We study numerically the interaction of surface acoustic waves propagating at the surface of a multilayer structure (the so-called Sezawa waves) with localized surface plasmons at the bottom of metallic pillars deposited on the substrate. The pillars are made of gold and the multilayer structure is constituted by a layer of gold on a silicon substrate and covered by a thin spacer of dielectric material. We are interested in localized plasmons, which are the analog of metal-dielectric-metal surface plasmons. The physical characteristics of these plasmons, such as their eigenvalues and absorption or reflection spectra, are modulated by surface acoustic waves due to deformation of the structure in the vicinity of the pillars. The surface waves are generated in the area in front of the pillars deposited on the substrate. Two types of acoustic modes are envisioned to interact with the localized plasmons, namely, confined modes associated with the local resonances of the pillars and the propagating Sezawa modes that deform the structure in the vicinity of the surface. We discuss the efficiency of phonon-plasmon coupling for both types of modes and select those that can be useful for an experimental realization based on this platform. Also, we show that, besides the plasmon frequency, the phonon-plasmon coupling magnitude strength is sensitive to the distance between the pillars, which reveals the role of interactions between the pillars on the coupling. The geometrical parameters are chosen such that the acoustic waves are in the sub-GHz range and the plasmons are around the telecommunication wavelength of 1.55 μm. This work may help with the design of optomechanical devices that can be controlled by fast coherent acoustics and facilitate applications involving enhanced sound-light interactions, such as light modulation.

Monolithic optical cavities employing total internal reflections are appealing compact platforms for a wide range of applications from fundamental physics to industry applications. We characterize the transverse patterns of a millimeter-size monolithic nonplanar ring vortex laser pumped by a Gaussian beam. Vortex beams carrying orbital angular momentum (OAM) up to $30$ together with vortex crystals and petal patterns are observed. The rigid nature of the monolithic cavity brings along its low-noise lasing performance for these beams. A frequency noise floor as low as $0.1\mathrm{Hz}/{\mathrm{Hz}}^{1/2}$ corresponding to sub-Hz instantaneous linewidth is measured. Furthermore, we demonstrate low-noise dual-frequency lasing with phase noise as low as $-118\mathrm{dBc}/\mathrm{Hz}$ at 10-kHz offset for the beatnote at 8.2 GHz. We expect that such versatile and compact OAM laser sources can facilitate multiple applications including optical communications, optical tweezers, and laser metrology such as dual-frequency Doppler lidar.

As opposed to metasurfaces, which can produce a single output waveform in response to a single input waveform, volumetric metamaterials have the ability to perform independent functions on many distinct input waveforms. Here, we present an experimental demonstration of this multiplexing capability using a volumetric metamaterial designed using the symphotic method, which realizes highly efficient multiplexing structures in the strong scattering limit. In contrast to perturbative design methods such as volume holography that are only applicable in weakly scattering media, we provide a comprehensive approach that takes into consideration design and fabrication constraints and that can be verified in simulations. We then demonstrate an experimental realization of a symphotic device operating at a frequency of 10 GHz, which has been optimized for three distinct input waveforms corresponding to three distinct output waveforms. The device is realized using a low-loss three-dimensionally printed material. The symphotic device consists of a lattice of dielectric cylindrical elements with varying radii, excited in a parallel-plate waveguide to enforce two-dimensional field symmetry. The experimental results show excellent agreement with analytical coupled-dipole-method simulations and finite-element simulations. The experiments further demonstrate the scalability of symphotic metamaterials and their viability for advanced rf and optical devices.

Highly directional and lossless surface waves have significant potential applications in two-dimensional photonic circuits and devices. Here, we experimentally demonstrate a selective Dyakonov surface wave coupling at the interface between a transparent polycarbonate material and a nematic liquid crystal 5CB. By controlling the anisotropy of the nematic liquid crystal with an applied magnetic field, a single ray at a certain incident angle from a diverged incident beam can be selectively coupled into a surface wave. The implementation of this property may lead to a new generation of on-chip integrated optics and two-dimensional photonic devices.

