In this Letter, we propose an E-shaped hole metasurface leveraging bound states in the continuum (BICs) for perfect absorption and ultrasensitive refractive index sensing. We can achieve 98.9% optical absorption at 909 nm in the symmetric metasurface through a symmetry-protected BIC mode. It is found that there is a squared relationship between the Q-factor and the asymmetry factor. More importantly, we successfully activate the quasi-BIC (Q-BIC) mode in a symmetry-broken structure at an 8° light incidence angle. Both the symmetry-protected BIC and Q-BIC modes show 1000 nm/RIU (refractive index unit) sensitivity in air, while the latter outperforms in organic solutions with a figure of merit (FOM) of 463.9. This platform offers a versatile solution for ultra-narrowband photonics and high-precision biosensing applications.

Optical angular momentum (AM), comprising spin angular momentum (SAM) and orbital angular momentum (OAM), is crucial in various applications, yet its flexible control remains challenging. This study proposes, to our knowledge, a novel method for manipulating SAM and OAM using spherical wave illumination and the Λ-shaped spiral aperture. By adjusting the spherical wave’s convergence or divergence, the sign of SAM and OAM can be switched, while the geometric topological charge of the aperture transfers to the optical AM due to AM conservation. The method is theoretically analyzed, simulated, and experimentally validated, offering a compact platform applicable to photonic systems, particle manipulation, and encryption.

Metasurfaces have revolutionized planar optics due to their prominent ability in light field manipulation. Recently, the incorporation of machine learning has further improved computational efficiency and reduced the reliance on professionals in designing various metasurfaces. However, the prevalent methods still suffer from configuration complexity and expensive training costs due to more than one model or a combination of rule-driven algorithms. This study proposes a deep learning-based paradigm using only one deep learning model for the end-to-end design of versatile metasurfaces. The adopted deep-enhanced RseNet acts both as the surrogate of the electromagnetic simulator in forward spectrum prediction and as the path for backward gradient descent optimization of the meta-atom structures in the paralleled calculation. Without loss of generality, a polarization-multiplexing holographic and a polarization-independent vortex metasurface were designed by this paradigm and successfully demonstrated in the terahertz range. The extremely simplified framework presented here will not only propel the design and application of metasurfaces in terahertz communication and imaging fields, but its universality will also accelerate the research and development of subwavelength planar optics across various wavelengths through artificial intelligence (AI)-enhanced design for optical devices.

Light-beam shifts accompanied by propagation between two media show potential in applications such as optical sensing, optical communication, and optical computing. However, existing work tends to focus on the static response of the device, i.e., the beam shift when the structural parameters and incident conditions are fixed. Here, we analyze the dynamics of beam shifting via photonic crystal slabs under refractive index variation. On the one hand, we investigate the trend of cross-polarized phase gradient under small changes in refractive index. Simulation results show that the direction of the beam shift can change by more than 50° for a refractive index change of only 0.06. On the other hand, we study the interaction of incident light with the far-field polarizations of bound states in the continuum in the presence of a refractive index jump in the phase-change material. In this case, simulation results show that the large change in the Pancharatnam–Berry phase gradient causes the beam to move widely, with a change in beam direction of 61.30° and a change in beam displacement of 15 µm. Furthermore, all displacement amounts are comparable to the radius of the incident beam (∼8 µm). Our work provides a new perspective on the study of beam shifts, which can advance practical applications of beam shift in sensing, intelligent detecting, and beam control.

It has long been a challenging task to improve the light collection efficiency of conventional image sensors built with color filters that inevitably cause the energy loss of out-of-band photons. Here, we demonstrate a pixelated spectral router based on a sparse meta-atom array, which can efficiently separate incident R (600–700 nm), G (500–600 nm), and B (400–500 nm) band light to the corresponding pixels of a Bayer image sensor, providing over 56% signal enhancement above the traditional color filter scheme. It is enabled by simple compound Si3N4 nanostructures, which are very suitable for massive production. Imaging experiments are conducted to verify the router’s potential for real applications. The complementary metal-oxide-semiconductor (CMOS)-compatible spectral router scheme is also found to be robust and can be freely adapted to image sensors of various pixel sizes, having great potential in building the new generation of high-performance image sensing components.

Micro-LED is one of the most promising technologies for naked-eye 3D display. However, due to challenges related to efficiency, resolution, viewing range, and structure integration, the 3D micro-LED display is still at the conceptual stage. In this work, we introduce a double-functioned metalens composed of highly symmetric unit cells into the 3D micro-LED system. The nonpolarized spotlight generated by the micro-LED is collimated and deflected through the designed metalens. Inspired by the compound eyes, metalens modules with varying deflection angles are spliced and penetrated together, enabling a wide viewing angle without sacrificing resolution. Additionally, the viewing position can be dynamically adjusted using adjustable subpixels. The results demonstrate that the proposed metalens and its optical system can reach a viewing angle ranging from -41.5° to 41.5° and an adjustable optimum viewing distance from 25 to 75 cm. The deflection efficiency exceeds 80%, with a resolution of 910 PPI (pixels per inch). Our design shows great potential for naked-eye 3D display.

In this Letter, by exploiting the spin-to-orbital angular momentum conversion capability and the polarization selectivity characteristic of cholesteric liquid crystal planar optical elements, an approach of combing two cholesteric liquid crystal layers with opposite handedness and independent surface patterns is proposed and investigated for generating arbitrary vector vortex beams on the hybrid-order Poincaré sphere. Furthermore, the intensity profiles and polarization distributions of typical vector vortex beams are experimentally demonstrated and analyzed, which shows good agreement with the theoretical prediction. The approach also suggests its advantages of operating light in the polychromatic spectral region of Bragg reflection. Our method presents a simple and direct way of phase and polarization manipulation, which also provides promising opportunities for developing advanced applications in structured light, high-resolution imaging, and information processing.

Defects are inevitably induced during the fabrication process of a metalens, which will affect the metalens’s yield and optical performances. Thus, investigations on the fabrication defects are becoming increasingly important for the mass production of metalenses. In this Letter, the optical performances of near-infrared metalenses with four types of fabrication defects are investigated. The results show that the process-induced defects obviously affect the focusing efficiency at λ = 940 nm, but they have less impact on the quality of the focal spot. This work provides fabrication guidance for large-scale manufacturing of metalenses in the future.

A gold nanohole/disk array-based plasmonic fiber end-facet sensing probe is proposed and demonstrated experimentally, where the hybrid plasmon mode on the top surface used for sensing is excited by the cooperative effect of the near-field coupling between the nanohole and the nanodisk, as well as the localized surface plasmon of the nanodisk. The high-quality integration of the nanohole/disk array on the fiber end facet is achieved by combining nanoimprint lithography on a planar substrate with fiber ultraviolet (UV)-curable adhesive transfer techniques. As a result, the fabricated fiber probe experimentally exhibits a moderately high bulk refractive index sensitivity of ∼196.91 nm/RIU and excellent surface sensitivity. Furthermore, the specific identification and determination of protein molecules verify their sensitivity analysis capabilities for future bioassays. This work provides a feasible plasmonic excitation strategy and enables batch-manufactured technology for nanostructure-based fiber probes to break through the current bottlenecks in biosensing applications.

Precise control of the polarization state of light on ultrafast time scales plays a key role in revealing the inherent chiral or anisotropic optical responses in various material systems, and it is crucial for applications that require complex polarization encoding. Here, we explore ultrafast polarization control enabled by silicon-based chiral bound state in the continuum (BIC) metasurfaces. By utilizing the intrinsic chiral mode, we achieve high-purity chiral reflection light (S3 ∼ -0.92) and rapid modulation (∼0.4 ps) of polarization states through all-optical methods. Unlike traditional polarization modulation techniques, our approach leverages the unique advantages of slanted etching dielectric chiral BIC metasurfaces, which facilitate high-Q resonance and exhibit narrow linewidths. These advantages allow swift alterations in polarization states with minimal modulation energy consumption, which should help for greater control of light in integrated photonic applications.

Polarization singularities beyond the bound states in the continuums (BICs) have garnered significant interest due to their potential for light manipulation. The conservation of topological charge has proven crucial in various photonic systems, and it guides the behavior of these singularities, including the generation and annihilation of BICs. This work theoretically reveals the simultaneous generation of two distinct polarization singularity types, which include off-Γ accidental BICs and Dirac-type band degeneracy points. The generation is driven by a quadratic degeneracy of symmetry-protected BICs in a photonic crystal slab. It should be noted that this is achieved through continuously tuning a geometric parameter without breaking symmetry. Importantly, the generation of both singularity types can be explained by the topological charge conservation law. This adherence ensures the stability of these singularities and allows for continuous tuning of their positions in momentum space by continuously tuning a geometric parameter while preserving symmetry. This study presents a novel framework for synthesizing and manipulating complex polarization states by combining polarization singularities from both BICs and band degeneracies and holds promise for application in other wave systems beyond photonics.

