We implement a whispering-gallery-mode microbubble resonator (MBR) as a multifunctional component for all-optical photoacoustic inspection of materials in a platform combining microfluidic design and high sensitivity to spectral absorbance. In our setup, the MBR acts both as the acoustic transducer and as the vial for a sample causing the photoacoustic event, thus dictating its operational frequency and removing any need for impedance-matching media. We challenge our setup in the assessment of the photostability of gold nanorods to demonstrate its validity. We develop a numerical model to qualitatively interpret the experimental signal as a transient shift of the resonance condition of the MBR. Possible implementations of the MBR in photoacoustic sensors are discussed.

Efficient microresonators simultaneously require a large quality factor $Q$ and a small volume $V$. However, the former is ultimately limited by bending losses, the unavoidable radiation of energy of a wave upon changing direction of propagation. Such bending losses increase exponentially as $V$ decreases and eventually result in a drop of $Q$. Therefore, circular cavities are generally designed with radii that are much larger than the optical wavelength. The same leakage of energy by radiation limits the sharpness of bends in photonic integrated circuits. In this paper, we present a way to reduce bending losses in circular microresonators. The proposed scheme consists of one or more external dielectric rings that are concentric with the cavity. These rings alter the field outside the cavity where radial oscillations set in and thus control the far-field radiation. As a result, the $Q$ factor can be increased by several orders of magnitude while keeping a small cavity volume.

Circularly polarized light can be obtained by using either polarization conversion or structural chirality. Here we reveal a fundamentally unrelated mechanism of generating circularly polarized light using coupled nonequilibrium sources. We show that thermal emission from a compact dimer of subwavelength, anisotropic antennas can be highly circularly polarized when the antennas are at unequal temperatures. Furthermore, the handedness of emitted light is flipped upon interchanging the temperatures of the antennas, thereby enabling reconfigurability of the polarization state lacked by most circularly polarized light sources. We describe the fundamental origin of this mechanism using rigorous fluctuational electrodynamic analysis and further provide practical examples for its experimental implementation. Apart from the technology applications in reconfigurable devices, communication, and sensing, this work motivates new inquiries of angular-momentum-related thermal-radiation phenomena using thermal nonequilibrium, without applying magnetic field.

The spontaneous decay rate of an excited molecule placed near a circular nanoaperture in a metal film of finite thickness and finite conductivity lying on a dielectric substrate (zero-mode waveguide, ZMW) is investigated. A significant influence of the molecule position and the presence of the substrate on the spontaneous emission rate of the molecule is shown. The asymptotes, which can be used to describe this process, are found. Total, radiative, and nonradiative channels of spontaneous decay rates of the excited molecule are extracted and analyzed. It is shown that a special choice of the substrate and the material of metal film leads to the appearance of “leaky” surface plasmon waves and substantial enhancement of the radiative decay rate. The results can be useful in the design of alternative ZMW optical nanodevices for effective detection of single-molecule fluorescence.

An optical vortex carrying orbital angular momentum (OAM) with a fractional topological charge (TC) is a good complement to a traditional integer optical vortex and has many potential applications. However, the fractional optical vortex (FOV), which breaks the orthogonality of OAM, makes the precision measurement of the FOV complicated and difficult when using a conventional method. Thus, we propose an accurate and effective FOV detection method by using a two-dimensional multifocal array consisting of different integer vortices. Theoretically, FOV beams could be decomposed into a Fourier series of integer optical vortex beams, and the average OAM of the FOV can be calculated via the energy of these integer optical vortices, which can be accurately measured by a commercial photoelectric detector. For improving the measurement accuracy, a compensation iteration algorithm is employed to correct the detected average OAM of the FOV. Experimental results show that the error of average OAM detection is less than 0.025. This alternative FOV TC detection method is sufficient for a wide range of real-time requirements in optical manipulation, imaging, and quantum optics.

