High-performance Metasurfaces Research Articles

Dielectric terahertz metasurface governed by symmetry-protected BIC for ultrasensitive sensing

The non-radiative bound states in the continuum (BIC) have attracted much attention in achieving theoretically infinite quality (Q) factor. In this paper, a dielectric terahertz metasurface with C 4v symmetry is proposed, and a toroidal dipole resonance is easily obtained under incident plane wave. Moreover, by slightly tuning the asymmetry parameter δ to break the in-plane symmetry of the structure (side length perturbation), a magnetic dipole BIC mode radiates as quasi-BIC (QBIC) with extremely narrow linewidth and ultrahigh Q of 1.2 × 104 at δ = 0.4 μm. It shows significant performance in THz sensing with the sensitivity around 446 GHz/RIU and figure of merit (FoM) up to 2267. The designed metasurface in the case of symmetry-breaking by position perturbation also achieves ultrasensitive sensing. Additionally, the effects of geometric parameters on the resonance modes have been comprehensively investigated. Our work provides a route to design symmetry-protected BIC metasurface with simple structure, and the Q factor as well as resonant frequency can be controlled using a single geometric parameter, which may facilitate designing high-performance metasurface in sensing applications.

Metatronics-inspired high-selectivity metasurface filter

Abstract Metatronic circuits extend the concept of subwavelength-scaled lumped circuitry from electronics to optics and photonics, providing a distinctive design paradigm for versatile optical nanocircuits. Here, based on the design of optical nanocircuits using metatronics concept, we introduce a general approach for dispersion synthesis with metasurface to achieve high-selectivity filtering response. We theoretically and numerically demonstrate how to achieve basic circuit lumped elements in metatronics by tailoring the dispersion of metasurface at the frequency of interest. Then, following the Butterworth filter design method, the meticulously designed metasurface, acting as lumped elements, are properly stacked to achieve a near-rectangular filtering response. Compared to the conventional designs, the proposed approach can simultaneously combine high selectivity with the theoretically widest out-of-band rejection in a considerably simple and time-efficient manner of circuit assembly, similar to electronic circuits, without extensive numerical simulations and complex structures. This dispersion synthesis approach provides exciting possibilities for high-performance metasurface design and future integrated circuits and chips.

Dual-Circularly Polarized and Flexible Metasurface Antenna Based on Graphene Assembled Film for Satellite Communications.

Traditional metal materials used in electronic devices are often problematic due to issues like bending resistance, oxidation leading to failure, and environmental pollution. To address these challenges, microwave electronic devices are constantly casting around for metal substitute materials with additional characteristics such as flexibility, anticorrosive, and eco-friendly. However, finding suitable materials that are accessible for radiofrequency (RF) applications is a difficult yet promising task. Consequently, a high-performance metasurface antenna based on highly conductive graphene films for satellite communications is developed in this paper. The proposed graphene assembled films (GAFs) have a conductivity of up to 1.13 × 106 S/m. Simulation and measurement results confirm the excellent performance of the designed antenna. Comparative experiments are also conducted on salt spray and mechanical bending between GAF antenna patterns and copper foil counterparts, further demonstrating the outstanding flexible property and corrosion resistance performance of prepared GAFs.