Randomness describes one inherent property of self-assembled metamaterials and greatly limits the practical applications of metamaterials based on bottom-up techniques, such as the microsphere lithography technique. Herein, by subtly utilizing the randomness in long-range disorder metasurfaces, we demonstrate a high-performance one-shot full-Stokes polarimeter in the visible waveband. The long-range disorder metasurfaces, i.e., chiral shells, were realized by depositing Ag on the self-assembled microsphere monolayer comprised of many micro-domains of random lattice directions and areas. The distinct optical anisotropy and chirality in different micro-domains can result in distinct photo-currents to the photodetector array placed underneath upon the injection of polarized lights. Through establishing the mapping relationship S^=f(I^) between the detected photo-currents I^ and the states of polarization (SoP) S^ with the convolutional neural network (CNN) algorithm, we realize a high-precision full-Stokes polarimeter in the waveband ranging from 500 to 650 nm, and the minimum mean square errors (MSEs) can reach about 0.37% (S1), 0.33% (S2), and 0.19% (S3) at 566 nm. The average MSEs in the investigated waveband are 0.49% (S1), 0.45% (S2), and 0.31% (S3), respectively. We have systematically investigated the macro- and micro-optical properties of chiral shells, the optical randomness of chiral shells in different domains, the reference SoP number, the exposure time and pixel number of the CCD, as well as the reliability and stability of the system.

We propose a modular designed over-coupled (OC) metasurface for the broadband surface-enhanced infrared absorption spectroscopy (SEIRAS) by analyzing the combined properties in the far field and near field. The customized sensors can independently modify the coupling mode, the resonance frequency, and the coupling efficiency by adjusting the vertical and horizontal structures and hybrid dielectric layers of the metasurface, respectively. Based on the independent regulation of the sensor properties, the influence of the detuning properties, the level of OC coupling, and the coupling efficiency of the signal amplification can be clearly presented through the single variable-controlling approach. These design principles are universal for customized sensors and herald possibilities for machine-learning-aided surface-enhanced infrared absorption (SEIRA) biosensing.

In this paper, we propose a new strategy based on the interconversion between two symmetry-protected bound states in the continuum (SP-BICs) to break the high symmetry dependency of the SP-BIC. The excitation of four high-Q quasi-BIC resonances is supported by disrupting both the translational and structural symmetry of the metasurface using spacing and length perturbations, respectively. Furthermore, interconversion between the two SP-BICs can be achieved via length perturbation, significantly diminishing the radiative attenuation rate of the quasi-BIC. Despite a relative asymmetric parameter reaching 97.2%, the Q-factor order of magnitude of the quasi-BIC can remain constant. Compared with previous studies, our approach significantly enhances the robustness of the Q-factor for the quasi-BIC by a minimum of two orders of magnitude, although our relative asymmetric parameter is approximately 10 times the corresponding work.

We propose a fast-printable and function-switchable metamaterial based on laser-induced graphene for terahertz (THz) wave modulation in the reflection mode. The design can modulate the linear polarization of the incoming wave fronts to its cross-polarization from 0.27 to 0.41 THz and linear polarization to circular polarization from 0.48 to 0.62 THz. The function of the device can also be switched from a polarization converter to an absorber by bending it with an angle of 57°. Experimental results showed a good agreement with those of the simulation. The proposed polarization converter may find its application in THz polarization control systems and sensing.

In this study, we investigate the intensity and spectral fluctuations in surface-enhanced Raman scattering (SERS) signals from individual plasmonic nanocavities. Extremely long-duration blinking components lasting up to minutes are observed in the SERS intensity fluctuation at room temperature, which can be characterized by successive and random appearance and eventual disappearance of multiple vibrational modes at different frequencies. Theoretical simulations show that multiple hotspots acting on different molecular sites are required to explain these multipeak features, suggesting that the source of long-duration blinking events is multiple atomic-scale protrusions interacting simultaneously with different sites of individual molecules or each with a different molecule.

Coherent interactions between excitons strongly coupled to plasmons are vital for quantum information devices. For practical applications, suppressing the incoherent dissipation pathways in the hybrid system is essential. Here, we report on a strong plasmon–exciton coupling in a monolayer WS2/graphene van der Waals heterostructure (WS2/Gr vdWhs) integrated with an Au nanocube (Au NC). The presence of graphene effectively suppresses the nonradiative decay pathway of neutral excitons in the vdWhs, resulting in a narrower photoluminescence (PL) linewidth. The further integration of the WS2/Gr vdWhs with the Au NC enables coherent interaction between the in-plane exciton and a tilted plasmonic dipole, delivering a Rabi splitting energy of 120 meV and an incoherent coupling strength of 1 meV. Our findings possess the potential to facilitate the advancement of quantum nanophotonic devices.

High-aspect-ratio structures with heights or depths significantly exceeding their lateral dimensions hold broad application potential across various fields. The production of these structures is challenging, requiring meticulous control over materials, scale, and precision. We introduce an economical and efficient approach for fabricating high-aspect-ratio nanostructures using a two-photon polymerization process. This approach achieves feature sizes of around 37 nm with an aspect ratio of 10:1 using commercial photoresists. Offering advantages over traditional techniques, our approach simplifies operation and enhances design flexibility, facilitating the creation of smaller, more complex, and high-aspect-ratio structures. The capabilities of this approach are demonstrated by producing arrays of three-dimensional microstructures that exhibit sub-micron scales, extensive periodicity, and pronounced aspect ratios. These developments open new possibilities for applications in biomedical, precision engineering, and optical microdevice manufacturing.

Atomic spectroscopy serves as the basis for quantum precision measurements, where frequency-stabilized lasers are crucial for obtaining accurate atomic spectra. This work introduces a compact laser frequency stabilization system that employs a multifunctional metasurface to adjust the polarization, amplitude, and propagation direction of incident light. By combining with a Rb atomic vapor cell, the system achieves a tunable sub-Doppler spectrum for laser frequency stabilization. The experimental result demonstrates that a laser frequency stability of 3 × 10-11 is attained from 1 to 200 s at 780 nm with the input power at 20 µW. The devices hold significant potential for compactness, integration, and mass production, making them highly suitable for quantum measurement applications.

Comparing the coupling strength with both the mean and the product of the square roots of the respective damping rates for the bright and dark modes is a crucial metric in the study of plasmon-induced transparency (PIT). The flip in the ratio determines whether the coupling state between the structural units is strong or weak and also applies to the group delay. Our study explores two primary coupling channels within PIT structures: the inter-resonator distance (d) between the split-ring resonators (SRRs) and the cut wire (CW) and the spacing (g) between the SRRs. In the simulations, photosensitive silicon is embedded in the openings of the dark mode SRR resonator, actively modulating the dispersion characteristics and the coupling strength. Furthermore, we methodically examine the influence of these coupling channels on the transition between the coupling states, as well as on the maximal group delay in the PIT effect. Theoretically, leveraging the parameter fitting via the Lorentz coupling resonator model identifies the dominant parameters governing coupling state flips and differential regulation mechanisms. Our findings contribute to a deeper understanding of PIT phenomena and offer insights into optimizing PIT structures for diverse applications.

Lenses with desired depth of focus have crucial applications in imaging systems. However, there is little theoretical guidance to extend the depth of focus beyond numerical optimization. The on-demand construction of the Jones matrix using the composite metasurface brings a powerful tool for polarization-multiplexed functionalities. Here, based on polarization-multiplexed focusing in four linear polarization channels, we propose a straightforward method to extend the depth of focus based on the coherent superposition of each linear polarization channel. The metalens shows long and uniform needle beam focusing with a depth of focus of 46λ in circularly polarized excitation in the experiment, which offers a promising tool to tailor the terahertz focal spot for imaging applications.

We design a terahertz (THz) biosensor supported by quasi-bound states in the continuum (QBICs) for lung cancer cell sensing. By destroying the in-plane symmetry of the bound state in the continuum (BIC), a QBIC with a high Q-factor is obtained. The designed biosensor exhibits excellent refractive index performance with a sensitivity of 354 GHZ/RIU. Unlike traditional detection schemes that require sample drying, a microfluidic liquid sample pool is utilized to detect different concentrations of lung cancer cells. As the cell concentration increases, the resonance frequency and intensity of the measured spectrum show significant changes. The designed sensor allows non-invasive real-time detection of living lung cancer cells, providing a potentially effective tool for early diagnosis and treatment of lung cancer.

Dual-polarized reconfigurable intelligent surfaces (RISs) increasingly play significant roles in reshaping wireless transmission environments. In this Letter, we propose a design method for dual-polarized RIS elements. This proposed method develops an equivalent multiport model to quickly calculate reflection electromagnetic (EM) responses of the elements containing multiple structural parameters. Moreover, the genetic algorithm (GA) is utilized to optimize the structural parameters to meet design specifications. A 1-bit dual-polarized RIS is implemented for verification. The simulated and experimental results show good consistency with the calculated results. The proposed method significantly conserves design resources, promoting the development of dual-polarized RISs.