The nitrogen-vacancy (N-V) center in diamond has been recognized as a high-sensitivity nanometer-scale metrology platform. Thermometry has been a recent focus, with attention largely confined to room-temperature applications. Temperature sensing at low temperatures, however, remains challenging as the sensitivity decreases for many commonly used techniques, which rely on a temperature-dependent frequency shift of the N-V center's spin resonance and its control with microwaves. Here we use an alternative approach that does not require microwaves, ratiometric all-optical thermometry, and demonstrate that it may be utilized to liquid-nitrogen temperatures without deterioration of the sensitivity. The use of an array of nanodiamonds embedded within a portable polydimethylsiloxane sheet provides a versatile temperature-sensing platform that can probe a wide variety of systems without the configurational restrictions needed for applying microwaves. With this device, we observe a temperature gradient over tens of microns in a ferromagnetic-insulator substrate (yttrium iron garnet) under local heating by a resistive heater. This thermometry technique provides a cryogenically compatible, microwave-free, minimally invasive approach capable of probing local temperatures with few restrictions on the substrate materials.

Spoof surface plasmon polaritons (SSPPs) can be converted to leaky-wave radiations by a spoof surface plasmon (SSP) waveguide with periodically modulated surface impedance, which is also known as a SSPP leaky-wave antenna (LWA). However, most of the reported SSPP LWAs can only radiate electromagnetic (em) waves with linear polarizations parallel to the SSP waveguide due to the limitation of perpendicular grooving structures, and the polarizations of LWA radiations are difficult to arbitrarily design. We propose an efficient method to achieve the leaky-wave radiations with arbitrarily customizable polarizations based on an alternative SSPP LWA, which is made of a periodically modulated SSP waveguide with bilateral 45°-tilted grooves to generate two orthogonally polarized em waves. The grooves on both sides of the SSPP LWA can be designed with proper displacement to provide the required phase difference between two orthogonally polarized radiation waves in order to customize the polarization of the leaky-wave radiations. Both simulation and experimental results are provided to validate the feasibility of tailoring arbitrary polarizations for the radiation waves based on the proposed SSPP LWA.

We perform picosecond absorption spectroscopy on $\mathrm{Ba}\mathrm{Br}\mathrm{Cl}$ samples that are undoped, doped with europium to make activated scintillators, and codoped with gold (as ${\mathrm{Au}\mathrm{Br}}_3$ in the melt) to increase light yield. The time-resolved measurements provide insights into the effects of interband excitation in $\mathrm{Ba}\mathrm{Br}\mathrm{Cl}$, including identification of self-trapped-exciton absorption bands, the mechanism of energy transport from the host to the $\mathrm{Eu}$ activator, and the effects of $\mathrm{Au}$ codoping. First-principles electronic structure calculations of self-trapped-exciton relaxations in $\mathrm{Ba}\mathrm{Br}\mathrm{Cl}$ are presented. $\mathrm{Ba}\mathrm{Br}\mathrm{Cl}$:$\mathrm{Eu}$,$\mathrm{Au}$ is a beneficiary of two material modifications for improvement of performance that have been intensely investigated recently: a mixed-halide solid solution that is further modified by aliovalent codoping.

We achieve reflection-type bound states in the continuum (BICs) in the compound grating waveguide structure by tuning the excitation of the guided modes. Assisted by the quasi-BICs with ultrahigh Q factors, the Goos-Hänchen (GH) shift can be greatly enhanced to larger than or equal to four orders of wavelength. In addition, different from the large GH shift based on the Brewster dip or transmission-type resonance, the maximum GH shift assisted by the reflection-type quasi-BIC is located at the reflectance peak with unity reflectance, which can be more easily detected in the experiment. Based on the quasi-BIC-assisted giant GH shift, we propose an ultrasensitive temperature sensor with a minimum resolution in the order of ${10}^{-4}$ °C. Our work provides a route, under the current experimental conditions, to design ultrasensitive sensors, light information storage devices, wavelength division de/multiplexers, optical switches, and polarization beam splitters based on this giant GH shift together with high reflectance.

Metasurfaces have attracted a lot of attention in recent years due to the alternative methods they provide for the efficient wavefront control and engineering of resonant transmission. We discuss an approach that allows the effective control of the appearance of sharp Fano resonances in metasurfaces associated with the bound states in the continuum. We demonstrate that by breaking the symmetry transversely, in the direction perpendicular to a metasurface with a complex unit cell, we can control the number, frequency, and type of high-$Q$ resonances originating from bound states in the continuum. As an example, we demonstrate experimentally the metasurfaces with magnetic dipole and toroidal dipole responses governed by the physics of multipolar bound states.

