Metallic Metasurfaces Research Articles

Broadband-tunable quarter wave plates based on paired metal nanowire grids

Ultrathin wave plates based on metallic and dielectric metasurfaces have attracted much attention for independently controlling the phase, amplitude, and polarization of incident light. However, the issues of narrow bandwidth and low polarization conversion need to be improved for their optical applications. In this study, we show that an array of paired metal nanowire grids (PMNG) covered with a dielectric layer controls the polarization state in a broadband of visible to mid-infrared wavelengths. By analyzing the phase difference and amplitude ratio of orthogonal electric-field components passing through the SiO2-covered PMNGs, it is observed that the wavelengths operating as a circular polarizer appear regularly in the range from 450 nm to 4.0 μm with a stable phase difference of π/2. Also, the array of PMNG continuously converts linear polarization into circular or elliptical polarization by rotating the polarization angle of the incident light. Then, we demonstrate experimentally accessible polarization control by investigating the change in polarization ellipse between input and output light for this PMNG structure. Moreover, the PMNG can play an effective role as a wire grid polarizer in the IR region. Therefore, the suggested PMNG platform can be variously applied to emerging fields requiring ultrathin, multifunctional, and high-performance planar polarization components.

Cylindrical metalens for multidimensional control of terahertz waves

Metalens, consisting of planar micro/nanostructure arrays, has shown great potentials in beam steering, subwavelength focusing and high-resolution imaging. Taking the advantages of ultrathin thickness and compact design, it can be integrated with a photonic chip to improve the collection efficiency of optoelectronic systems. However, it remains challenging to realize the multidimensional control of terahertz (THz) waves by using a metalens, although the convergence properties have been widely studied. Here, we design and experimentally demonstrate a multifunctional cylindrical metalens by using a tri-layer metallic metasurface. By modulating the amplitude, phase and polarization states of terahertz waves independently, the cylindrical metalens can generate two focal lines at different positions for transmitted x- and y- polarized terahertz waves with varying amplitude distribution. Importantly, up to 64.7% focusing efficiency has been obtained at the frequency of 0.34 THz with a numerical aperture (NA) of 0.75. Our study offers a novel method for implementing multifunctional cylindrical metalens which lays the foundation of advanced integration with arrayed terahertz detection chips.

A low radar cross section antenna array based on a liquid metal metasurface

AbstractA low RCS (radar cross section) slot array antenna based on a liquid metal metasurface is proposed. The metasurface consists of 6 × 6 nested ring‐shaped element. The inner ring with a checkerboard distribution can realise 180° ± 37° phase difference, which can effectively reduce the scattering performances for the slot array. The outer and inner ring together can excite more energy from the slot array, thus resulting in increased radiation gains. Therefore, by changing the layout of liquid metal on the metasurface, scattering and radiation enhancement modes can be switched. The measured results show that in the scattering mode, the RCS can be reduced from 8.1 to 17 GHz. While in the radiation enhancement mode, the proposed array with the metasurface can achieve a −10‐dB impedance bandwidth of 9.4–11.2 GHz (17.4%) with a maximum gain of 8.12 dBi. Compared with the slot array without the metasurface, the realised gain is enhanced by 1.2 dBi from 6.9 dBi. The measured results correspond well with the simulated ones.

All-Dielectric Integrated Meta-Antenna Operating in 6G Terahertz Communication Window.

Efficient transceivers and antennas at terahertz frequencies are leading the development of 6G terahertz communication systems. The antenna design for high-resolution terahertz spatial sensing and communication remains challenging, while emergent metallic metasurface antennas can address this issue but often suffer from low efficiency and complex manufacturing. Here, an all-dielectric integrated meta-antenna operating in 6G terahertz communication window for high-efficiency beam focusing in the sub-wavelength scale is reported. With the antenna surface functionalized by metagrating arrays with asymmetric scattering patterns, the design and optimization methods are demonstrated with a physical size constraint. The highest manipulation and diffraction efficiencies achieve 84.1% and 48.1%. The commercially accessible fabrication method with low cost and easy to implement has been demonstrated for the meta-antenna by photocuring 3D printing. A filamentous focal spot is measured as 0.86λ with a long depth of focus of 25.3λ. Its application for integrated imaging and communication has been demonstrated. The proposed technical roadmap provides a general pathway for creating high-efficiency integrated meta-antennas with great potential in high-resolution 6G terahertz spatial sensing and communication applications.

