Multimode entanglement is an essential resource for quantum information in continuous-variable systems. Light-based quantum technologies will arguably not be built upon table-top bulk setups, but will presumably rather resort to integrated optics. Sequential bulk opticslike proposals based on cascaded integrated interferometers are not scalable with the current state-of-the-art low-loss materials used for continuous variables. We analyze the multimode continuous-variable entanglement capabilities of a compact currently available integrated bulk-optics analog: the array of nonlinear waveguides. We theoretically demonstrate that this simple and compact structure, together with a reconfigurable input pump distribution and multimode coherent detection of the output modes, is a versatile entanglement synthesizer in the spatial domain. We exhibit this versatility through analytical and numerically optimized multimode squeezing, entanglement, and cluster-state generation in different encodings. Our results re-establish spatial encoding as a contender in the game of continuous-variable quantum information processing.

Resonant excitation of solid-state quantum emitters enables coherent control of quantum states and generation of coherent single photons, which are required for scalable quantum-photonics applications. However, these systems can often decay to one or more intermediate dark states or spectrally jump, resulting in the lack of photons on resonance. Here we present an optical co-excitation scheme that uses a weak nonresonant laser to reduce transitions to a dark state and amplify the photoluminescence from quantum emitters in hexagonal boron nitride (h-BN). Using a two-laser repumping scheme, we achieve optically stable resonance fluorescence of h-BN emitters and an overall increase of on time of an order of magnitude compared with only resonant excitation. Our results are important for the deployment of atomlike defects in h-BN as reliable building blocks for quantum-photonics applications.

Non-Hermitian systems exploiting the synergy between the properties of closed-conservative systems and open-dissipative (gain-loss) systems have recently become the playground to uncover unusual physical phenomena. Indeed, the spatial symmetry breaking in such systems allows tailoring of the wave propagation at will. Inspired by such a property, we propose a feasible approach based on local Hilbert transform to control the field flows in two- or higher-dimensional non-Hermitian systems. Moreover, we invent an iterative procedure to reduce the dimensionality of complex refractive-index parameter space to two, one, or zero dimensions, restricting the complex refractive index within practical limits. The proposal provides a flexible way to systematically design locally non-Hermitian systems realizable with a limited collection of realistic materials.

Fano resonance has attracted great attention in nanophotonics attributed to its unique properties. In this work, we study the imaging function of a Fano-resonant silicon metasurface governed by quasi--bound states in the continuum (quasi-BICs). First, by breaking the in-plane symmetry of nanodisks, a symmetry-protected quasi-BIC is excited with the emergence of a sharp Fano resonance. The near-field distributions, multipole contributions, and radiation patterns of the metasurface are investigated to uncover the mechanism and characteristics of this resonance. In addition, we investigate the imaging function of this Fano-resonant metasurface assisted by phase-change material ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ (GST). Through selective modification of different units from $a$-GST to $c$-GST, the produced transmitted image well reconstructs the target letter. Our findings may provide a route to achieve efficient metasurface-based imaging and fast spatial modulations.

Vector vortex beams are a class of optical beams with singularities in their space-variant polarization. Vector vortex beam lasers have applications in many areas including imaging and communication, where vertical-cavity lasers emitting Gaussian beams have been most widely used so far. Generation of vector vortex beams from vertical-cavity lasers has required external control or modulation. Here, by utilizing a polarization-selective subwavelength grating as one of the reflectors in a vertical semiconductor microcavity, we design the spin textures of the polariton mode and demonstrate polariton lasing in a single-mode, radially polarized vector vortex beam. Polarization and phase distributions of the emission are characterized by polarization-resolved imaging and interferometry. This method of vector vortex laser beam generation allows low threshold power, stable single-mode operation, scalability, and on-chip integration, all of which are important for applications in imaging and communication.

We present an analysis of wave mixing in the recently developed Josephson traveling-wave parametric amplifier (JTWPA). Circuit simulations performed using WRSPICE show the full behavior of the JTWPA, allowing propagation of all tones. The coupled mode equations (CMEs) containing only pump, signal, and idler propagation are shown to be insufficient to completely capture complex mixing behavior in the JTWPA. Extension of the CMEs through additional state vectors in the analytic solutions allows closer agreement with WRSPICE. We consider an ordered framework for the systematic inclusion of extended eigenmodes and make a comparison with WRSPICE at each step. The agreement between the two methods validates both approaches and provides insight into the operation of the JTWPA.