In this paper, a new strategy is proposed based on arbitrary selection of perturbation in a dielectric metasurface to achieve multiple quasi-bound states in the continuum (BICs) with identical modes under dual polarizations. Three distinct symmetry-broken perturbations are discussed. By selecting an arbitrary perturbation, triple quasi-BICs can be induced in transverse magnetic polarization modes at wavelengths of 1071.18, 1098.8, and 1199.6 nm, respectively. Simultaneously, double quasi-BICs at wavelengths of 1375.9 and 1628.5 nm are generated in transverse electric polarization modes. Moreover, the excited quasi-BICs exhibit excellent sensing performance with a maximum sensitivity of 900 nm/RIU, which is better than similar previous studies.

Bound states in the continuum (BICs) have gained significant attention in recent years for enhancing light–matter interaction. Here, we numerically and experimentally demonstrate quasi-BICs in a terahertz photonic crystal (PhC) slab induced by breaking the structural symmetry. The terahertz PhC slab can support four symmetry-protected BICs, exhibiting multipole properties in the near fields and vector vortex characteristics in the far fields. By altering the shape of the holes to break the in-plane inversion symmetry, the quasi-BICs can be excited in the PhC slab under normal incidence. Furthermore, by elaborately adjusting the asymmetry parameter, accidental BICs can also be created in the asymmetric PhC slab. Experimental fabrication of both symmetric and asymmetric terahertz PhC slabs confirms the observation of quasi-BICs in the PhC slabs. The high-Q quasi-BICs in the asymmetric terahertz PhC slab show promise for applications in terahertz sensors, filters, and modulators.

Mode-division multiplexing technology leveraging diverse spatial modes has advanced to sustain capacity expansion in fiber-optic communications. The intelligent recognition of spatial modes using ultra-compact devices and low-complexity designs is crucial for mode visualization and system miniaturization. In this work, we theoretically design and experimentally demonstrate a neural network-optimized metasurface capable of dual-mode pattern recognition through dual-channel image display. Our framework offers three key advantages: device compatibility, design flexibility, and function scalability by integrating neural networks and metasurfaces into mode-division multiplexing platforms. Our framework enhances research and applications of intelligent metasurface-driven pattern recognition and object classification, as well as information encoding and decoding.

We propose a heterogeneous all-dielectric photonic molecule comprising a Mie nano-resonator (MNR) and a photonic crystal nanocavity (PCNC), forming a strongly coupled system. The coupling mechanism is rigorously analyzed using the coupled mode theory, unveiling key optical phenomena, including Fano resonance, mode splitting, and Rabi oscillation. By precisely tuning the spatial position of the MNR relative to the PCNC and the structural parameters of the MNR, we achieve modulation of near-field mode interactions and far-field radiation. Performance evaluation reveals highly directional radiation and tunable spectral properties, facilitating efficient light manipulation at the nanoscale. This study establishes a versatile platform for advancing quantum optics, integrated photonics, and optical antennas, with promising applications in high-purity quantum light sources, ultra-sensitive sensing, and low-threshold nano lasers.

In this Letter, we explore the interplay between topological defects and resonant phenomena in photonic crystal slabs, focusing on quasi-flatband resonances and bound states in the continuum (BICs). We identify anisotropic quasi-flatband resonances and isotropic quasi-flatband symmetry-protected BICs that exist in coupled topological defects characterized by nontrivial 2D Zak phases, originating from monopole, dipole, and quadrupole corner modes within second-order topological insulator systems. These topological defect modes, whose band structures are described using a tight-binding model, exhibit distinctive radiative behavior due to their symmetry and multipolar characteristics. Through far-field excitation analysis, we demonstrate the robustness and accessibility of these modes in terms of angular and spectral stability. Furthermore, we investigate potential applications of the quasi-flatband resonances in light–matter interactions, including optical forces, second-harmonic generation, and strong coupling, which exhibit robust performance under varying illumination angles. These findings offer new opportunities for precise control over light–matter interactions.

In recent years, the perfect vortex beam with independent wavefront spiral correlation has attracted extensive attention since its beam diameter is independent of topological charge. Perfect vortex beams are expected to make significant progress in optical fiber communications, particle manipulation, quantum information, and other areas. Traditional optical devices are difficult to integrate into the system due to their large size. In this paper, we design and realize a perfect vortex beam with a high reflection efficiency of 90.17% by an all-dielectric metasurface through a Pancharatnam–Berry (PB) phase modulation structure. The cross-polarization conversion efficiency measured by experiment is 89.81%. By modulating the parameter r0 in the phase function, we can achieve flexible manipulation of topological charges and ring diameters. In addition, we also demonstrate the generation of a four-channel perfect vortex beam array based on the Dammann grating, with a beam uniformity of 40%. Our research will be of great significance for the realization of compact and multifunctional on-chip integrated photonic devices.

Lithium-niobate-on-insulator (LNOI) chips have shown outstanding performance in various photonic devices including modulators, lasers, nonlinear converters, and quantum sources. LNOI-based edge couplers are quite important for further promotion of the above devices in practical applications, especially for large-scale multiport photonic uses, where efficient and mode-selective coupling between chips and fibers is of necessity. Previously, several LNOI edge couplers have been demonstrated, but they mainly focus on achieving high coupling efficiency of the fundamental mode, and sub-wavelength etched lithium-niobate (LN) structures are normally needed, which increases fabrication complexity. Here we propose a new type of edge coupler with direct mode-selective excitation ability, using only SiON cladding grating structures without additional etching of LN. By introducing a cladding waveguide with periodic structures on the uniform LNOI waveguide, high-efficiency excitation of multiple modes can be realized directly with easier fabrication. For a specific simulation here, TE00, TM00, and TE10 core modes can be excited, respectively, at optimized periods and grating lengths with a tunable central wavelength, at the launch of the TM cladding mode. The periods of the needed SiON gratings are all over 2 µm, which is feasible with i-line UV lithography. Our results provide a low-cost edge coupler for LNOI photonic circuits with the ability of flexible spatial mode selectivity, which may promote LNOI devices in large-scale multiport photonic integrated circuits in the future.

Recently, polarization conversion based on the equivalent magnetic field of Weyl semimetals (WSMs) has gained significant attention. Based on a twin WSM-layer structure, we transplant the concept of twist from Twistronics to obtain a dynamically tunable polarization converter with multifunctions and nonreciprocity. One-way conversion of linear polarization to its orthogonal state can be obtained and tuned by twisting the converter. Moreover, nonreciprocal conversion from linear polarization to elliptical polarization (with ellipticity linearly tuned from -1 to 1 by twisting the converter) in one way and to quasi-linear polarization in the other way can be obtained. Calculations show that the converter achieves 95% efficiency, high optical isolation (100 dB), and low insertion loss (<2 dB). The proposal may find applications in tunable and nonreciprocal devices for integrated photonics.

Orbital angular momentum (OAM) waveguides are critical for multi-channel photonic-on-chip applications. However, current large-mode-area waveguides pose a challenge for OAM device miniaturization. Here, a novel hybrid plasmonic waveguide is theoretically proposed to decrease the OAM mode area by two orders of magnitude. Benefiting from the Ag-As2S3-SiO2-As2S3-Ag five-layer cylindrical structure, the guided OAM mode realizes a larger figure of merit of 88. Based on this waveguide, an OAM coupler with a record-breaking small footprint (0.68 µm × 5.7 µm) is designed. The proposed waveguide enables subwavelength OAM light transmission, which provides a key building block for high-density OAM photonics circuits.

The optical properties of hybrid core–shell nanostructures composed of a metallic core and an organic shell of molecular J-aggregates are determined by the electromagnetic coupling between plasmons localized at the surface of the metallic core and Frenkel excitons in the shell. In cases of strong and ultra-strong plasmon–exciton coupling, the use of the traditional isotropic classical oscillator model to describe the J-aggregate permittivity may lead to drastic discrepancies between theoretical predictions and the available experimental spectra of hybrid nanoparticles. We show that these discrepancies are not caused by limitations of the classical oscillator model itself, but by considering the organic shell as an optically isotropic material. By assuming a tangential orientation of the classical oscillators of the molecular J-aggregates in a shell, we obtain excellent agreement with the experimental extinction spectra of TDBC-coated gold nanorods, which cannot be treated with the conventional isotropic shell model. Our results extend the understanding of the physical effects in the optics of metal–organic nanoparticles and suggest an approach for the theoretical description of such hybrid systems.

The miniaturization of spectrometers has received much attention in recent years. The rapid development of metasurfaces has provided a new avenue for creating more compact and lightweight spectrometers. However, most metasurface-based spectrometers operate in the visible light region, with much less research on near-infrared wavelengths. This is possibly caused by the lack of effective metasurface filters for the near-infrared light. We design and fabricate a polarization-insensitive amorphous silicon metasurface that exhibits unique transmission spectra in parts of the visible and near-infrared wavelengths. By passing the light to be measured through a metasurface filter array and measuring the transmitted power, we achieve the precise reconstruction of unknown spectra in the visible and near-infrared range (450–950 nm) using an algorithm matched to the filter model. Our approach is a step towards miniaturized spectrometers within the visible-to-near-infrared range based on metasurface filter arrays.