All-dielectric metasurface two-parameter sensor based on quasi-bound states in continuum

<sec>Refractive index sensors based on metal metasurfaces are commonly limited by their low quality factors due to significant Ohmic losses in the metal material. In contrast, sensors based on all-dielectric metasurfaces can overcome this disadvantage. Currently, all-dielectric metesurface sensors based on symmetry-protected bound states in the continuum (BIC) have aroused intense research interest due to their ability to achieve ultrahigh quality factors. Such a metasurface sensor is mainly based on single BIC and single form of symmetry breaking. There are few studies on metasurface sensors of multiple BICs and multiple forms of symmetry breaking. In additon, the refractive-index sensors commonly neglect the influence of temperature fluctuation and thus suffer the crosstalk between the refractive index and temperature of the environment.</sec><sec>In this work, an all-dielectric metasurface composed of a periodic array of asymmetric semicircular-cylinder pairs is designed and two quasi-bound states in the continuum (QBIC) with high quality factors are obtained. By choosing three different forms of symmetry breaking (two in-plane and one out-of-plane), two robust QBIC modes can be generated in the selected near-infrared frequency band, and their resonance wavelengths, quality factors and polarization dependences exhibit different characteristics. Full-wave simulation results show that by measuring the resonance wavelengths of the two QBICs (denoted by QBIC1 and QBIC2), two-parameter sensing of refractive index and temperature can be achieved, which then solves the problem of crosstalk between the refractive index and temperature of the environment in refractive-index sensing. The dependence of quality factor on asymmetric parameters follows an inverse quadratic relation for the two QBICs. By adjusting the asymmetric parameters, the quality factor can be theoretially increased to any value, so that the sensing performance can be improved and adjusted. For refractive-index sensing, the QBIC1 can achieve a sensitivity of 194.7 nm/RIU and a highest figure of merit (FOM) of 8197 (corresponding to a quality factor of 45829); the QBIC2 can achieve a sensitivity of 170 nm/RIU, and a highest FOM of 4970 (corresponding to a quality factor of 28097). For temperature sensing, the QBIC1 can achieve a sensitivity of 7.77 pm/℃, and the QBIC2 can achieve a sensitivity of 24 pm/℃.</sec>

Dual‐Band Terahertz Metamaterial Absorber Based on LC Resonance and Dipolar Response and Its Applications

AbstractThis study presents a new dual‐band terahertz absorber (DBTA) with ease of production consisting of a metallic base plane of a certain thickness, a dielectric layer on this plane, and a hexagonal metallic metasurface structure. The proposed absorber exhibits absorption band peaks above 99% at two distinct frequencies (7.51 and 8.08 THz). The absorption origin is explained as a dual‐band response using the LC resonance and dipolar response. In this way, the presented absorber facilitates production by improving absorption responses with a much simpler structure, unlike studies with complex geometries. The proposed dual‐band absorber results in sensitivity to the polarization of the incident light. Thus, it also makes the proposed structure very helpful for manipulating wave polarization and detecting waves with particular polarization. The hexagonal structure of the absorber is scaled to millimeter dimensions, and its transformation into another dual‐band absorber that can also operate in microwave frequency regions. Similarly, the hexagonal structure is transformed into a circular structure with the same geometric dimensions, and a different dual‐band absorber is obtained in the terahertz (THz) region. Moreover, the proposed concept can be extended for various applications such as bolometric sensing, thermal imaging, energy harvesting, sensing, and selective thermal emitters.