Graphene plasmons provide a compelling avenue toward chip-scale dynamic tuning of infrared light. Dynamic tunability emerges through controlled alterations in the optical properties of the system defining graphene’s plasmonic dispersion. Typically, electrostatic induced alterations of the carrier concentration in graphene working in conjunction with mobility have been considered the primary factors dictating plasmonic tunability. We find here that the surrounding dielectric environment also plays a primary role, dictating not just the energy of the graphene plasmon but so too the magnitude of its tuning and spectral width. To arrive at this conclusion, poles in the imaginary component of the reflection coefficient are used to efficiently survey the effect of the surrounding dielectric on the tuning of the graphene plasmon. By investigating several common polar materials, optical phonons (i.e., the Reststrahlen band) of the dielectric substrate are shown to appreciably affect not only the plasmon’s spectral location but its tunability, and its resonance shape as well. In particular, tunability is maximized when the resonances are spectrally distant from the Reststrahlen band, whereas sharp resonances (i.e., high-$Q$) are achievable at the band’s edge. These observations both underscore the necessity of viewing the dielectric environment in aggregate when considering the plasmonic response derived from two-dimensional materials and provide heuristics to design dynamically tunable graphene-based infrared devices.

Metaphotonics, combining metamaterial (MM) and nanophotonic concepts, offers a unique platform for obtaining an unusual and/or advantageous electromagnetic response on a scale much less than the wavelength, although several practical applications are strongly hampered by the complexity of fabrication of MM devices and the intrinsic subwavelength inhomogeneous response of the MM composite. Recently, much research effort has been focused on the study of photonic devices for magnetic resonance imaging (MRI), one of the cornerstone diagnostics techniques in life science. In the MRI context, we introduce the use of magnetic surface excitations (magnetic localized surface plasmons) supported by a negative-magnetic-permeability MM sphere that results in a significant local enhancement of the MRI signal-to-noise ratio. Using the Mie resonance theory, we show that an increase in the signal-to-noise ratio boost can be obtained by replacing the MM sphere with a homogeneous high-permittivity one of the same radius. Overcoming some of the main MM limiting factors, our results suggest a simple approach for the realization of radio-frequency magnetic metadevices.

Optical networks implementing single-qudit quantum gates may exhibit superior properties to those for qubits as each optical element in the network can work in parallel on many optical modes simultaneously. We present a class of such networks that implements in a deterministic and efficient way the quantum Fourier transform (QFT) in an arbitrarily high dimension. These networks redistribute the initial quantum state into the orbital angular momentum (OAM) and path degrees of freedom and offer two modes of operation. Either the OAM-only QFT can be implemented, which uses the path as an internal auxiliary degree of freedom, or the path-only QFT is implemented, which uses the OAM as the auxiliary degree of freedom. The resources for both schemes scale linearly $O(d)$ with the dimension $d$ of the system, beating the best known bounds for the path-encoded QFT. While the QFT of the orbital-angular-momentum states of single photons has been applied in a multitude of experiments, these schemes require specially designed elements with nontrivial phase profiles. In contrast, we propose a different approach that utilizes only conventional optical elements.

Characterizing the radiated or received acoustic field of ultrasonic transducers using the spatial impulse response (SIR) represents an important step in testing, design, and optimization of ultrasonic transducers. However, for the ultrahigh-frequency acoustic field, conventional methods, such as the hydrophone method and the small-ball reflection method, are limited by narrow bandwidth and poor spatial resolution. Here, we propose a method to obtain the transducer’s SIR through its response to photoacoustic waves, which allows high-precision acoustic field measurements, with spatial resolution as fine as 1.7 µm. We subsequently measure the SIRs of two focused ultrasonic transducers, with 20 and 93 MHz center frequencies, and confirm that the three-dimensional acoustic fields can be accurately reconstructed using the angular spectrum approach. More importantly, this method is unique to receive-only ultrasonic detectors, the SIR of which could not be measured previously with conventional methods, and it could facilitate ultrasonic transducer design, as well as other related fields, such as nondestructive evaluation, biomedical imaging, and particle manipulation.