In this work, a simple fabrication method of germanium-based metasurfaces is proposed, where the deposited Al2O3 layer with high selectivity is chosen as the hard mask and retained after the dry etching process. The simulation and experimental characterization results verify the feasibility of the fabrication method. The experimental study on the fabrication methods of germanium-based metasurfaces is very significant as the meta-atoms with a higher refractive index can achieve 0 to 2π transmission phase variation with a smaller period under the same thickness-to-period ratio, which is consistent with the requirement of the period miniaturization in some cases.

We present a tunable terahertz (THz) spectrum analyzer with hyperspectral resolution formed from electrically tunable metamaterial and plasmonic structures. As few as eight encoders based on four detectors are needed to recover 396 spectral bands. The incident spectra in the range of 1–5 THz can be reconstructed with a localization precision of 0.3 GHz and a minimum average mean squared error (MSE) of 6.9 × 10-5. Our proposed analyzers are faster and more portable than those based on frequency combs and power meters, and more accurate than existing Fourier transform techniques, showing promising applications in pathology, biomedical imaging, and many other areas.

In the field of long-wave infrared (LWIR) thermal imaging, vital for applications such as military surveillance and medical diagnostics, metalenses show immense potential for compact, lightweight, and low-power optical systems. However, to date, the development of LWIR broadband achromatic metalenses with dynamic tunable focus, which are suitable for both coaxial and off-axis applications, remains a large unexplored area. Herein, we have developed an extensive database of broadband achromatic all-As2Se3 microstructure units for the LWIR range. Utilizing this database with the particle swarm optimization (PSO) algorithm, we have designed and demonstrated LWIR broadband achromatic metalenses capable of coaxial and off-axis focusing with three dynamic tunable states. This research may have potential applications for the design of compact, high-performance optical devices, including those with extreme depth-of-field and wide-angle imaging capabilities.

In this work, we propose a novel approach that combines a bidirectional deep neural network (BDNN) with a multifunctional metasurface absorber (MMA) for inverse design, which can effectively address the challenge of on-demand customization for absorbers. The inverse design of absorption peak frequencies can be achieved from 0.5 to 10 terahertz (THz), covering the quasi-entire THz band. Based on this, the BDNN is extended to broadband absorption, and the inverse design yields an MMA at the desired frequency. This work provides a broadly applicable approach to the custom design of multifunctional devices that can facilitate the evaluation and design of metasurfaces in electromagnetic absorption.

Tunability, ultracompact design, high group index, low loss, and broad bandwidth are desired properties for integrated optical delay lines (ODLs). However, those properties are challenging to achieve simultaneously in the visible region. This paper proposes a tunable hexagonal boron nitride topological optical delay line (ODL) in the visible region based on valley photonic crystals. The topological edge state from the beard-type boundary allows the achievement of an ultralow group velocity close to zero, which results in a large group index of 629 at 645 nm. Moreover, we demonstrate tuning of the slow-light wavelength and optical delay times with electrically tunable liquid crystals by applying external voltage. The device has an ultracompact size of 5 µm × 2.7 µm with an optical delay distance of 25a (a is the lattice constant) and a delay time of 12 ps. Our design can provide a new possibility for designing ODLs working in the visible region for optical communication and quantum computing systems.

The metalens has attracted remarkable attention due to its ultra-thin and ultra-light characteristics, which indicate great potential for compact imaging. However, the limited efficiency at a large angle incidence severely hinders the application of wide-angle focusing and imaging, which is pursued in the fast-developing imaging systems. Therefore, new strategies to improve the lens performance at large incident angles are in demand. In this work, we propose tilted structures for large-angle focusing with improved efficiency. Metalenses based on dynamic phase and geometric phase are designed and systematically characterized by numerical simulations. We show that tilted structures of unit cells significantly improve the lens performance at oblique incidences. In detail, the focusing efficiency of the metalens with tilted structures is increased over 25% at 30° incidence, as well as the modulation transfer function. In addition, we develop a hybrid metalens array achieving highly efficient wide-angle imaging up to 120°. We believe this design provides a feasible route toward wide-field and high-performance imaging applications.

Active metasurfaces have recently attracted more attention since they can make the light manipulation be versatile and real-time. Metasurfaces-based holography possesses the advantages of high spatial resolution and enormous information capacity for applications in optical displays and encryption. In this work, a tunable polarization multiplexing holographic metasurface controlled by an external magnetic field is proposed. The elaborately designed nanoantennas are arranged on the magneto-optical intermediate layer, which is placed on the metallic reflecting layer. Since the non-diagonal elements of the dielectric tensor of the magneto-optical material become non-zero values once the external magnetic field is applied, the differential absorption for the left and right circularly polarized light can be generated. Meanwhile, the amplitude and phase can be flexibly modulated by changing the sizes of the nanoantennas. Based on this, the dynamic multichannel holographic display of metasurface in the linear and circular polarization channels is realized via magnetic control, and it can provide enhanced security for optical information storage. This work paves the way for the realization of magnetically controllable phase modulation, which is promising in dynamic wavefront control and optical information encryption.

Lithium niobate is a material that exhibits outstanding electro-optic, nonlinear optical, acousto-optic, piezoelectric, photorefractive, and pyroelectric properties. A thin-film lithium niobate photonic crystal can confine light in the sub-wavelength scale, which is beneficial to the integration of the lithium niobate on-chip device. The commercialization of the lithium niobate on insulator gives birth to the emergence of high-quality lithium niobate photonic crystals. In order to provide guidance to the research of lithium niobate photonic crystal devices, recent progress about fabrication, characterization, and applications of the thin-film lithium niobate photonic crystal is reviewed. The performance parameters of the different devices are compared.

Vortex waves with orbital angular momentum (OAM) are a highly active research topic in various fields. In this paper, we design and investigate cylindrical metagratings (CMs) with an even number of unit cells that can efficiently achieve vortex localization and specific OAM selective conversion. The multifunctional manipulation of vortex waves and the new OAM conservation law have further been confirmed through analytical calculations and numerical simulations. In addition, we qualitatively and quantitatively determine the OAM range for vortex localization and the OAM value of vortex selective conversion and also explore the stability for performance and potential applications of the designed structure. This work holds potential applications in particle manipulation and optical communication.

Optical tweezers have proved to be a powerful tool with a wide range of applications. The gradient force plays a vital role in the stable optical trapping of nano-objects. The scalar method is convenient and effective for analyzing the gradient force in traditional optical trapping. However, when the third-order nonlinear effect of the nano-object is stimulated, the scalar method cannot adequately present the optical response of the metal nanoparticle to the external optical field. Here, we propose a theoretical model to interpret the nonlinear gradient force using the vector method. By combining the optical Kerr effect, the polarizability vector of the metallic nanoparticle is derived. A quantitative analysis is obtained for the gradient force as well as for the optical potential well. The vector method yields better agreement with reported experimental observations. We suggest that this method could lead to a deeper understanding of the physics relevant to nonlinear optical trapping and binding phenomena.

We demonstrate a high-Q perfect light absorber based on all-dielectric doubly-resonant metasurface. Leveraging bound states in the continuum (BICs) protected by different symmetries, we manage to independently manipulate the Q factors of the two degenerate quasi-BICs through dual-symmetry perturbations, achieving precise matching of the radiative and nonradiative Q factors for degenerate critical coupling. We achieve a narrowband light absorption with a >600 Q factor and a > 99% absorptance at λ0 = 1550 nm on an asymmetric germanium metasurface with a 0.2λ0 thickness. Our work provides a new strategy for engineering multiresonant metasurfaces for narrowband light absorption and nonlinear applications.

Polarizers have always been an important optical component for optical engineering and have played an indispensable part of polarization imaging systems. Metasurface polarizers provide an excellent platform to achieve miniaturization, high resolution, and low cost of polarization imaging systems. Here, we proposed freeform metasurface polarizers derived by adjoint-based inverse design of a full-Jones matrix with gradient-descent optimization. We designed multiple freeform polarizers with different filtered states of polarization (SOPs), including circular polarizers, elliptical polarizers, and linear polarizers that could cover the full Poincaré sphere. Note that near-unitary polarization dichroism and the ultrahigh polarization extinction ratio (ER) reaching 50 dB were achieved for optimized circular polarizers. The multiple freeform polarizers with filtered polarization state locating at four vertices of an inscribed regular tetrahedron of the Poincaré sphere are designed to form a full-Stokes parameters micropolarizer array. Our work provides a novel approach, we believe, for the design of meta-polarizers that may have potential applications in polarization imaging, polarization detection, and communication.