High-density communication through optical fiber is made possible by wavelength-division multiplexing (WDM), which is the simultaneous transmission of many discrete signals at different optical frequencies. Vast quantities of data may be transmitted without interference using this scheme but flexible routing of these signals requires an electronic middle step, which carries a cost in latency. We present a technique for frequency conversion across the entire WDM spectrum with a single device, which removes this latency cost. Using an optical waveguide in lithium niobate and two infrared pump beams, we show how to maximize the conversion efficiency between arbitrary frequencies by analyzing the role of dispersion in cascaded nonlinear processes. The technique is presented generally and may be applied to any suitable nonlinear material or platform and to classical or quantum optical signals.

The gain-bandwidth product is a fundamental figure of merit that restricts the operation of optical amplifiers. Here, we introduce a design paradigm based on exceptional points, which relaxes this limitation and allows for the building of a new generation of optical amplifiers that exhibits a better gain-bandwidth scaling. Additionally, our results can be extended to other physical systems such as acoustics and microwaves.

Superconducting-nanowire single-photon detectors promise efficient (approximately $100\mathrm{\%}$) and fast (approximately ${10}^9$ counts/s) detection of light at the single-photon level. They constitute one of the building blocks to realize integrated quantum optical circuits in a waveguide architecture. The optical response of single-photon detectors, however, is limited to measure only the presence of photons. It lacks the capability to resolve the spectrum of a possible broadband illumination. In this work, we propose the optical design for a superconducting-nanowire single-photon spectrometer in an integrated optical platform. We use a cascade of cavities with different resonance wavelengths side-coupled to a photonic crystal bus waveguide. This allows us to demultiplex different wavelengths into different spatial regions, where individual superconducting nanowires that measure the presence of single photons are placed next to these cavities. We use temporal coupled-mode theory to derive the optimal conditions to achieve a high absorption efficiency in the nanowire with fine spectral resolution. It is shown that the use of a mirror at the end of the cascaded system that terminates the photonic crystal bus waveguide increases the absorption efficiency up to unity, in principle, in the absence of loss. The expected response is demonstrated by full-wave simulations for both two-dimensional and three-dimensional structures. Absorption efficiencies of about $80\mathrm{\%}$ are achieved both in two-dimensional structures for four cascaded cavities and in three-dimensional structures for two cascaded cavities. The spectral resolution achieved is about $1\mathrm{nm}$. We expect that the proposed setup, both analytically studied and numerically demonstrated in this work, will offer a great impetus for future quantum nanophotonic on-chip technologies.

How to measure the optical conductivity of atomically thin crystals is an important but challenging issue due to the weak light-matter interaction at the atomic scale. The photonic spin Hall effect, as a fundamental physical effect in light-matter interaction, is extremely sensitive to the optical conductivity of atomically thin crystals. Here we report a precision measurement of the optical conductivity of graphene, where the photonic spin Hall effect acts as a measurement pointer. By use of the weak-value-amplification technique, the optical conductivity of monolayer graphene taken as a universal constant of $(0.993\pm 0.005){\sigma }_0$ is detected, and a high measurement resolution of $1.5\times {10}^{-8}{\mathrm{\Omega }}^{-1}$ is obtained. For few-layer graphene without a twist, we find that the conductivities increase linearly with the number of layers. Our idea could provide an important measurement technique for probing other parameters of atomically thin crystals, such as the magneto-optical constant, circular dichroism, and optical nonlinear coefficient.

As the counterpart of $\mathcal{P}\mathcal{T}$ symmetry, for anti-$\mathcal{P}\mathcal{T}$-symmetry abundant phenomena and potential applications have been predicted or demonstrated theoretically. However, experimental realization of the coupling required in anti-$\mathcal{P}\mathcal{T}$ symmetry is difficult. By our coupling two yttrium iron garnet spheres commonly to a microwave cavity, the large cavity dissipation rate makes the magnon-magnon coupling dissipative and purely imaginary. Thereby, the hybrid magnon-cavity system obeys a two-dimensional anti-$\mathcal{P}\mathcal{T}$ Hamiltonian. In terms of the magnon-readout method, the method adopted here, we demonstrate the validity of our method in constructing an anti-$\mathcal{P}\mathcal{T}$ system and present the counterintuitive level-attraction process. Our work provides a platform to explore the anti-$\mathcal{P}\mathcal{T}$-symmetry properties and paves the way to study dynamical evolution and topological properties around exceptional points in multimagnon-cavity systems.