High efficiency design of metal–insulator–metal metasurface by ResNets-10

Deep learning prediction of metasurface has been a widely discussed issue in recent years. However, the prediction accuracy is still one of the challenges to be solved. In this work, we proposed using the ResNets-10 model to predict plasmonic metasurface S11 parameters. The two-stage training was performed by the k-fold cross-validation and small learning rate. After the training was complete, the predicted logarithmic losses for aluminum, gold, and silver metal–insulator–metal metasurfaces were −48.45, −46.47, and −35.54, respectively. Due to the ultralow error value, the proposed network can efficiently replace the traditional computing methods within a certain structural range. The ResNets-10 can complete training within 1100 iterations, which is highly efficient. The ResNets-10 model we proposed can also be used to design meta-diffractive devices and meta-resonance biosensors, thereby reducing the time required for the simulation process. The ultralow lose value of the network indicates that this work contributes to the development of future artificial intelligence electromagnetic devices computing software.

Electrically tunable VO2–metal metasurface for mid-infrared switching, limiting and nonlinear isolation

Electrically tunable VO2–metal metasurface for mid-infrared switching, limiting and nonlinear isolation

Enhanced second harmonic generation from a quasi-periodic silver dendritic metasurface

The preparation of the vast majority of nonlinear optical metal metasurfaces currently relies on complex top-down methods such as electron beam or ion beam etching, which are expensive and difficult to meet the requirement of large area preparation. In this paper, an easily prepared quasi-periodic silver dendritic metasurface model with high Q factor is established in the near-infrared band based on a simple and easy-to-operate electrochemical deposition method. The simulations prove that the silver dendritic metasurface has a high Q factor (exceeds 104) because of its strong electric field localization ability, which is analogous to the superposition of multiple split-ring resonators. It is demonstrated that the second harmonic generation (SHG) intensity of the dendritic metasurface at a large incident angle (such as 85°) is about 30 times that of the metasurface at a small incident angle when the x-polarized pump light is incident obliquely to break the centrosymmetry of the metasurface. The influences of the incident angle or dendritic structure’s dimensions on the Q factor and SHG efficiency have also been researched through a lot of simulation. This easily prepared quasi-periodic silver dendritic metasurface SHG device may provide a new avenue for the development and application of miniature, integratable nonlinear optical devices.

Optimization of the Geometrical Design for an All-Dielectric Metasurface Sensor with a High Refractive-Index Response

This study aims to develop a refractive-index sensor operating in the visible region using an all-dielectric metasurface, which was chosen for its advantages of low optical loss and narrow spectral bandwidth, compared to those of conventional metallic metasurfaces. COMSOL software was utilized as a calculation tool to simulate the resonant properties of an all-dielectric metasurface composed of a circular nanohole-structured titanium oxide (TiO2) thin film, with the aim of enhancing the sensitivity of the refractive index for sensing targets. The simulation focused on finding the best geometrical conditions for the all-dielectric metasurface to achieve high sensitivity. Two resonance modes observed in this metasurface were considered: the quasi-bound-state-in-the-continuum (qBIC) mode and the perfect-reflection (PR) mode. The simulated results demonstrated that high sensitivities of 257 nm/RIU at the PR mode and 94 nm/RIU at the qBIC mode in the visible spectral range could be obtained by periodically constructing the metasurface with a unit cell having a lattice constant of 350 nm, a nanohole radius of 160 nm, and a nanohole depth of 250 nm. Furthermore, the study showed that the resonance mode that enabled high sensitivity was the PR mode, with a sensitivity nearly three times larger than that of the qBIC mode and the ability to reach the highest reflectance at the resonance wavelength. The optimized feature had the highest reflectance at a resonant wavelength of 570.19 nm, and although the quality factor was 25.50, these designed parameters were considered sufficient for developing a refractive index biosensor with high sensitivity and optical efficiency when operating in the visible spectral range.

Ultrathin Pyroelectric Photodetector with Integrated Polarization-Sensing Metasurface.