A designed arrangement of nanometer-sized holes in a thin dielectric film is presented to create a gradient-index plasmonic metasurface Gutman lens (PMGL) for controlling surface plasmon polaritons. We introduce two distinct designs for PMGL: one features a periodic rectangular hole array and the other features a dodecagonal quasicrystal array. Upon comparing their focusing properties, we find that, despite the superior rotational symmetry of the dodecagonal structure, the rectangular array outperforms in terms of the focusing properties of the Gutman lens.

We theoretically demonstrate electrically controlled light focusing using a tunable metasurface employing thin film lithium niobate (TFLN). The designed metasurface features a high-quality factor guided-mode resonance with an electrically controllable resonant wavelength, resulting in a high extinction ratio of transmittance at the operational wavelength by changing the applied voltage. A reconfigurable one-dimensional Fresnel zone plate with a focusing efficiency of around 15% has been realized through spatial modulation of transmitted light intensity, whose focal position can be electrically tunable in both longitudinal and lateral directions. Our approach reveals the great potential of metasurfaces using TFLN for electrically controlled light focusing.

Vector vortex beams (VVBs), novel structured optical fields that combine the polarization properties of vector beams and phase characteristics of vortex beams, have garnered widespread attention in the photonics community. Capitalizing on recently developed metasurfaces, miniaturized VVB generators with advanced properties have been implemented. However, metasurface-empowered VVB generators remain static and can only generate one pre-designed structured light. Here, we propose a kind of phase change metasurface for tunable vector beam generation by utilizing anisotropic Ge2Sb2Se4Te1 (GSST) unit cells with tunable phase retardation when GSST transits between two different phase states. By properly rotating the orientations of the tunable GSST unit structures that transit between quarter-wave plates and half-wave plates, we can effectively transform incident plane waves into vector beams with distinct topological charges and polarization states. When GSST is in the amorphous state, the designed metasurface can transmit circularly polarized light into VVBs. In the crystalline state, the same GSST metasurface converts linearly polarized light into second-order radially polarized (RP) and azimuthally polarized (AP) beams. Our phase-change metasurface paves the way for precise control over the polarization patterns and vortex characteristics of beams, thereby enabling the exact manipulation of beam structures through the alteration of their phase states.

The study of strong coupling between photonic cavities and excitons has brought about significant advances, varying from fundamental physics to applied science. However, there are several challenges hindering its further development, including obtaining photonic modes with both low room-temperature loss and high electric field (EF) enhancements, the difficulty of precisely transferring exciton materials into the photonic cavity, and the urgent need for additional manipulation approaches. In order to overcome these challenges simultaneously, we present a theoretical strong coupling system based on the chiral metasurfaces that are built by the excitonic van der Waals material of WSe2 and can support the quasi-bound states in the continuum (q-BIC) mode. The q-BIC mode can sustain EF enhancements over 80 times with loss smaller than 10 meV, and the strong coupling between q-BIC mode and WSe2 excitons can be naturally realized without material transferring. Furthermore, a large chirality beyond 0.98 can be obtained in this strong coupling system, making the circular polarization of excitation light an effective parameter to control the generation of coherent states in this metasurface system. Our results can benefit the further development of strong coupling research, shedding light onto the exploration of new quantum devices.

The diffractive optical element (DOE) is an important component of three-dimensional (3D) imaging systems based on structured light. In this work, we designed the metasurface-driven DOEs based on generalized Rayleigh–Sommerfeld diffraction theory to project large field of view (FOV) pseudo-random dot array for 3D imaging. We measured an efficiency of 61.04% and root-mean-square error (RMSE) of 0.45 for the 60° FOV sample and an efficiency of 42.96% and RMSE of 0.75 for the 144° FOV sample. Because the pattern is designed based on the generalized Rayleigh–Sommerfeld diffraction theory, the projected pattern is similar to the target pattern and has even intensity.

By its unparalleled capacity to manipulate optical parameters, metasurfaces demonstrate the ability to simultaneously manipulate the amplitude and phase of incident light. Exhibiting both near-field nanoprinting images and far-field holography images is a quintessential illustration of this capability. In preceding investigations, image multiplexing commonly transpires within the single polarization state or orthogonal polarization states, thereby exhibiting a deficiency in terms of information security when contrasted with the nonorthogonal polarization states. In this research, a multifunctional metasurface with the capability of exhibiting four-channel images has been proposed by using a nanobrick as a quarter-wave plate. Through the adjustment of the orientation angles of each nanobrick, nanoprinting can be displayed under both linearly and circularly polarized light. Building on this, the propagation phase is combined with the geometric phase to generate diverse phase delays, enabling the metasurface to be multiplexed under two nonorthogonal polarization states to achieve four-channel image displays. Intriguingly, bidirectional nanoprinting and bidirectional holography can be achieved by altering the direction of incidence polarization states. The proposed metasurface platform can open new possibilities for creating compact multifunctional optical devices, while also enhancing applications in multichannel image displays, information anticounterfeiting, and encryption.

All-dielectric metasurfaces are usually limited because of their static functionality and small scale. In this paper, we use an easy nanofabrication technique to fabricate all-dielectric metasurfaces with the advantages of having dynamic tunability and a large area. Using an anodized aluminum oxide (AAO) template as an evaporation mask, a large-area metasurface embedded in polydimethylsiloxane (PDMS) (>2 cm2) is fabricated. The metasurface exhibits remarkable electric dipole (ED) and magnetic dipole (MD) resonances. Based on the solvent-swelling effect of PDMS in 20% toluene, the ED/MD resonance peak shifts dynamically ∼40 nm to red. So far, to the best of our knowledge, a large-area metasurface embedded in PDMS and achieved by using the AAO template method has not appeared.

SrMoO3 (SMO) thin films are deposited on LaAlO3 substrates by magnetron sputtering. The effects of ambient temperature on the structural, electrical, and optical properties of the films are investigated. As the temperature increases from 23°C to 800°C, the SMO film exhibits high crystallinity and low electrical resistivity, and the real part of dielectric functions becomes less negative in the visible and near-IR wavelength range, and the epsilon near zero (ENZ) wavelength increases from 460 nm to 890 nm. The optical loss of the SMO film is significantly lower than that of Au, and its plasmonic performance is comparable to or even higher than TiN in the temperature range of 23°C to 600°C. These studies are critical for the design of high-temperature SMO-based plasmonic devices.

Noble metallic nanostructures with strong electric near-field enhancement can significantly improve nanoscale light–matter interactions and are critical for high-sensitivity surface-enhanced Raman spectroscopy (SERS). Here, we use an azimuthal vector beam (AVB) to illuminate the plasmonic tips circular cluster (PTCC) array to enhance the electric near-field intensity of the PTCC array, and then use it to improve SERS sensitivity. The PTCC array was prepared based on the self-assembled and inductive coupled plasmon (ICP) etching methods. The calculation results show that, compared with the linearly polarized beam (LPB) and radial vector beam excitations, the AVB excitation can obtain stronger electric near-field enhancement due to the strong resonant responses formed in the nanogap between adjacent plasmonic tips. Subsequently, our experimental results proved that AVB excitation increased SERS sensitivity to 10-13 mol/L, which is two orders of magnitude higher than that of LPB excitation. Meanwhile, the PTCC array had excellent uniformity with the Raman enhancement factor calculated to be ∼2.4×108. This kind of vector light field enhancing Raman spectroscopy may be applied in the field of sensing technologies, such as the trace amount detection.

An ultrathin angle-insensitive color filter enabling high color saturation and a wide color gamut is proposed by relying on a magnesium hydride-hydrogenated amorphous silicon (MgH2-a-Si:H) lossy dielectric layer. Based on effective medium theory, the MgH2-a-Si:H layer with an ultrathin thickness can be equivalent to a quasi-homogeneous dielectric layer with an effective complex refractive index, which can be tuned by altering the thickness of MgH2 to obtain the targeted value of the imaginary part, corresponding to the realization of high color saturation. It is verified that the proposed color filter offers highly enhanced color saturation in conjunction with a wide color gamut by introducing a few-nanometer thick MgH2 layer. As the MgH2-a-Si:H layer retains the advantages of high refractive index and tiny thickness, the proposed color filter exhibits large angular tolerance up to ±60°. In addition, MgH2 with an unstable property can interconvert with Mg under a dehydrogenation/hydrogenation reaction, which empowers the proposed color filter with dynamically tunable output color. The proposed scheme shows great promise in color printing and ultracompact display devices with high color saturation, wide gamut, large angular tolerance, and dynamic tunability.

We propose a chip-integratable cylindrical vector (CV) beam generator by integrating six plasmonic split ring resonators (SRRs) on a planar photonic crystal (PPC) cavity. The employed PPC cavity is formed by cutting six adjacent air holes in the PPC center, which could generate a CV beam with azimuthally symmetric polarizations. By further integrating six SRRs on the structure defects of the PPC cavity, the polarizations of the CV beam could be tailored by controlling the opening angles of the SRRs, e.g., from azimuthal to radial symmetry. The mechanism is governed by the coupling between the resonance modes in SRRs and PPC cavity, which modifies the far-field radiation of the resonance mode of the PPC cavity with the SRR as the nano-antenna. The integration of SRRs also increases the coupling of the generated CV beam with the free-space optics, such as an objective lens, promising its further applications in optical communication, optical tweezer, imaging, etc.

