB, one photon of added noise). The system exhibits quantitative agreement with the theoretical prediction. Based on a gradiometric superconducting quantum-interference device with $\mathrm{Nb}/\mathrm{Al}\text{−}\mathrm{Al}{\mathrm{O}}_x/\mathrm{Nb}$ Josephson junctions, the FPJA is first-order insensitive to flux noise and can be operated without magnetic shielding at low temperature. Owing to its flexible design and compatibility with existing superconducting fabrication techniques, the FPJA offers a straightforward route toward on-chip integration with superconducting quantum circuits such as qubits and microwave optomechanical systems.

Chip-scale optical microresonators with integrated planar optical waveguides are useful building blocks for linear, nonlinear, and quantum-optical photonic devices alike. Loss reduction through improving fabrication processes results in several integrated microresonator platforms attaining quality ($Q$) factors of several millions. Beyond the improvement of the quality factor, the ability to operate the microresonator with high coupling ideality in the overcoupled regime is of central importance. In this regime, the dominant source of loss constitutes the coupling to a single desired output channel, which is particularly important not only for quantum-optical applications such as the generation of squeezed light and correlated photon pairs but also for linear and nonlinear photonics. However, to date, the coupling ideality in integrated photonic microresonators is not well understood, in particular, design-dependent losses and their impact on the regime of high ideality. Here we investigate design-dependent parasitic losses described by the coupling ideality of the commonly employed microresonator design consisting of a microring-resonator waveguide side coupled to a straight bus waveguide, a system which is not properly described by the conventional input-output theory of open systems due to the presence of higher-order modes. By systematic characterization of multimode high-$Q$ silicon nitride microresonator devices, we show that this design can suffer from low coupling ideality. By performing 3D simulations, we identify the coupling to higher-order bus waveguide modes as the dominant origin of parasitic losses which lead to the low coupling ideality. Using suitably designed bus waveguides, parasitic losses are mitigated with a nearly unity ideality and strong overcoupling (i.e., a ratio of external coupling to internal resonator loss rate $>9$) are demonstrated. Moreover, we find that different resonator modes can exchange power through the coupler, which, therefore, constitutes a mechanism that induces modal coupling, a phenomenon known to distort resonator dispersion properties. Our results demonstrate the potential for significant performance improvements of integrated planar microresonators for applications in quantum optics and nonlinear photonics achievable by optimized coupler designs.

We report on the implementation of quantum frequency conversion between infrared and ultraviolet (UV) wavelengths by using single-stage up-conversion in a periodically poled potassium-titanyl-phosphate waveguide. Because of the monolithic waveguide design, we manage to transfer a telecommunication-band input photon to the wavelength of the ionic dipole transition of ${\mathrm{Yb}}^{+}$ at 369.5 nm. The external (internal) conversion efficiency is around 5% (10%). The high-energy pump used in this converter introduces a spontaneous parametric down-conversion process, which is a cause for noise in the UV mode. Using this process, we show that the converter preserves nonclassical correlations in the up-conversion process, rendering this miniaturized interface a source for quantum states of light in the UV.

Acoustic-wave devices provide a promising chip-scale platform for efficiently coupling radio frequency (rf) and optical fields. Here, we use an integrated piezo-optomechanical circuit platform that exploits both the piezoelectric and photoelastic coupling mechanisms to link 2.4-GHz rf waves to 194-THz (1550 nm) optical waves, through coupling to propagating and localized 2.4-GHz acoustic waves. We demonstrate acousto-optic modulation, resonant in both the optical and mechanical domains, in which waveforms encoded on the rf carrier are mapped to the optical field. We also show optoacoustic gating, in which the application of modulated optical pulses interferometrically gates the transmission of propagating acoustic pulses. The time-domain characteristics of this system under both pulsed rf and pulsed optical excitation are considered in the context of the different physical pathways involved in driving the acoustic waves, and modeled through the coupled mode equations of cavity optomechanics.

Both polycrystalline and single-crystal films of iron-substituted strontium titanate, $\mathrm{Sr}({\mathrm{Ti}}_{0.65}{\mathrm{Fe}}_{0.35}){\mathrm{O}}_{3-\delta }$, prepared by pulsed laser deposition, show room-temperature magnetism and Faraday rotation, with the polycrystalline films exhibiting higher saturation magnetization and Faraday rotation. The magnetic properties vary with the oxygen pressure at which the films are grown, showing a maximum at pressures of approximately $4\mu \mathrm{Torr}$ at which the unit-cell volume is largest. The results are discussed in terms of the oxygen stoichiometry and corresponding Fe valence states, the structure and strain state, and the presence of small-volume fractions of metallic Fe in single-crystal films grown at the optimum deposition pressure. Integration of magneto-optical polycrystalline films on an optical-waveguide device demonstrates a nonreciprocal phase shift.

We report the generation of twin beams by the parametric amplification four-wave mixing process and triplet beams by the parametric amplification six-wave mixing (PA SWM) process associated with the multiorder fluorescence signals in a ${\mathrm{Pr}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal. The intensity noise correlation and intensity-difference squeezing result from the nonlinear gain, which can be well controlled by the polarized dressing effect. The correlation value at the resonant position increases due to the double dressing effect; however, such correlation decreases if the triple dressing effect works. Specifically, correlation and squeezing between Stokes and anti-Stokes signals can be also switched by the relative nonlinear phase shift. The generated triplet beams from the PA SWM process have potential applications in three-mode all-optical information processing that can be used in on-chip photonic devices.