Nanostructured and bulk silicon carbide ($\mathrm{Si}\mathrm{C}$) materials are relevant for electronics, nano- and micromechanical systems, and biosensing applications. $\mathrm{Si}\mathrm{C}$ has recently emerged as an alternative platform for nanophotonics and quantum applications due to its intra-band-gap point defects, emitting from the visible to the near-infrared, which are ideal for photoluminescent probes. Here, we use a single-molecule localization microscope to study the photoluminescence (PL) properties of $\mathrm{Si}\mathrm{C}$ point defects in bulk, quantum dots, and nanoparticles of different sizes in the 3C and $4H\text{−}\mathrm{Si}\mathrm{C}$ polytypes using intra-band-gap excitation. We study the PL dynamics by using different excitation wavelengths, and we use the point-defect PL intermittency to achieve superresolved images, with a resolution of 20 nm and a minimum distance between emitters of 40 nm. We observe that, while 561 nm is an ideal excitation wavelength to obtain a sufficient blinking behavior, 638 nm is mostly quenching the PL. We further incubate $4H\text{−}\mathrm{Si}\mathrm{C}$ nanoparticles with MCF10A cells in vitro and observe superresolved images of the nanoparticles in cells by combining 561 and 638 nm excitation. This approach is very promising for the application of $\mathrm{Si}\mathrm{C}$ fluorescent nanocrystals and their quantum dots, hosting a combination of intra-band-gap color centers and surface defects, for quantum nanophotonics, magnetic sensing, and biomedical imaging, paving the way for single-particle tracking combined with spin sensing within a cellular environment using near-infrared emission.

We propose a perfect photonic router based on a specially designed chiral bimetasurface membrane for spin-polarized point light sources. Due to the mirror symmetry breaking in the chiral metamembrane, the radiation power flux of the clockwise and counterclockwise spinning dipoles to the opposite sides of the slab becomes different. We show that spinning dipoles in the specially designed chiral $D_4$-symmetrical bimetasurface membrane can emit light either upward or downward depending on their rotation direction. We attribute this phenomenon to the Fano-resonance effect, which is a result of the guided modes coupling with the far field. We show the advantage of $D_4$-symmetrical structures for the achievement of 100% routing efficiency. This phenomenon can find applications in spintronics for spin-selective interchip coupling or as a measurement tool of spin polarization in memory cells.

We analyze the properties of femtosecond-laser-written waveguides in silicon. By using a mix of semianalytical and numerical methods, we find that the transverse index profile is accurately modeled by a W-shaped function supporting nonleaky guided modes. The maximum change in the refractive index is about $4\times {10}^{-3}$, whereas the central lobe is about 2.5 $\mu \mathrm{m}$ wide for a writing wavelength of 1.55 $\mu \mathrm{m}$. Waveguide shape and maximum index variation both saturate for input pulse energies larger than 100 nJ and a pulse duration of 860 fs. We find that the performance of the waveguides are limited by the presence of randomly distributed scattering centers situated in the waveguide core. To confirm our findings, we probed the waveguide response as the input beam is kept normal at the input interface but shifted perpendicularly to the waveguide axis, resulting in a variable offset with respect to the center of the waveguide. The behavior of the energy coupled in the waveguide versus the beam shift is in agreement with theoretical predictions. We also used a simple analytical model to describe the nonlinear propagation of the writing beam.

We demonstrate voltage-tunable thermo-optic bi- and tristability in silicon photonic microring resonators with lateral $p$-$i$-$n$ junctions and present a technique for characterizing the thermo-optic transient response of integrated optical resonators. Our method for thermo-optic transient response measurement is applicable to any integrated photonics platform and uses standard equipment. Thermo-optic relaxation in encapsulated waveguides is found to be approximately logarithmic in time, consistent with the analytical solution for two-dimensional heat diffusion. We develop a model for thermo-optic microring multistability and dynamics that agrees with experimental data over a wide range of operating conditions. Our work highlights the fundamental connection in semiconductor waveguides between active free-carrier removal and thermo-optic heating, a result of particular relevance to Kerr soliton-state stability and on-chip frequency-comb generation. The devices studied here are fabricated in a CMOS foundry process and as a result our model is useful for the design of silicon photonic waveguide devices.