Metasurfaces have exhibited great capabilities to control electromagnetic waves, and many multifunctional metasurfaces were recently proposed. However, although angle-multiplexed meta-devices were successfully realized in reflection geometries, their transmission-mode counterparts are difficult to achieve due to the additional requirements. Here, we design and fabricate a transmissive angle-multiplexed meta-polarizer in the microwave regime based on a multilayer metasurface. Coupled-mode-theory analyses reveal that the device exhibits distinct angle-dependent transmissive responses under excitations with different polarizations, and such differences are further enhanced by multiple scatterings inside the device. Microwave experimental results are in good agreement with numerical simulations and theoretical analyses.

The metasurface is a platform with a small footprint and abundant functionalities. With propagation phase and geometric phase, polarization multiplexing is possible. However, different response behaviors of propagation phase and geometric phase to wavelength have not been fully employed to widen the capabilities of metasurfaces. Here, we theoretically demonstrate that metasurfaces can achieve near-field and far-field decoupling with the same polarization at two wavelengths. First, we found a set of pillars whose propagation phase difference between two wavelengths covers the full range of 2π. Then, by rotating pillars to control the geometric phase, the phase at both wavelengths can cover the full range of 2π. Finally, by means of interference principle, arbitrary independent coding for the near field and far field of dual wavelengths is realized. In addition, when the far-field function is focusing, the focused spot is close to the diffraction limit, and, when the NA of the lens is very small, the final output focal length is four times of initial input focal length. This work circumvents the strong wavelength-dependent limitation of planar devices and paves the way toward designing multi-wavelength and multi-functional metadevices for scenarios such as AR applications, fluorescence microscopy, and stimulated emission depletion microscopy.

Wide field-of-view (FOV) optics are widely used in various imaging, display, and sensing applications. Conventional wide FOV optics rely on complicated lens assembly comprising multiple elements to suppress coma and other Seidel aberrations. The emergence of flat optics exemplified by metasurfaces and diffractive optical elements (DOEs) offers a promising route to expand the FOV without escalating complexity of optical systems. To date, design of large FOV flat lenses has been relying upon iterative numerical optimization. Here, we derive, for the first time, to the best of our knowledge, an analytical solution to enable computationally efficient design of flat lenses with an ultra-wide FOV approaching 180°. This analytical theory further provides critical insights into working principles and otherwise non-intuitive design trade-offs of wide FOV optics.

Topological photonic states have promising applications in slow light, photon sorting, and optical buffering. However, realizing such states in non-Hermitian systems has been challenging due to their complexity and elusive properties. In this work, we have experimentally realized a topological rainbow in non-Hermitian photonic crystals by controlling loss in the microwave frequency range for what we believe is the first time. We reveal that the lossy photonic crystal provides a reliable platform for the study of non-Hermitian photonics, and loss is also taken as a degree of freedom to modulate topological states, both theoretically and experimentally. This work opens a way for the construction of a non-Hermitian photonic crystal platform, will greatly promote the development of topological photonic devices, and will lay a foundation for the real-world applications.

In this paper, we propose an ultrabroadband chiral metasurface (CMS) composed of S-shaped resonator structures situated between two twisted subwavelength gratings and dielectric substrate. This innovative structure enables ultrabroadband and high-efficiency linear polarization (LP) conversion, as well as asymmetric transmission (AT) effect in the microwave region. The enhanced interference effect of the Fabry–Perot-like resonance cavity greatly expands the bandwidth and efficiency of LP conversion and AT effect. Through numerical simulations, it has been revealed that the cross-polarization transmission coefficients for normal forward (-z) and backward (+z) incidence exceed 0.8 in the frequency range of 4.13 to 17.34 GHz, accompanied by a polarization conversion ratio of over 99%. Furthermore, our microwave experimental results validate the consistency among simulation, theory, and measurement. Additionally, we elucidate the distinct characteristics of ultrabroadband LP conversion and significant AT effect through analysis of polarization azimuth rotation and ellipticity angles, total transmittance, AT coefficient, and electric field distribution. The proposed CMS structure shows excellent polarization conversion properties via AT effect and has potential applications in areas such as radar, remote sensing, and satellite communication.

We propose and experimentally demonstrate a high quality (Q)-factor all-silicon bound state in the continuum (BIC) metasurface with an imperforated air-hole array. The metasurface supports two polarization-insensitive BICs originated from guided mode resonances (GMRs) in the frequency range of 0.4 to 0.6 THz, and the measured Q-factors of the two GMRs are as high as 334 and 152, respectively. In addition, the influence of the thickness of the silicon substrate on the two resonances is analyzed in detail. The proposed all-silicon THz metasurface has great potential in the design and application of high-Q metasurfaces.

Hybrid metal-dielectric structures combine the advantages of both metal and dielectric materials, enabling high-confined but low-loss magnetic and electric resonances through deliberate arrangements. However, their potential for enhancing magnetic emission has yet to be fully explored. Here, we study the magnetic and electric Purcell enhancement supported by a hybrid structure composed of a dielectric nanoring and a silver nanorod. This structure enables low Ohmic loss and highly-confined field under the mode hybridization of magnetic resonances on a nanoring and electric resonances on a nanorod in the optical communication band. Thus, the 60-fold magnetic Purcell enhancement and 45-fold electric Purcell enhancement can be achieved. Over 90% of the radiation can be transmitted to the far field. For the sufficiently large Purcell enhancement, the position of emitter has a tolerance of several tens of nanometers, which brings convenience to experimental fabrications. Moreover, an array formed by this hybrid nanostructure can further enhance the magnetic Purcell factors. The system provides a feasible option to selectively excite magnetic and electric emission in integrated photonic circuits. It may also facilitate brighter magnetic emission sources and light-emitting metasurfaces with a more straightforward design.

In this Letter, we report on the investigations of nonlinear scattering of plasmonic nanoparticles by manipulating ambient environments. We create different local thermal hosts for gold nanospheres that are immersed in oil, encapsulated in silica glass and also coated with silica shells. In terms of regulable effective thermal conductivity, silica coatings are found to contribute significantly to scattering saturation. Benefitting from the enhanced thermal stability and the reduced plasmonic coupling provided by the shell-isolated nanoparticles, we achieve super-resolution imaging with a feature size of 52 nm (λ/10), and we can readily resolve pairs of nanoparticles with a gap-to-gap distance of 5 nm.

We report the modulation of epsilon-near-zero (ENZ) wavelength and enhanced third-order nonlinearity in indium tin oxide (ITO)/Au multilayer films. The samples consisting of five-layer 40 nm ITO films spaced by four-layer ultrathin Au films of different thickness, i.e., ITO(40 nm)/[Au(x)/ITO(40 nm)]4, were prepared by magnetron sputtering at room temperature. The ENZ wavelength in the multilayer films is theoretically calculated and experimentally confirmed. The nonlinear refractive index and nonlinear absorption coefficient of the samples of x=0, 2, 3, 4 nm were determined using the Z-scan method at a wavelength of 1.064 µm. The large nonlinear refractive index n2=1.12×10-13 m2/W and nonlinear absorption coefficient β=-1.78×10-7 m/W in the sample of x = 4 nm are both four times larger than those in the single-layer ITO film. The large optical nonlinearity due to the ENZ enhancement and carrier concentration is discussed. The results indicate that the ITO/Au multilayer films are promising for advanced all-optical devices.

The chiral feature of an optical field can be evaluated by the parameter of g-factor enhancement, which is helpful to enhance chiroptic signals from a chiral dipole. In this work, the superchiral spot has been theoretically proposed in metal-insulator-metal waveguides. The g-factor enhancement of the superchiral spot can be enhanced by 67-fold more than that of circularly polarized light, and the spot is confined in the deep wavelength scale along each spatial dimension. Moreover, the position of the superchiral spot can be tuned by manipulating the incident field. The tunable superchiral spot may find applications in chiral imaging and sensing.

Perfect absorbers (PAs) are devices that can efficiently absorb electromagnetic waves. Great attention has been attracted since metamaterial PAs (MPAs) were first proposed in 2008. In recent years, with the development of nanophotonics and the improvement of nanomanufacturing technology, considerable progresses have been achieved in designing MPAs using new materials and new structures. In this review, we summarized first the latest developments of PAs from five directions: dual-band, multi-band, wideband, narrow-band, and tunable light absorption. The shortcomings of the previous PAs and the latest improvements were introduced as well. Then, the application of perfect absorption in solar cells, sensors, switches, and structural colors was discussed. Finally, we presented the main challenges and prospects in these fields. Novel PAs for applications in a wide field of opto-electronic devices will continuously progress with breakthrough advances in absorbers related technology and science.