It has recently been highlighted that nonreciprocal surface waves can be stopped in suitably designed waveguides, leading to the formation of hotspots wherein the electromagnetic fields are massively enhanced. Here, we prove that this phenomenon has a topological origin and is related to a breakdown of the bulk-edge correspondence in electromagnetic continua with no spatial cutoff. Our theoretical analysis shows that nonreciprocal electromagnetic continua can be used to realize energy sinks with ultrasingular fields that absorb all the energy generated in their surroundings. Moreover, we prove that similar energy sinks may be formed in fully reciprocal optical platforms with a parity-time-duality ($\mathcal{P}\mathcal{T}\mathcal{D}$) symmetry.

We study the optical properties of tetravalent-vanadium impurities in $4H$ silicon carbide. Light emission from two crystalline sites is observed at wavelengths of 1.28 and $1.33\mu \mathrm{m}$, with optical lifetimes of 163 and 43 ns, respectively, which remains stable up to 50 and 20 K, respectively. Moreover, spectrally broad photoluminescence is observed up to room temperature. Group-theory and ab initio density-functional supercell calculations enable unequivocal site assignment and shed light on the spectral features of the defects. Specifically, our numerical simulations indicate that the site assignment is reversed with respect to previous assumptions. Our calculations show that vanadium in silicon carbide has highly favorable properties for the generation of single photons in the telecommunication wavelength regime. Combined with the available electronic and nuclear degrees of freedom, vanadium presents all the ingredients required for a highly efficient spin-photon interface.

We construct a theory for the quantum efficiency of radiation in bulk semiconductor crystals as observed in PL measurements. The general form of the external quantum efficiency (EQE), which is directly measured from the PL intensity, is formulated by considering the self-absorption in the sample. For a better understanding of the physical concept behind the EQE, we derive a simple analytical formula for EQE in a weak photoexcitation condition and discuss the relation between EQE and internal quantum efficiency (IQE). We find that, in contrast to IQE, EQE is affected by two kinds of light-extraction efficiencies, ${\eta }_{\mathrm{opt}}$ and ${\eta }_{\mathrm{self}}$. ${\eta }_{\mathrm{opt}}$ gives the fraction of luminescence passing through the sample surface without the optical Fresnel reflection. Meanwhile, ${\eta }_{\mathrm{self}}$ gives the fraction of luminescence reaching the sample surface that overcomes the self-absorption process inside the sample. ${\eta }_{\mathrm{self}}$ strongly depends on the spatial carrier distribution, because of which carrier-diffusion phenomena have a large impact on EQE. We also show an example calculation of the theoretical calibration curve of EQE to determine the concentration of nonradiative recombination centers (NRCs) in semiconductor samples. The NRC concentration estimated from the calibration curve is consistent with the recent omnidirectional photoluminescence measurements, and the sensitivity is high enough to quantify defect concentrations for power-device applications.

Nondiffracting Bessel, Mathieu, and Weber vector beams are generated by using ring-shaped plasmonic geometric metasurfaces. The nondiffracting vector beam is produced by the superposition of two off-axis right-handed and left-handed circularly polarized nondiffracting scalar beams described by the Whittaker integral, which are simultaneously generated by a single metasurface with the ring-shaped phase profile. The polarization states of the generated nondiffracting vector beams are analyzed by the Stokes parameters and the orbital angular momentum states are measured by the beam interference. In addition, the selfhealing properties of nondiffracting vector beams are further demonstrated, showing that not only the beam profiles but also the polarization states can be recovered behind the small opaque obstacle. The demonstrated ring-shaped plasmonic metasurfaces provide a compact and efficient platform to produce complex nondiffracting vector beams and pave the way to many promising applications related to spin and orbital angular momentum conversion, quantum information processing, optical manipulation, and optical communication.