The spectral theory of thermal fields subject to arbitrary boundary surface flux conditions developed in this work establishes a universal approach to non-steady-state (photo)thermal responses of solids under transient or modulated thermal excitation through a combined Fourier-Laplace formalism. The self-consistent evolution from the nonsteady state to the steady state under single frequency or arbitrary pulsed or multifrequency thermal-wave excitation waveforms allows for defining experimental criteria in terms of excitation waveform repetition periods to attain steady state in frequency-modulated photothermal (and other general experimental) systems. This approach is crucial for ensuring accurate material property measurements using lock-in or correlation and spectral analysis demodulation of thermal waves, especially under linear frequency modulation (LFM) when the modulated steady state is time dependent. The theory is validated using experimental photothermal radiometry LFM responses from black anodized aluminum. We also define the associated thermal diffusion length in terms of linear superpositions of partial thermal waves (wavelets), which are shown to result in improved depth, lateral, and axial resolution in thermal-wave radar imaging applications compared with sequential single-modulation frequency detection.

Periodic photonic structures have attracted much interest due to their versatility for controlling light propagation. The dispersion of photon modes in photonic lattices exhibits an energy band structure analogous to the electronic one in crystals. An important consequence of the photonic band structure is the localization of light in a band gap, which can be engineered by breaking the translational symmetry introducing a defect in the lattice. Here, we demonstrate experimentally the localization of light in a two-dimensional periodic system of polaritons confined in a patterned microcavity. We generate a self-trapping of light by optically inducing a local breaking of the strong-coupling regime of excitons to photons. In the photon lasing regime we show the existence of confined modes that can be controlled by the shape of the generated defect. We demonstrate single on-site localization with lasing mode at the edge of the Brillouin zone similar to a gap soliton. Our results pave the way for useful tools to optically control localization and propagation of light towards the generation of microlasers.

Artificial intelligence is currently attracting unprecedented attention for its ability to tackle hard problems with huge datasets, which have been rendered tractable by the giant computational power and amount of training data available today. Photonic inverse design, in which one seeks to find objects of desired electromagnetic response, belongs to this class of complex problems that can greatly benefit from these ideas. In this work, artificial intelligence concepts are applied to advance the quest for invisible particles that do not perturb the background field; in particular, a fully connected neural network is proposed to address such a problem by learning the dynamics of visible-light interaction with low-scattering multilayered nanospheres. By swapping the roles between inputs and outputs, the same network can then be used for the inverse design of invisible nanoparticles, obtaining superior performance with respect to the best elements of the training set. The proposed approach can be generalized to approximate Maxwell interactions by simulating the electromagnetic response of more complicated optical configurations, and accomplish their inverse design directly, without successive iterations.

We present a quantum-optics approach for describing stimulated parametric down-conversion in the two-type-I crystal “sandwich” configuration, which allows for parametric interaction of vector vortex beams. We analyze the conditions for which phase conjugation of the seed vector beam occurs. We then use two strategies for defining generalized Stokes parameters to describe phase conjugation of vector vortex beams. These allow for geometrical representations, such as higher-order Poincaré spheres. Our results are useful for the description and design of stimulated and spontaneous parametric down-conversion experiments with vector vortex beams.

Tunable optical phased arrays and metasurfaces play an important role in a diverse range of applications from imaging and remote sensing to communications and displays. However, as the number of tunable elements grows, the required control architecture becomes insurmountably complex. Here, we discuss the concept of perimeter-controlled tuning to shape far-zone radiation. We discuss applications of our approach to beam forming, holography, and image projection. We show that, with a proper design, the complexity of a control architecture may be dramatically simplified. We further discuss the use of our method to time-sharing image projection and holography. Our concept is applicable to a variety of systems, including phased array optical antennas and metasurfaces.