Quantum efficiency is a critical piece of information of a quantum emitter and regulates the emitter’s fluorescence decay dynamics in an optical environment through the Purcell effect. Here, we present a simple way to experimentally probe fluorescence quantum efficiency of single dibenzoterrylene molecules embedded in a thin anthracene microcrystal obtained through a co-sublimation process. In particular, we correlate the fluorescence lifetime change of single dibenzoterrylene molecules with the variation of the matrix thickness due to natural sublimation. With the identification of the molecule emission dipole orientation, we could deduce the near-unity intrinsic quantum efficiency of dibenzoterrylene molecules in the anthracene matrix.

We introduce a simple one-dimensional (1D) structure in the design of 1D color splitters (1D-CSs) with RGB unit cells for color imaging and propose a single-to-double-layer design in 1D-CSs. Based on inverse design metasurfaces, we demonstrate numerically a single-layer 1D-CS with a full-color efficiency of 46.2% and a double-layer 1D-CS with a full-color efficiency of 48.2%; both of them are significantly higher than that of traditional color filters. Moreover, we demonstrate a 1D-CS that has application value by evaluating the double-layer 1D-CS’s performances in terms of incident angle sensitivity, polarization angle sensitivity, and assembly tolerance.

Photon nanosieves, as amplitude-type metasurfaces, have been demonstrated usually in a transmission mode for optical super-focusing, display, and holography, but the sieves with subwavelength size constrain optical transmission, thus leading to low efficiency. Here, we report reflective photon nanosieves that consist of metallic meta-mirrors sitting on a transparent quartz substrate. Upon illumination, these meta-mirrors offer the reflectance of ∼50%, which is higher than the transmission of visible light through diameter-identical nanoholes. Benefiting from this configuration, a meta-mirror-based reflective hologram has been demonstrated with good consistence between theoretical and experimental results over the broadband spectrum from 500 nm to 650 nm, meanwhile exhibiting total efficiency of ∼7%. Additionally, if an additional high-reflectance layer is employed below these meta-mirrors, the efficiency can be enhanced further for optical anti-counterfeiting.

Palladium-based hydrogen sensors have been typically studied due to the dielectric function that changes with the hydrogen concentration. However, the development of a reliable, integral, and widely applicable hydrogen sensor requires a simple readout mechanism and an optimization of the fast detection of hydrogen. In this work, optical fiber hydrogen sensing platforms are developed using an optimized metasurface, which consists of a layer of palladium nanoantennas array suspended above a gold mirror layer. Since the optical properties of these palladium nanoantennas differ from the traditional palladium films, a high reflectance difference can be achieved when the sensor based on the metasurface is exposed to the hydrogen atmosphere. Finally, the optimized reflectance difference ΔR of ∼0.28 can be obtained when the sensor is exposed in the presence of hydrogen. It is demonstrated that this integrated system architecture with an optimized palladium-based metasurface and a simple optical fiber readout system provides a compact and light platform for hydrogen detection in various working environments.

Phase-modulated metasurfaces that can implement the independent manipulation of co- and cross-polarized output waves under circularly polarized (CP) incidence have been proposed. With this, we introduce one particular metasurface composed of meta-atoms with a phase difference of 2π/3 to generate specific elliptically polarized waves under various polarized incidences. Furthermore, a metasurface composed of these above meta-atoms and the meta-atoms with a phase difference of π/3 arranged in a certain rule can realize polarization conversion function between linearly polarized and CP states. The designs shed new light on multifarious optical devices and may further promote the development of metasurface polarization optics.

The cylindrical vector beam (CVB) has been extensively studied in recent years, but detection of CVBs with on-chip photonic devices is a challenge. Here, we propose and theoretically study a chiral plasmonic lens structure for CVB detection. The structure illuminated by a CVB can generate single plasmonic focus, whose focal position depends on the incident angle and the polarization order of CVB. Thus, the incident CVB can be detected according to the focal position and incident angle and with a coupling waveguide to avoid the imaging of the whole plasmonic field. It shows great potential in applications including CVB-multiplexing integrated communication systems.

A counter-surface plasmon polariton lens (CSPPL) is proposed to perform stable nanoparticle trapping by providing up to 120kbT optical potential depth. The optical potential depth is related to the incident angle and phase difference of the light incident on two gratings of CSPPL. The depth of optical potential can be manipulated with negligible displacement by the incident angle less than 20°. Both the depth and the center position of the optical potential well can be manipulated by the incident phase difference. The study of stable and manipulatable optical potential on the CSPPL promotes the integration of optical tweezers.

The optical vortex beam has widely been studied and used because of its unique orbital angular momentum (OAM). To generate and control OAM in compact and integrated systems, many metallic metasurface devices have been proposed, however, most of them suffer from the low efficiency. Here, we propose and experimentally verify a high-efficiency monolayer metallic metasurface composed of semicircular nano-grooves distributed with detour phase. The metasurface can generate single or an array of OAM with spin-sensitive modulation and achieve the maximum efficiency of 60.2% in theory and 30.44% in experiment. This work has great potential in compact OAM detection and communication systems.

Lithium niobate (LN) metasurfaces have emerged as a new platform for manipulating electromagnetic waves. Here, we report a fabrication technique for LN nano-grating metasurfaces by combining focused ion beam (FIB) milling with inductively coupled plasma reactive ion etching (ICP-RIE). Steep sidewalls with angles larger than 80° are achieved. Sharp quasi-bound states in the continuum are observed from our metasurfaces. The measured transmission spectra show good agreement with the numerical simulations, confirming the high quality of the fabricated metasurfaces. Our technique can be applied to fabricate the LN metasurfaces with sharp resonances for various applications in optical communications, on-chip photonics, laser physics, sensing, and so on.

A stimulated emission depletion is capable of breaking the diffraction limit by exciting fluorescent molecules with a solid Gaussian beam and quenching the excited molecules with another donut beam through stimulated emission. The coincidence degree of these two beams in three dimensions will significantly influence the spatial resolution of the microscope. However, the conventional alignment approach based on raster scanning of gold nanoparticles by the two laser beams separately suffers from a mismatch between fluorescence and scattering modes. To circumvent the above problems, we demonstrate a fast alignment design by scanning the second beam over the fabricated sample, which is made of aggregation-induced emission (AIE) dye resin. The relative positions of solid and donut laser beams can be represented by the fluorescent AIE from the labeled spots in the dye resin. This design achieves ultra-high resolutions of 22 nm in the x/y relative displacement and 27 nm in the z relative displacement for fast spatial matching of the two laser beams. This study has potential applications in scenarios that require the spatial matching of multiple laser beams, and the field of views of different objectives, for example, in a microscope with high precision.

Light beams carrying orbital angular momentum (OAM) have inspired various advanced applications, and such abundant practical applications in turn demand complex generation and manipulation of optical vortices. Here, we propose a multifocal graphene vortex generator, which can produce broadband angular momentum cascade containing continuous integer non-diffracting vortex modes. Our device naturally embodies a continuous spiral slit vortex generator and a zone plate, which enables the generation of high-quality continuous vortex modes with deep depths of foci. Meanwhile, the generated vortex modes can be simultaneously tuned through incident wavelength and position of the focal plane. The elegant structure of the device largely improves the design efficiency and can be fabricated by laser nanofabrication in a single step. Moreover, the outstanding property of graphene may enable new possibilities in enormous practical applications, even in some harsh environments, such as aerospace.

Metasurfaces are ultrathin metamaterials constructed by planar meta-atoms with tailored electromagnetic responses. They have attracted tremendous attention owing to their ability to freely control the propagation of electromagnetic waves. With active elements incorporated into metasurface designs, one can realize tunable and reconfigurable metadevices with functionalities controlled by external stimuli, opening up a new platform to dynamically manipulate electromagnetic waves. In this article, we review the recent progress on tunable and reconfigurable metasurfaces, focusing on their operation principles and practical applications. We describe the approaches to the engineering of reconfigurable metasurfaces categorized into different classes based on the available active materials or elements, which can offer uniform manipulations of electromagnetic waves. We further summarize the recent achievements on programmable metasurfaces with constitutional meta-atoms locally tuned by external stimuli, which can dynamically control the wavefronts of electromagnetic waves. Finally, we discuss time-modulated metasurfaces, which are meaningful to exploit the temporal dimension by applying a dynamic switching of the coding sequence. The review is concluded by our outlook on possible future directions and existing challenges in this fast developing field.

We proposed a multifunctional terahertz metasurface based on a double L-shaped pattern and a vanadium dioxide (VO2) film separated by polyimide. When the VO2 film is an insulator, a dual-band electromagnetically induced transparency effect is obtained, and the physical mechanism is investigated based on the current distribution and “two-particle” model. When the VO2 film is a metal, a dual-band linear-to-circular polarization converter, in which the y-polarized linear wave can be effectively converted to left-handed circularly polarized (LCP) and right-handed circularly polarized simultaneously in different bands, can be achieved. By arranging the metal pattern rotating 30°, a multifunctional antenna can be obtained. When the VO2 is an insulator, the radiation of the LCP wave is divided into four beams, with two beams reflected and two beams transmitted. When the VO2 is in the metallic state, we can only get the co-polarized reflected wave with a 21° angle. Moreover, in our design, the VO2 film does not need lithography to obtain certain patterns, which improves the convenience of fabrication and experiment. Our design opens a new way for the development of multifunctional terahertz devices and has potential applications in the terahertz communication field.