The coupling between a superconducting qubit and a control line inevitably results in radiative decay of the qubit into the line. We propose a Josephson quantum filter (JQF), which protects the data qubit (DQ) from radiative decay through the control line without reducing the gate speed on the DQ. The JQF consists of a qubit strongly coupled to the control line of the DQ and its working principle is a subradiance effect that is characteristic of waveguide quantum electrodynamics setups. The JQF is a passive circuit element and is therefore suitable for integration in a scalable superconducting qubit system.

Isotropic leaky-wave holograms have great potential in the synthesis of single-beam and multibeam radiations in the desired directions. However, conventional methods are based only on the phase synthesis of the object wave front and do not have the ability to control the polarization of waves. A solution to this problem is the use of anisotropic holograms that provide more degrees of freedom. In this paper, the aperture field estimation theory is proposed for the synthesis of anisotropic holograms. Its advantage is the accurate prediction of far-field wave front characteristics, including phase, amplitude, and polarization. This method is used in the design of two different types of holograms in order to generate the pencil beam and vortex beam radiations. The pencil beam hologram is designed to have a high polarization purity across the visible region. In addition, a surface-wave reflector is implemented for eliminating the unwanted backward modes and adding a scannability property from 16 to 19 GHz to the hologram with an appropriate level of cross-polarization across the bandwidth. In the second section, the theoretical equations for the synthesis of vortex beam hologram with circular polarization is developed. As an example, an anisotropic hologram with topological charge of $l=2$ is synthesized. Both theoretical and numerical results show that the anisotropic leaky-wave holograms are capable of generating orbital angular momentum (OAM) mode with the desired topological charge and polarization, which are simpler and more compact than other methods of generating OAM mode. For the proof of concept, a pencil beam hologram with a surface-wave reflector is fabricated and measured.

We report cavity-enhanced Raman scattering from a single-crystal diamond membrane embedded in a highly miniaturized fully tunable Fabry-Perot cavity. The Raman intensity is enhanced 58.8-fold compared to the corresponding confocal measurement. The strong signal amplification results from the Purcell effect. We show that the cavity-enhanced Raman scattering can be harnessed as a narrow-band high-intensity internal light source. The Raman process can be triggered in a simple way by using an optical excitation frequency outside the cavity stop band and is independent of the lateral positioning of the cavity mode with respect to the diamond membrane. The strong Raman signal emerging from the cavity output facilitates in situ mode matching of the cavity mode to single-mode collection optics; it also represents a simple way of measuring the dispersion and spatial intensity profile of the cavity modes. The optimization of the cavity performance via the strong Raman process is extremely helpful in achieving efficient cavity outcoupling of the relatively weak emission of single color centers such as nitrogen-vacancy centers in diamond or rare-earth ions in crystalline hosts with a low emitter density.

Benefiting from prominent performance in various scientific fields, metasurface research has been flourishing over the past decade. Huygens’ metasurface, as an emerging subcategory of metasurface, can theoretically achieve full control of electric field distribution within the subwavelength scale. Driven by its outstanding manipulation capacity, the three-dimensional holography utilizing broadband Huygens’ metasurface is demonstrated in the microwave regime in this paper. For the broadband working properties, two types of equivalent circuit are integrated into the design of Huygens’ meta-atom. Besides, for high image quality, the modified weighted Gerchberg-Saxton (GSW) algorithm is implemented for specific interfacial phase distribution. The proof-of-concept experiments show notable microwave holographic images, with an 86.2% transmittance efficiency, a 62.7% imaging efficiency, and an 88.1 signal-to-noise ratio at 12 GHz. The proposed holography extends the route to broadband microwave applications in security, data storage, and antenna systems.