Quantum networks are the ultimate target in quantum communication, where many connected users can share information carried by quantum systems. The keystones of such structures are the reliable generation, transmission, and manipulation of quantum states. Two-dimensional quantum states, qubits, are steadily adopted as information units. However, high-dimensional quantum states, qudits, constitute a richer resource for future quantum networks, exceeding the limitations imposed by the ubiquitous qubits. The generation and manipulation of such $D$-level systems have been improved over the last ten years, but their reliable transmission between remote locations remains the main challenge. Here, we show how a recent air-core fiber supporting orbital angular momentum (OAM) modes can be exploited to faithfully transmit $D$-dimensional states. Four OAM quantum states and their superpositions are created, propagated in a 1.2-km-long fiber and detected with high fidelities. In addition, three quantum-key-distribution protocols are implemented as concrete applications to assert the practicality of our results. This experiment enhances the distribution of high-dimensional quantum states, attesting the orbital angular momentum as a vessel for the future quantum network.

A method to generate spatiotemporally controllable plasma lattice structures via dielectric barrier discharge is proposed by utilizing a latticed water electrode. A variety of plasma lattice structures including triangle, hexagon, honeycomb, and complex superlattices are obtained by changing three parameters including the applied voltage, the gas pressure, and the gas compositions. Moreover, these plasma lattices can be quickly reconstructed, allowing for active control both in space and time. Two-dimensional particle-in-cell simulations are carried out to demonstrate the formation of plasma structures under different voltages, which are in good agreement with experimental observations. Our method provides a unique way for the fabrication of controllable plasma lattice structures, which may enable tunable control of microwave radiation for wide applications.

We report a hybrid multichannel metal-cladded slab-capillary microcavity, which supports simultaneous red, green, and blue lasing from an individual microcavity at room temperature. White and tunable lasing over the full range of visible colors is achieved through controlled injection of different dye solutions into the metal-cladded capillaries of the microcavity. The results show that RGB lasing beams with a line width of approximately 1 nm are observed from the metal-cladded slab-capillary microcavity. The experimental observations show the realization of white light and lasing simultaneously in a single hybrid microcavity. Moreover, the improvement in the laser structure enables the elimination of background noise. Our work describes a simplification in fabricating microlasers with dynamically color-controllable emissions and provides an important route toward the potential of realizing a microlaser device generating both white and full color light directly from a single monolithic structure.

Universal unitary photonic devices can apply arbitrary unitary transformations to a vector of input modes and provide a promising hardware platform for fast and energy-efficient machine learning using light. We simulate the gradient-based optimization of random unitary matrices on universal photonic devices composed of imperfect tunable interferometers. If device components are initialized uniform randomly, the locally interacting nature of the mesh components biases the optimization search space toward banded unitary matrices, limiting convergence to random unitary matrices. We detail a procedure for initializing the device by sampling from the distribution of random unitary matrices and show that this greatly improves convergence speed. We also explore mesh architecture improvements such as adding extra tunable beam splitters or permuting waveguide layers to further improve the training speed and scalability of these devices.

There has been recent interest in neuromorphic photonics, a field with the promise to access pivotal and unexplored regimes of machine intelligence. Progress has been made on isolated neurons and analog interconnects; nevertheless, this renewal of interest has yet to produce a demonstration of a silicon photonic neuron capable of interacting with other like neurons. We report a modulator-class photonic neuron fabricated in a conventional silicon photonic process line. We demonstrate the behaviors of transfer-function configurability, fan-in, inhibition, time-resolved pulse processing, and, crucially, autaptic cascadability—a sufficient set of behaviors for a device to act as a neuron participating in a network of like neurons. The silicon photonic modulator neuron constitutes the final piece needed to make photonic neural networks fully integrated on currently available silicon photonic platforms.

Optical nanofiber waveguides are widely used for near-field delivery and measurement of light. Despite their versatility and efficiency, nanofibers have a critical drawback—their inability to maintain light’s polarization state on propagation. Here, we design a directional coupler consisting of two crossed nanofibers to probe the polarization state at the waist region. Directionality of coupling occurs due to asymmetric dipolar emission or spin locking when the evanescent field pattern breaks the mirror symmetry of the crossed-nanofiber system. We demonstrate that, by monitoring the outputs from the directional coupler, two nonorthogonal polarization states can be prepared at the nanofiber waist with a fidelity higher than 99%. Based on these states, we devise a simple and reliable method for complete control of the polarization along a nanofiber waveguide.