An optoelectronic Coupler based on graphene patterns and SU-8 photoresist

PurposeThe purpose of this research is to efficiently separate incident terahertz (THz) waves into distinct transmission and reflection channels by minimizing the absorption ratio. So, the optical systems operating within the THz frequency range can developed. To achieve a multi-band response, four different periodic arrays of graphene patterns are used. These arrays are strategically stacked on both sides of three SU-8 photoresists, serving as dielectric materials. Consequently, each layer exhibits a unique influence on the device's response, and by applying four external bias voltages, the behavior of the device can be precisely controlled and adjusted.Design/methodology/approachA novel optoelectronic device operating in the THz frequency range is introduced, using periodic arrays of graphene patterns and SU-8 photoresist dielectrics. The design of this device is based on meta-surface principles, using both the equivalent circuit model (ECM) and transmission line concept. The output of the device is a THz coupler implemented by analyzing the reflection and transmission channels. The structure is characterized using the ECM and validated through comprehensive full-wave simulations. By representing the electromagnetic phenomenon with passive circuit elements, enabling the calculation of absorption, reflection and transmission through the application of the theory of maximum power transfer.FindingsBased on simulation results and theoretical analysis, the proposed device exhibits sensitivity to gate biasing, enabling efficient reflection and transmission of THz waves. The device achieves reflection and transmission peaks exceeding across the five distinct THz bands 90%, and its behavior can be tuned by external gate biasing. Moreover, the device's sensitivity to variations in geometrical parameters and chemical potentials demonstrates its reliable performance. With its outstanding performance, this high-performance meta-surface emerges as an ideal candidate for fundamental building blocks in larger optical systems, including sensors and detectors, operating within the THz frequency band.Originality/valueThe proposed device covers a significant portion of the THz gap through the provision of five adjustable peaks for reflection and transmission channels. Additionally, the ECM and impedance matching concept offers a simplified and time-efficient approach to designing the meta-surface. Leveraging this approach, the proposed device is effectively represented using passive circuit elements such as inductors, capacitors and resistors, while its performance is validated through the utilization of the finite element method (FEM) as a full-wave simulation tool. This combination of circuit modeling and FEM simulation contributes to the robustness and accuracy of the device's performance evaluation.

Switchable multi-wavelength coherent polarization manipulation component based on single-layered hollowed S-shaped metasurface

High-performance metasurface polarization manipulation components have important applications in the fields of polarization imaging, polarization detection, and communication. Single-layered metasurface components have attracted a great deal of interest, but it is still a challenge to realize high-efficiency and dynamic polarization manipulation. Combined with coherent control techniques, we propose a single-layered hollowed S-shaped metasurface design that may accomplish multi-wavelength polarization switching in the optical to near-infrared band. The single-layered metasurface enables a real-time switchable effect between dual-wavelength and single-wavelength polarization properties by controlling the phase difference between two coherent counter-propagating lights. The physical mechanism of switchable multi-wavelength coherent polarization manipulation is elucidated based on the electric field distribution of the hollowed S-shaped structure. The effect of metal layer thickness on the switchable coherent polarization properties is investigated in detail. The proposed hollowed S-shaped metasurface significantly improves the polarization performance of the single-layered structure, which is expected to facilitate the development of switchable and compact polarization components.

An Efficient Design Method for a Metasurface Polarizer with High Transmittance and Extinction Ratio

In fields such as polarization navigation and imaging, there is a demand for polarizers to simultaneously possess a high transmittance and extinction ratio. However, most studies focus solely on a single metric for polarizer design. To address this issue, this paper proposed a metasurface polarizer with a transition layer and its efficient design method, aiming to enhance the polarizers’ transmittance and extinction ratio performance simultaneously. An all-dielectric metasurface polarizer with a TiO2 transition layer was designed, and a tandem neural network was constructed by combining forward prediction and inverse design networks. The network is successfully trained by utilizing structural parameters and spectral response Tx and Ty datasets collected through simulation software. A high-performance metasurface polarizer was designed under the desired metrics with a transmittance of 90% and an extinction ratio of 40 dB. The proposed method in this paper made progress in the comprehensive optimization of polarizer performance. It is more accurate and efficient compared to other design methods and is better suited for practical applications.