Spin angular momentum (SAM), a fundamental property of light, is associated with the right-hand and left-hand circular polarizations. It has never been trivial to engineer a light beam with desired SAM, and especially at the frequency regime other than the visible region. A widely used approach is to build periodic arrays of plasmonic nanostructures that host two orthogonal dipole resonances. As the unit cell, its structure design is crucial, but the design principle remains obscure. We present a model of two orthogonal resonant Lorentz dipoles to quantitatively describe the creation of SAM in such plasmonic quarter-wave plates. The equal amplitude and $\pi /2$ phase difference are essential. The requirements can be achieved by structure modulation and, in addition, the amplitude can be manipulated by varying the polarization angle of incident waves. In practice, the resonant frequency difference of two dipoles $\mathrm{\Delta }\omega =|{\omega }_A-{\omega }_B|$, and their damping rate ${\kappa }_A$ and ${\kappa }_B$, should meet a basic criterion, such as $\mathrm{\Delta }\omega ⩾\sqrt{{\kappa }_A{\kappa }_B}$. As an illustration, we design a cross-shaped circular polarization convertor, and show its structure and incident angle optimization. We find the optimal working frequency ${\omega }_{\mathrm{opt}}$ is roughly equal to $\sqrt{{\omega }_A{\omega }_B}$ at the critical condition $\mathrm{\Delta }\omega =\sqrt{{\kappa }_A{\kappa }_B}$. Our results provide good guidance on the design of SAM-enabled plasmonic devices for versatile applications.

Dielectric multilayer photonic-band-gap structures, called one-dimensional photonic crystals (1DPCs), have drawn considerable attention in the fields of physics, chemistry, and biophotonics. Here, experimental results verify the feasibility of a 1DPC working as a substrate for switchable manipulations of colloidal microparticles. The optically induced thermal convective force on a 1DPC can assemble colloidal particles that are dispersed in a water solution, while the photonic scattering force on the same 1DPC caused by propagating evanescent waves can guide these particles. Additionally, in the 1DPC, one internal mode can be excited that has seldom been noticed previously. This mode shows an ability to assemble particles over large areas even when the incident power is low. The assembly and guidance of colloidal particles on the 1DPC are switchable just through tuning the polarization and angle of the incident laser beam. Numerical simulations are carried out, which are consistent with these experimental observations.

We present the design and experimental demonstration of an impedance-matched circular-polarization-selective surface that also offers spin-selective phase modulations at microwave frequencies. We achieve this by leveraging the theory of Pancharatnam-Berry phase shift and cascading four tensor impedance layers, each comprising an array of crossed meander lines. These meander lines are precisely tuned and rotated to implement particular tensor surface impedance values to satisfy the impedance-matching condition for the transmitted right-handed circularly polarized field while inducing Pancharatnam-Berry phase shift for only the reflected left-handed circularly polarized field. We present a detailed numerical synthesis technique to obtain the required impedance values to satisfy the impedance-matching condition, and demonstrate spin-selective phase modulations based on Pancharatnam-Berry phase shifts. To verify the proposed idea, we experimentally demonstrate nearly reflectionless transmission of right-handed circular polarization at broadside and reflection of left-handed circular polarization at ${33}^{\circ }$ off broadside at 12 GHz. For this purpose, a free-space quasioptical setup and a near-field measurement system are used to measure the transmitted and reflected circularly polarized fields, respectively.

In this paper, a momentum-space geometrical structure in helical evanescent electromagnetic waves is revealed. It is shown that for every helical evanescent wave on a helicity-dependent half tangent line in momentum space, the orientation of each of its field, spin, and Poynting vectors is the same. This geometric structure is revealed in a remarkable relation between the far-field and near-field components of the angular spectrum. Any general evanescent wave vector is linked to two points on the $k_{\rho }=k_0$ circle of propagating wave vectors via two helicity-dependent tangent lines. Knowing the field of a general dipolar source on the $k_{\rho }=k_0$ circle is sufficient to determine its entire evanescent angular spectrum. By applying this concept, we gain insights into near-field directionality by showing that every zero in the angular spectrum is a helicity singularity where two half tangent lines of opposite helicity intersect. A powerful method for the synthetic design of near-field directional sources is also devised, using structured helical illumination to gain full control of the near-field directionality. The results provide fundamental insight into helical evanescent waves and have implications in areas where chiral light-matter interaction plays a central role.