Flat optics has demonstrated great advances in miniaturizing conventional bulky optical elements due to recent developments in metasurface design. Specific applications of such designs include spatial differentiation and the compression of free-space. However, metasurfaces designed for such applications are often polarization-dependent and are designed for a single functionality. In this work, we introduce a polarization-independent metasurface structure by designing guided resonances with degenerate band curvatures in a photonic crystal slab. Our device can perform both free-space compression and spatial differentiation when operated at different frequencies at normal incidence. This work demonstrates the promise of dispersion engineering in metasurface design to create ultrathin devices with polarization-independent functionality.

Phase-gradient metagratings have exhibited unconventional abilities to manipulate electromagnetic waves owing to functionality inherited from metasurface and different physics. In this paper, by integrating homogeneous dielectric materials with gradient thickness into periodical metallic slits, a passive phased metagrating based on mode conversion is designed in which each unit provides nearly perfect broadband transmission. To validate the effectiveness of our scheme, anomalous scattering and planar metalenses with excellent efficiency in a finite band are designed theoretically and proved numerically. Good performance makes the implementation of other metasurface functions possible, providing a platform for functions from holography to caustic engineering. More interestingly, the high fluidity of this structure promises potential application in other physical fields, such as acoustics and water waves.

The height dependence of Raman light scattering on organic molecules deposited onto thick one-dimensional metal-dielectric gratings is investigated theoretically and experimentally. We observe oscillations of the intensity of Raman light scattering as a function of the height of the metastructure’s strips and demonstrate that these oscillations are explicitly associated with a type of Fabry-Perot effect. We show that the intensity of surface-enhanced Raman scattering can be additionally increased by local field enhancement (by an order of magnitude) at resonances of both pump and Stokes as well as anti-Stokes frequency, due to coupling of the surface plasmon polaritons on the top and bottom metal parts of the gratings via the vertical Fabry-Perot resonances over their middle dielectric part. A semianalytical one-mode model to describe the effect qualitatively is proposed. A significant deviation of the dispersion of Fabry-Perot-coupled plasmon polaritons from the surface-plasmon-polariton dispersion just folded into the first Brillouin zone is demonstrated.

We reveal that a synthetic photonic lattice based on coupled optical loops can be utilized as a feed-forward neural network for processing of optical pulse sequences in time domain. As a proof of concept, we train the optical system to restore an initial shape of the pulse train from the signal distorted due to linear dispersion in a fiber-optic link. We also show efficient training of the optical network with an intrinsic Kerr-type nonlinearity for the realization of target nonlinear transmission functions and inference functionality for the discrimination of different pulse sequences. The theoretical modeling is performed under practical conditions and can guide future experimental realizations.

Few-layer metasurfaces, which are planar artificial arrays composed of more than one functional layer, have been showing unprecedented capabilities for the implementation of integrated and miniaturized optical devices with high efficiency and broad working bandwidth. However, the rich design freedoms of few-layer metasurfaces severely challenge their design and optimization. A universal strategy for the design of few-layer metasurfaces with different desired optical functionalities and an arbitrary number of layers, which can lower the design complexity and the time cost for structural optimization, is still eagerly anticipated by the scientific community. Here, we demonstrate an inverse design strategy based on deep-learning technology for the design of few-layer metasurfaces. By combining the matrix theory of multilayer optics, the proposed algorithm can predict the entire scattering matrix of a few-layer metasurface in tens of seconds with an acceptable accuracy and realize the inverse design of few-layer metasurfaces with different desired functionalities. Thus, the proposed inverse design strategy provides an efficient solution for the reduction of the design complexity of few-layer metasurfaces and significantly lowers the time cost for the structural optimization when compared with the numerical simulation methods based on an iterative process of trial and error, which will be of benefit to and expand the related research.

When midinfrared light interacts with nanoscale polar dielectric structures, optical phonon propagation cannot be ignored, leading to a rich nonlocal phenomenology that we have only recently started to uncover. In properly crafted nanodevices this includes the creation of polaritonic excitations with hybrid longitudinal-transverse nature, which are predicted to allow energy funneling from longitudinal electrical currents to far-field transverse radiation. In this work we study the physics of these longitudinal-transverse polaritons in a dielectric nanolayer in which the nonlocality strongly couples epsilon-near-zero modes to longitudinal phonons. After having calculated the system’s spectrum solving Maxwell’s equations, we develop an analytical polaritonic theory able to transparently quantify the nonlocality-mediated coupling as a function of the system parameters. Such a theory provides a powerful tool for the study of longitudinal-transverse polariton interactions and we use it to determine the conditions required for the hybrid modes to appear.