Genetic algorithm assisted meta-atom design for high-performance metasurface optics

Genetic algorithm assisted meta-atom design for high-performance metasurface optics

Inverse-designed metasurfaces for highly saturated transmissive colors

Advances in augmented reality and virtual reality platforms have sparked interest in high-performance metasurface color filters with elevated resolution, saturation, and durability. However, the predominant use of either dielectrics or metals prevents the realization of efficient “transmissive” color filters for displays. Here, we propose a novel, to our knowledge, approach combining dielectric and metallic components, optimizing complex structures using inverse design with height restrictions on the layers of red, green, and blue. The optimized structure achieved full coverage of the sRGB color space and surpassed 70% efficiency. Experimental validation demonstrated the potential of the inverse design for enhancing the performance of complex structures.

End-to-End Diverse Metasurface Design and Evaluation Using an Invertible Neural Network.

Employing deep learning models to design high-performance metasurfaces has garnered significant attention due to its potential benefits in terms of accuracy and efficiency. A deep learning-based metasurface design framework typically comprises a forward prediction path for predicting optical responses and a backward retrieval path for generating geometrical configurations. In the forward design path, a specific geometrical configuration corresponds to a unique optical response. However, in the inverse design path, a single performance metric can correspond to multiple potential designs. This one-to-many mapping poses a significant challenge for deep learning models and can potentially impede their performance. Although representing the inverse path as a probabilistic distribution is a widely adopted method for tackling this problem, accurately capturing the posterior distribution to encompass all potential solutions remains an ongoing challenge. Furthermore, in most pioneering works, the forward and backward paths are captured using separate models. However, the knowledge acquired from the forward path does not contribute to the training of the backward model. This separation of models adds complexity to the system and can hinder the overall efficiency and effectiveness of the design framework. Here, we utilized an invertible neural network (INN) to simultaneously model both the forward and inverse process. Unlike other frameworks, INN focuses on the forward process and implicitly captures a probabilistic model for the inverse process. Given a specific optical response, the INN enables the recovery of the complete posterior over the parameter space. This capability allows for the generation of novel designs that are not present in the training data. Through the integration of the INN with the angular spectrum method, we have developed an efficient and automated end-to-end metasurface design and evaluation framework. This novel approach eliminates the need for human intervention and significantly speeds up the design process. Utilizing this advanced framework, we have effectively designed high-efficiency metalenses and dual-polarization metasurface holograms. This approach extends beyond dielectric metasurface design, serving as a general method for modeling optical inverse design problems in diverse optical fields.

Transmissive Pancharatnam-Berry metasurfaces with stable amplitude and precise phase modulations using dartboard discretization configuration.

Metasurfaces are ultra-thin artificial structures capable of flexibly manipulating electromagnetic (EM) waves. Among various applications, phase modulation of electromagnetic (EM) waves using metasurfaces holds great significance. The Pancharatnam-Berry (P-B) metasurfaces provides a complete 2π phase modulation by simply rotating the meta-atom. However, the fixed lattice in rotation employed by traditional P-B metasurfaces often results in unstable amplitude and imprecise P-B phase, leading to performance degradation. In this work, we demonstrate transmissive P-B metasurfaces with stable amplitude and precise phase modulation. To ensure stable amplitude and precise P-B phase, we adopt a dartboard discretization configuration with a hexagonal lattice for the meta-atom design. By applying topology optimization to the encoding sequence formed by surface pixels and dimensions, we significantly enhancing the high transmissive bandwidth of the optimized meta-atom. Furthermore, the optimized meta-atom exhibits a stable amplitude and precise P-B phase for each rotation angle. As proof-of-concept demonstrations, two metasurfaces for single and multiplexed vortex beams generating are designed utilizing the optimized meta-atom. Both the simulated and measured results indicate high mode purity of generated vortex beams. The design method can also be readily extended to other high performance metasurfaces with stable amplitude and precise phase manipulations, which can enhance the efficiency and capacity of metasurface-assisted holographic imaging and 6 G wireless communication systems.