Metalenses are expected to play an increasingly important role in miniaturized and integrated optical imaging components/systems. However, devising broadband achromatic metalenses with high focusing efficiencies is still quite challenging. In this work, we proposed an aperture-shared partition phase cooperative manipulation approach for designing a high-efficiency broadband achromatic metalens composed of two concentric sub-metalenses. As a proof-of-concept, an achromatic polarization-independent metalens is successfully designed for the visible and near-infrared range from 450 nm to 1400 nm with the focusing efficiency over 70% for the wavelength range of 600 nm to 1400 nm. The approach reported here provides a possibility for designing a high-performance metalens, which has great potential applications in integrated optics.

We introduce a nanoplasmonic isolator that consists of a cylindrical resonator placed close to a metal-dielectric-metal (MDM) waveguide. The material filling the waveguide and resonator is a magneto-optical (MO) material, and the structure is under an externally applied static magnetic field. We theoretically investigate the properties of the structure and show that the cavity mode without MO activity splits into two modes when the MO activity is present. In addition, we find that the presence of the MDM waveguide leads to a second resonance due to the geometrical asymmetry caused by the existence of the waveguide. We also show that, when MO activity is present, the cavity becomes a traveling wave resonator. Thus, the transmission of the structure depends on the direction of the incident light, and the proposed structure operates as an optical isolator.

The excitation of a surface-plasmon-polariton (SPP) wave guided by a columnar thin film (CTF) deposited on a one-dimensional metallic surface-relief grating was investigated for sensing the refractive index of a fluid infiltrating that CTF. The Bruggemann homogenization formalism was used to determine the relative permittivity scalars of the CTF infiltrated by the fluid. The change in the refractive index of the fluid was sensed by determining the change in the incidence angle for which an SPP wave was excited on illumination by a p-polarized plane wave, when the plane of incidence was taken to coincide with the grating plane but not with the morphologically significant plane of the CTF. Multiple excitations of the same SPP wave were found to be possible, depending on the refractive index of the fluid, which can help increase the reliability of results by sensing the same fluid with more than one excitation of the SPP wave.

In this paper, we experimentally demonstrate ultrafast optical control of slow light in the terahertz (THz) range by combining the electromagnetically induced transparency (EIT) metasurfaces with the cut wire made of P+-implanted silicon with short carrier lifetime. Employing the optical-pump THz-probe spectroscopy, we observed that the device transited from a state with a slow light effect to a state without a slow light effect in an ultrafast time of 5 ps and recovered within 200 ps. A coupled oscillator model is utilized to explain the origin of controllability. The experimental results agree very well with the simulated and theoretical results. These EIT metasurfaces have the potential to be used as an ultrafast THz optical delay device.

We show that inhomogeneous waveguides of slowly varied parity-time (PT) symmetry support localized optical resonances. The resonance is closely related to the formation of exceptional points separating exact and broken PT phases. Salient features of this kind of non-Hermitian resonance, including the formation of half-vortex flux and the discrete nature, are discussed. This investigation highlights the unprecedented uniqueness of field dynamics in non-Hermitian systems with many potential adaptive applications.

Plasmonic structural colors have plenty of advantages over traditional colors based on colorants. The pulsed laser provides an important method generating plasmonic structural colors with high efficiency and low cost. Here, we present plasmonic color printing Al nanodisc structures through curvature-driven shape transition. We systematically study the mechanism of morphologic evolution of the Al nanodisc below the thermal melting threshold. A multi-pulse-induced accumulated photothermal effect and subsequent curvature-driven surface atom diffusion model are adopted to explain the controllable shape transition. The shape transition and corresponding plasmonic resonances of the nanodisc can be independently and precisely modulated by controlled irradiations. This method opens new ways towards high-fidelity color prints in a highly efficient and facile laser writing fashion.

Phase carried by two orthogonal polarizations can be manipulated independently by controlling both the geometric size and orientation of the dielectric nanopost. With this characteristic, we demonstrate a novel multifunctional metasurface, which converts part of the incident linearly polarized light into its cross-polarization and encodes the phase of the two orthogonal polarizations independently. A beam splitter and a bifocal metalens were realized in a single-layer dielectric metasurface by this approach. We fabricated the bifocal metalens and demonstrated that two focal spots in orthogonal polarizations can be separated transversely or longitudinally at will. The proposed approach shows a new route to design multifunctional metasurfaces with various applications in holography and three-dimensional display.

A retroreflector that reflects light along its incident direction has found numerous applications in photonics, but the available metasurface schemes suffer from the issue of narrow bandwidth and/or a single angle of incidence. Here, a retroreflector using double layers of achromatic gradient metasurfaces is reported, which can realize retroreflection over a continuous range of incidence angles within a wide spectral band. The first metasurface serves as a transmissive achromatic lens that performs a broadband spatial Fourier transform and its inverse, while the second metasurface works as a reflective achromatic lens that undergoes wavelength- and position-dependent phase dispersions. Using this design strategy, a near-infrared retroreflector comprised of silicon nanopillars with the cross sections of square pillars and square holes is numerically demonstrated, providing a high-performance retroreflection for polarization-independent incident light waves over a continuous range of incidence angles from 0° to 16° within an extremely broad wavelength range between 1.35 and 1.95 μm. The scheme herein can offer a design strategy of broadband retroreflectors and impact numerous photonics applications.

The strong coupling between vibrational modes of molecules and surface plasmon resonance (SPR) modes in graphene makes them an ideal platform for biosensor techniques. In this paper, a new optical biosensor for molecule detection based on silver metallic nanoparticles (MNPs) and graphene/gold MNPs in a terahertz frequency range is achieved. It is established that the nonlinear electrical properties of graphene can play a major role in realizing a biosensor for molecule detection. The performance parameters of the proposed device are reported with respect to the chemical potential μ of graphene, noting that the sensitivity of our device passes from 255 nm/RIU (nanometers/refractive index unit) for μ=1.21 eV to 2753 nm/RIU for μ=0.21 eV. Finally, this structure exhibits an optical sensing region that can be adjusted to meet the requirements of optical detection.

We investigate the strong coupling from 5,5’,6,6’-tetrachloro-1,1’-diethyl-3,3’-di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC) molecules near pure nano-triangular Ag prisms or Ag@Au hollow nanoshells. When TDBC molecules are deposited on pure Ag nanoprisms or Ag@Au hollow nanoshells with the plasmonic resonance peak, matching very closely with the absorption band of TDBC J-aggregates, obvious Rabi splitting can be observed due to the strong coupling regime. Meanwhile, the photoluminescence intensity decreased with the increasing of the temperature, verifying the decreasing plasmon–exciton coupling interaction in the higher temperature. Our experimental results are coincident with the simulation results calculated by finite-difference time-domain method.

Multifrequency superscattering is a phenomenon in which the scattering cross section from a subwavelength object simultaneously exceeds the single-channel limit at multiple frequency regimes. Here, we achieve simultaneously, within a graphene-coated subwavelength structure, multifrequency superscattering and superscattering shaping with different engineered scattering patterns. It is shown that multimode degenerate resonances at multiple frequency regimes appearing in a graphene composite structure due to the peculiar dispersion can be employed to resonantly overlap electric and magnetic multipoles of various orders, and, as a result, effective multifrequency superscattering with different engineered angular patterns can be obtained. Moreover, the phenomena of multifrequency superscattering have a high tolerance to material losses and some structural variations. Our work should anticipate extensive applications ranging from emission enhancing, energy harvesting, and antenna design with improved sensitivity and accuracy due to multifrequency operation.

Recently reported plasmon-induced transparency (PIT) in metamaterials endows the optical structures in classical systems with quantum optical effects. In particular, the nonreconfigurable nature in metamaterials makes multifunctional applications of PIT effects in terahertz communications and optical networks remain a great challenge. Here, we present an ultrafast process-selectable modulation of the PIT effect. By incorporating silicon islands into diatomic metamaterials, the PIT effect is modulated reversely, depending on the vertical and horizontal configurations, with giant modulation depths as high as 129% and 109%. Accompanied by the enormous switching of the transparent window, remarkable slow light effect occurs.

Nanogap plasmonic structures with strong coupling between separated components have different responses to orthogonal-polarized light, giving rise to giant optical chirality. Here, we proposed a three-dimensional (3D) nanostructure that consists of two vertically and twistedly aligned nanogaps, showing the hybridized charge distribution within 3D structures. It is discovered that the structure twisted by 60° exhibits plasmonic coupling behavior with/without gap modes for different circular-polarized plane waves, showing giant chiral response of 60% at the wavelength of 1550 nm. By controlling the disk radius and the insulator layer, the circular dichroism signal can be further tuned between 1538 and 1626 nm.