Periodic arrays of nanoholes perforated in metallic thin films interact strongly with light and produce large electromagnetic near-field enhancements in their vicinity. As a result, the optical response of these systems is very sensitive to changes in their dielectric environment, thus making them an exceptional platform for the development of compact optical sensors. Given that these systems already operate at the shot-noise limit when used as optical sensors, their sensing capabilities can be enhanced beyond this limit by probing them with quantum light, such as squeezed or entangled states. Motivated by this goal, here, we present a comparative theoretical analysis of the quantum enhanced sensing capabilities of metallic nanohole arrays with one and two holes per unit cell. Through a detailed investigation of their optical response, we find that the two-hole array supports resonances that are narrower and stronger than its one-hole counterpart, and therefore have a higher fundamental sensitivity limit as defined by the quantum Cramér-Rao bound. We validate the optical response of the analyzed arrays with experimental measurements of the reflectance of representative samples. The results of this work advance our understanding of the optical response of these systems and pave the way for developing sensing platforms capable of taking full advantage of the resources offered by quantum states of light.

In this paper we study the electromagnetic scattering, absorption, and performance bounds for short time modulated pulses that impinge on a time-varying lossy layer that is sandwiched between vacuum and a perfect electric conductor. The electric characteristics of the layer, namely, the conductivity, permittivity, and permeability, are assumed to change abruptly or gradually in time. We demonstrate numerically that a time-varying absorbing layer that undergoes temporal switching of its permittivity and conductance can absorb the power of a modulated, ultrawideband, as well as a quasimonochromatic, pulsed wave beyond what is dictated by the time-invariant Rozanov bound when integrating over the whole frequency spectrum. We suggest and simulate a practical metamaterial realization that is constructed as a three-dimensional array of resistor loaded dipoles. By switching only the dipole’s load resistance, desired effective media properties are obtained. Furthermore, we show that Rozanov’s bound can be bypassed with abrupt and a more practical gradual, soft, switching, thus overcoming some possible causality issues in abrupt switching.

Except for a few cases to which sophisticated techniques are applicable, it is difficult to observe experimentally local electric fields or near fields associated with excitation of electromagnetic modes supported by building blocks of nanostructures. Although elucidating the behavior of the local electric fields or near fields is very useful to develop potential applications of the nanostructures, in many cases, their behaviors have been only speculated from simulations based on the electromagnetic theory. Here, we develop a simple technique that allows us to study experimentally the average behavior of the local electric fields inside two planar waveguide layers interacting with each other in all-dielectric multilayer structures. The study is made possible by doping the two waveguide layers with two different fluorophores. Fluorescence intensities detected at two different wavelengths allow us to study separately the behaviors of the local electric fields inside each waveguide layer. Angle-scan fluorescence excitation spectra are measured for two different series of multilayer samples by exciting the samples in an attenuated total reflection geometry. The line shapes observed are in good agreement with theoretical ones calculated based on the electromagnetic theory, confirming that the experimental fluorescence excitation spectra reflect well the average behaviors of the local electric fields. Applying the developed technique, we investigate systematically the changes in the local electric fields inside the waveguide layers caused by the change in the strength of coupling between the two waveguide modes. In one series of the samples, a gradual transition in the behavior of the local electric fields, from a Fano-resonant behavior to a double-peak behavior, is demonstrated with increasing the coupling strength. In another series of the samples, the coupling strength is widely changed and the formation of the coupled modes in a strong coupling regime is clearly observed. The observed changes in the local electric fields can be reproduced very well by simple analytical calculations based on a coupled oscillator model. The model is helpful to gain physical insights into the coupling of the waveguide modes, which cannot be obtained by the electromagnetic calculations.

Based on the self-consistent calculations of the eight-band $\mathbf{k}\cdot \mathbf{p}$ model and Poisson equation, we theoretically demonstrate that the intersubband optical absorption of $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ type-II quantum wells (QWs) is effectively tuned by the electric-field-driven band-inversion transition. By utilizing this property, $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ QWs can be made into highly efficient electroabsorption modulators (EAMs) for TM-polarized mid-infrared radiation (MIR) (approximately $4.2$–$12\mu \mathrm{m}$) and TE-polarized far-infrared radiation (FIR) (approximately $12$–$100\mu \mathrm{m}$). At low temperature, an example $p$-$i$-$n$ waveguide EAM composed of six-period $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ QWs achieves a drive voltage of $<0.5$ V, a 3-dB cutoff bandwidth of approximately $24.6$ GHz, and a dynamic power consumption of $\le 0.06$ mW/GHz for a 20-dB on:off ratio modulation of $4.46$-$\mu \mathrm{m}$ TM-polarized radiation. Although the modulation efficiencies drop by 70%–80% when the temperature increases, the performance of $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ EAMs is (at least) comparable with conventional $1.55$-$\mu \mathrm{m}$ EAMs at 300 K. These results suggest that $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ EAMs are promising devices for high-speed and low-power-consumption MIR (or FIR) free-space optical communications.