Optimally designed tunable phase change material-based narrowband perfect absorber

In recent years, there has been a growing interest in active metasurfaces. In particular, phase change material-based metasurfaces offering all-optical reconfigurability are being explored. Despite recent progress, further improvement in device reconfiguration energies and optical contrast achievable between the amorphous and crystalline states is desirable. In this work, we demonstrate that using a mirror-backed chalcogenide-based narrowband perfect absorber metasurface can significantly improve the device’s reflection contrast at much lower energies than its mirrorless case. By considering a GST225 metasurface operating in the near IR, our systematic numerical study finds improved reflection contrast (up to −32 dB, Q-factor 19.22 compared with 9.59 dB, Q-factor 11 for the mirrorless case). For the mirrored case, the thermal study finds faster crystallization (up to 6 times) at reduced reconfiguration thresholds (72 times lower) compared with the mirrorless case. This results in a more than 2 orders of magnitude higher device figure of merit [defined as the change in reflection contrast (in dB) to a corresponding change in optical energy (in nJ)] compared with the mirrorless case. The results are promising for high-performance metasurfaces at reduced switching energies.

Design of metasurfaces with decoupled amplitude and phase response for spatial light modulation.

Spatial light modulators based on metasurfaces have attracted great attention due to their abilities of amplitude and phase modulation. However, the traditional one degree of freedom (1-DOF) tunable metasurfaces are limited by incomplete phase coverage and coupled amplitude and phase modulation. Here, we propose an optimization method for 2-DOF tunable metasurfaces within the framework of temporal coupled mode theory. As a validation of the proposed method, we present a germanium antimony tellurium (GST)-alloy-based 2-DOF tunable reflective metasurface. Full-wave simulation shows that independent modulation of amplitude and phase is realized with full phase coverage and amplitude range from 0 to 0.55. Our proposed design scheme for a 2-DOF tunable metasurface may facilitate the development of high-performance metasurface devices.

Multifunctional Full-Space Metahologram Employing a Monolayer Phase-Encoding Metasurface

Full-space metasurfaces exhibit considerable capacities in the fields of electromagnetic wave-front tailoring due to their functional and spatial integration, and extensive applications in modern communication systems. However, almost all metasurfaces for wave-front tailoring are implemented by multilayer dielectric cascade structures, which not only present imperfections in the cost and manufacturing process, but are also detrimental to integration. As such, it is highly desirable to actualize a high-performance metasurface that merely relies on elaborated designs and simple manufacturing process. This study combines frequency multiplexing, spatial channel multiplexing, and polarization multiplexing to propose a full-space metahologram with four different information channels based on a thin double-face copper-cladded metasurface. Exploiting the propagation phase, linear-polarized incident waves can be independently manipulated for the reflection semispace. Integrating the Pancharatnam-Berry (PB) phase, circular-polarized incident waves are tailored for the transmission semispace. To validate the concept, a metahologram sample is fabricated and tested. The experimental results are fully consistent with theoretical calculations and numerical simulation results. The metasurface has a useful role in multifunctional metadevices, and paves the way for potential applications in the fields of antennas and communication systems.

Topology optimization of the azimuth-rotation-independent polarization conversion metasurface for bandwidth enhancement.

Metasurfaces offer an unprecedented opportunity for flexible manipulation of electromagnetic wave. The azimuth-rotation-independent (ARI) polarization conversion metasurface (PCM) is an ultrathin device, which could convert an arbitrary linearly-polarized incident wave to its cross-polarized state. However, the bandwidth of an ARI PCM with a high cross-polarized transmission is usually limited. Here, a topology optimization method of multi-feature points based on the differential evolution (DE) algorithm is adopted to enhance the bandwidth of the traditional ARI PCM while maintaining a high transmission and polarization conversion ratio. The simulated results of the optimized structure indicate a 2.08 times bandwidth expansion in the cross-polarization conversion compared with the original structure. In addition, the measured results are consistent with the simulated ones and the ARI characteristic is validated. The proposed method provides a promising route for efficient high-performance metasurface designs.

Multifunctional terahertz metasurface utilizing Dirac semi-metal and vanadium dioxide hybrid metamaterials

We have proposed a novel multifunctional terahertz metasurface based on Dirac semi-metal and VO2 hybrid metamaterials that can solve many shortcomings of typical metal-medium-metal electromagnetic metamaterials, such as a narrow band, untunablility and single functionality. By adjusting the conductivity of VO2 and the Fermi energy of the Dirac semimetal, the metasurface can switch between the dual-band perfect absorber and Half Wave Plate (HWP)/Quarter Wave Plate (QWP). Numerical simulation results show that two perfect absorption peaks appear at 4.094 THz and 4.894 THz when vanadium dioxide is in a fully metallic state. Interestingly, the position and value of the two absorption peaks are only slightly effected by the Fermi energy of the resonant structure composed of Dirac semimetal in the middle layer. Moreover, the metasurface switches to a polarization converter, and it is worth noting that the metasurface can switch between QWP and HWP as the Fermi energy moves from 85 meV to 180 meV when VO2 is in an insulating state. A broadband perfect QWP has been obtained when the Fermi energy of Dirac semimetal is 85 meV, which can transform an incident linearly polarized wave to Left-handed circularly wave with an ellipticity greater than 0.95 in the frequency range of 1.71–2.73 THz. Moreover, the functionality of the metasurface switches from QWP to HWP when the Fermi energy of Dirac semimetal increases to 180 meV. This change can transform the incident linearly polarized wave to its orthogonal polarized wave with polarization conversion ratio exceeding 0.95 and a bandwidth of 1.06 THz in the frequency range of 1.82–2.88 THz. In addition, we practically investigated the influence of the incident angle to verify the potential of the designed metasurface. Such a multi-functional, broadband and high-performance metasurface has excellent application potential in sensing, imaging and super lenses.

Generating ultraviolet perfect vortex beams using a high-efficiency broadband dielectric metasurface.

Due to the topological charge-independent doughnut spatial structure as well as the association of orbital angular momentums, perfect vortex beams promise significant advances in fiber communication, optical manipulation and quantum optics. Inspired by the development of planar photonics, several plasmonic and dielectric metasurfaces have been constructed to generate perfect vortex beams, instead of conventional bulky configuration. However, owing to the intrinsic Ohmic losses and interband electron transitions in materials, these metasurface-based vortex beam generators only work at optical frequencies up to the visible range. Herein, using silicon nitride nanopillars as high-efficiency half-wave plates, broadband and high-performance metasurfaces are designed and demonstrated numerically to directly produce perfect vortex beams in the ultraviolet region, by combining the phase profiles of spiral phase plate, axicon and Fourier transformation lens based on geometric phase. The conversion efficiency of the metasurface is up to 86.6% at the design wavelength. Moreover, the influence of several control parameters on perfect vortex beam structures is discussed. We believe that this ultraviolet dielectric generator of perfect vortex beams will find many significant applications, such as high-resolution spectroscopy, optical tweezer and on-chip communication.

Automatic Inverse Design of High-Performance Beam-Steering Metasurfaces via Genetic-type Tree Optimization.

We introduce a genetic-type tree search (GTTS) algorithm combined with unsupervised clustering for the automatic inverse design of high-performance metasurfaces. With the proposed method, we realize highly directive beam-steering metasurfaces via the cooptimization of the amplitude and phase. In comparison with previous topology optimization approaches, the developed GTTS algorithm optimizes the organization of subwavelength nanoantennas and, thus, is applicable to the design of both passive and active metasurfaces. The optimized beam-steering metasurface specifically exhibits a nearly constant directivity when the steering angle varies from 5° to 30°. Furthermore, the optimized nonintuitive reflectance and phase profiles assist in achieving highly directive beam steering when the phase modulation range is <180°, which was previously challenging. Our approach can diminish the requirements of scattering light properties with substantially enhanced angular resolution of beam-steering metasurfaces, which enables the realization of high-performance metasurfaces that will be promising for a wide range of advanced nanophotonic applications.