Quantum key distribution (QKD), which enables the secure distribution of symmetric keys between two legitimate parties, is highly useful in future network security. An access network that connects multiple end users with one network backbone can be combined with QKD to build security for end users in a scalable and cost-effective way. In this paper, we show that a downstream-access network can be implemented using continuous-variable (CV) QKD. The security analysis is conducted independently with other network nodes, which greatly relax the trustfulness requirement on the network nodes. The security is then obtained against other network parties even when the quantum signals are broadcast throughout the network. The passive beam splitter is sufficient to deterministically distribute the quantum signals, and standard CV QKD systems can be directly fitted in the downstream-access network, which makes it more applicable for practical implementations. Numerous simulation results are provided to demonstrate the performance of the CV QKD downstream-access network, where up to 64 end users are shown to be feasible to access the network. We provide the security-analysis framework for realizing CV QKD in the downstream-access network, which will boost the diversity for constructing practical CV QKD networks.

We study the impact of experimental imperfections in integrated optical circuits. We discuss the emergence of a moderate biased error in path encoding, and investigate its correlation with properties of the optical paths. Our analysis connects and deepens previous studies in this direction, revealing potential issues in high-precision tests and optical implementations of machine learning.

The canonical way to sustain photonic topological edge states relies on the interface formed by topologically trivial and nontrivial photonic crystals that share a common band gap to prevent the coupling of edge states from the radiation continuum of free space. Herein, we report a scheme to achieve bound topological edge states in the continuum at the air edge of a single topologically nontrivial photonic crystal utilizing the physics of bound states in the continuum. These localized states, though existing in the continuum, exhibit topological features inherited from the nontrivial topology of bulk photonic bands, such as pseudospin-momentum locking unidirectional propagation. Microwave experiments are provided to further validate the theoretical predictions. Our work not only has practical importance but also opens an avenue where the nontrivial topology of bulk photonic bands could be exploited to enrich the physics of bound states in the continuum.

The goal of quantum metrology is the precise estimation of parameters using quantum properties such as entanglement. This estimation usually consists of three steps: state preparation, time evolution during which information of the parameters is encoded in the state, and readout of the state. Decoherence during the time evolution typically degrades the performance of quantum metrology and is considered to be one of the major obstacles to realizing entanglement-enhanced sensing. We show, however, that under suitable conditions, this decoherence can be exploited to improve the sensitivity. Assume that we have two axes, and our aim is to estimate the relative angle between them. Our results reveal that the use of Markovian collective dephasing to estimate the relative angle between the two directions affords Heisenberg-limited sensitivity. Moreover, our protocol based on Markovian collective dephasing is robust against environmental noise: it is possible to achieve the Heisenberg limit by applying the collective dephasing even under the effect of independent dephasing. Our counterintuitive proposal with the decoherence leads to alternative applications in quantum metrology.

The emergence of a photonic band gap in $\mathrm{Ge}$-on-$\mathrm{Si}$ micropillars ordered in a two-dimensional square lattice is demonstrated by the finite-element method. Candidate architectures are fabricated through epitaxy and the opening of the photonic band gap experimentally proved by photoluminescence spectroscopy. When the direct-gap emission of $\mathrm{Ge}$ is resonantly driven into the photonic gap, light propagation in the lattice plane is inhibited. Emission is eventually funneled out of plane, yielding a giant increase, i.e., about one order of magnitude, in the observed intensity. The demonstration of light routing in microcrystals’ lattices opens interesting possibilities for $\mathrm{Si}$ photonics. The epitaxial self-assembled microstructures introduced here can be monotonically integrated on $\mathrm{Si}$ to improve the performances of group-IV lasers or engineered to optimize the working wavelength of future quantum photonic circuits.

Hyperbolic metamaterials (HMMs) support propagating waves with arbitrarily large wavevectors over broad spectral ranges, and are uniquely valuable for engineering radiative thermal transport in the near field. Here, by employing a rational-design approach based on the electromagnetic local density of states, we demonstrate the ability of HMMs to substantially rectify radiative heat flow. Our idea is to establish a forward-biased scenario where the two HMM-based terminals of a thermal diode feature overlapped hyperbolic bands which result in a large heat current, and suppress the reverse heat flow by creating spectrally mismatched density of states as the temperature bias is flipped. As an example, we present a few high-performance thermal diodes by pairing HMMs made of polar dielectrics and metal-to-insulator transition materials in the form of periodic nanowire arrays, and considering three representative kinds of substrates. Upon optimization, we theoretically achieve a rectification ratio of 324 at a 100-nm gap, which remains greater than 148 for larger gap sizes up to 1 µm over a wide temperature range. The maximum rectification represents an almost 1000-fold increase compared with a bulk diode using the same materials, and is twice that of state-of-the-art designs. Our work highlights the potential of HMMs for rectifying radiative heat flow, and may find applications in advanced thermal-management and energy-conversion systems.

Optical high-finesse cavities are a well-known means to enhance light-matter interactions. Despite much progress in the realization of strongly coupled light-matter systems, the controlled positioning of single solid emitters in cavity modes remains a challenge. We pursue the idea of using nanofibers with subwavelength diameter as a substrate for such emitters. This paper addresses the question of how strongly optical nanofibers influence the cavity modes. We analyze the influence of the fiber position for various fiber diameters on the finesse of the cavity and on the shape of the modes.