Design of High-Performance Scattering Metasurfaces Through Optimization-Based Explicit RCS Reduction

The recent advances in the development of coding metasurfaces created new opportunities in realization of radar cross section (RCS) reduction. Metasurfaces, composed of optimized geometries of meta-atoms arranged as periodic lattices, are devised to obtain desired electromagnetic (EM) scattering characteristics. Despite potential benefits, their rigorous design methodologies are still lacking, especially in the context of controlling the EM wavefront through parameter tuning of meta-atoms. One of the practical obstacles hindering efficient design of metasurfaces is implicit handling of RCS performance. To achieve essential RCS reduction, the design task is normally formulated in terms of phase reflection characteristics of the meta-atoms, whereas their reflection amplitudes—although contributing to the overall performance of the structure—is largely ignored. As a result, the conventional approaches are unable to determine truly optimum solutions. This article proposes a novel formulation of the metasurface design task with explicit handling of RCS reduction at the level of meta-atoms. Our methodology accounts for both the phase and reflection amplitudes of the unit cells. The design objective is defined to directly optimize the RCS reduction bandwidth at the specified level (e.g., 10 dB) w.r.t. the metallic surface. The benefits of the presented scheme are twofold: (i) it provides a reliable insight into the metasurface properties even though the design process is carried out at the level of meta-atoms, (ii) the obtained design requires minimum amount of tuning at the level of the entire metasurface. None of these is possible for phase-response-based approach fostered in the literature. For practical purposes, the design is conducted using a surrogate-assisted procedure involving kriging metamodels, which enables global optimization at a low computational cost. To corroborate the utility of our formulation, a high-performance metasurface incorporating crusader-cross-shaped meta-atoms has been developed. The obtained results indicate that the system characteristics predicted at the design stage are well aligned with those of the EM-simulated structure (which is not the case for the traditional design approach). The metasurface features 10-dB RCS reduction in the frequency range of 16.5 GHz to 34.6 GHz, as validated both numerically and experimentally.

Electrically switchable metasurface for beam steering using PEDOT polymers

Switchable and active metasurfaces allow for the realization of beam steering, zoomable metalenses, and dynamic holography. To achieve this goal, one has to combine high-performance metasurfaces with switchable materials that exhibit high refractive index contrast and high switching speeds. In this work, we present an electrochemically switchable metasurface for beam steering where we use the conducting polymer poly(3,4-ethylenedioxythiophene) as an active material. We show beam diffraction with angles up to 10° and a change of the intensities of the diffracted and primary beams employing an externally applied cyclic voltage between −1 V and +0.5 V. With this unique combination, we realize switching speeds in the range of 1 Hz, and the extension to typical display frequencies in the region of tens of hertz is possible. Our findings have immediate implications for the design and fabrication of future electronically switchable and display nanotechnologies, such as dynamic holograms.

Design and verification of a single band metasurface based on cross-shaped metal array

A single-band terahertz (THz) high performance metasurface absorber is designed and verified based on the ohmic loss of the cross-shaped metal array. In the first set of experiments, three samples with structural parameters w=2.4μm, w=3.8μm, and w=4.2μm are prepared and measured. The absorption peak is enhanced from 80.2% to 96.8%, and the resonance frequency is reduced from 35.96 THz to 31.06 THz. In the second set of experiments, three other samples with structural parameters L=8.0μm, L=8.4μm, and L=8.8μm are prepared and measured (other structural parameters remained unchanged, and the test conditions remained unchanged). The absorption peak is increased from 80.2% to 88.8%, and resonance frequency is shifted to 27.28 THz. In the third set of experiments, samples with structural parameters w=2.4μm are placed in three different gases. Three high sensitivities are achieved by this samples: SNO=1421Hz/RIU, SCO=1674Hz/RIU, and SCO2=1897Hz/RIU. This proposed metasurface absorber shows the potential in gas sensing.