An abundance of metallic metasurfaces have been realized with miniscule, intricate features capable of tailored scattering, reflection, and absorption; however, high losses through heat limit their use in optoelectronics. Here, codesign of a detector and a polarization-sensing metasurface overcomes this challenge by utilizing the heat generation for integrated pyroelectric detection of the incoming light polarization. Using a nanogap metasurface with asymmetric metallic elements, polarization-sensitive photodetection exhibits high extinction ratios up to 19 for orthogonally polarized light and allows extraction of Stokes parameters with <12% deviation from theoretical values. This polarization-sensitive photodetector is ultrathin, consisting of active layers of only 290 nm, and exhibits fast response times of ∼2 ns. The structure is fully integrated, requiring no external cameras, detectors, or power sources, and points toward the creation of layered, multifunctional devices that utilize exotic metasurface properties for novel and compact sensing and imaging.

A terahertz wideband linear-to-circular polarization converter based on meandering-structured metasurface

In the terahertz (THz) band, this paper proposes a transmissive linear-to-circular (LTC) polarization converter based on a bilayer metallic meandering-structured metasurface. In this design, the LTC polarization conversion is achieved by utilizing the meandering structure, and the bandwidth is greatly expanded by inserting a wire grid into the y direction of the back metal layer. Furthermore, this metasurface converter is characterized by its simple structure and ultra-thin thickness. In detail, the axial ratio of the proposed converter is less than 3 dB in the frequency range of 0.4–1.21 THz signifying 100% relative bandwidth. Meanwhile, the emulation polarization conversion efficiency exceeds 60% in the frequency range of 0.68–1.21 THz, and the experimental results are in general agreement with the simulation ones. The proposed converter provides a new idea for the LTC polarization conversion in a wide THz band.

Nondepleted Time-Domain Method for Simulating Difference Frequency Generation From Metallic Metasurfaces

Difference frequency generation (DFG) is an essential phenomenon for generating far-infrared (FIR) and broadband terahertz (THz) waves, which cannot be generated by either alternating current flow-driven antennas for microwave source or pumped materials for higher frequency lights. Metallic nonlinear metasurfaces (NLMSs) as a new platform for generating THz sources were intensively developed. In this work, a nondepleted time-domain model derived from the Maxwell-hydrodynamic model is proposed where linear and nonlinear responses in metallic structures under an intense excitation are separated. By using the proposed model, only linear and low-frequency DFG signals will be considered, which makes this model suitable for unconditionally stable time-domain algorithms. A finite-difference time-domain (FDTD) method is employed to solve a set of multiphysics equations based on the proposed model. The stability of the algorithm is apparent. We validate our method by simulating THz generations from metallic NLMSs and benchmark the approach with previous numerical studies as well as our recent experimental results. In addition, the implementation of solving the DFG response can be remarkably accelerated by using the unconditionally stable algorithm.

Graphene Hybrid Metasurfaces for Mid-Infrared Molecular Sensors.

We integrated graphene with asymmetric metal metasurfaces and optimised the geometry dependent photoresponse towards optoelectronic molecular sensor devices. Through careful tuning and characterisation, combining finite-difference time-domain simulations, electron-beam lithography-based nanofabrication, and micro-Fourier transform infrared spectroscopy, we achieved precise control over the mid-infrared peak response wavelengths, transmittance, and reflectance. Our methods enabled simple, reproducible and targeted mid-infrared molecular sensing over a wide range of geometrical parameters. With ultimate minimization potential down to atomic thicknesses and a diverse range of complimentary nanomaterial combinations, we anticipate a high impact potential of these technologies for environmental monitoring, threat detection, and point of care diagnostics.

Ultrahigh Sensitivity Solar‐Blind UV Detection via Multistage‐Concentric‐Annulus Architecture Metasurface

AbstractHigh‐sensitivity photodetection capability has become an indispensable requirement for detecting feeble solar‐blind ultraviolet (UV) light. Micro‐nano metallic metasurface structures are employed to enhance the detector photoelectric performances. However, existing approaches are confronted with bottlenecks of structure simplification and efficacy mediocrity. Here, a distinctive diamond solar‐blind UV photodetector with a synthetic multistage‐concentric‐annulus (MCA) architecture metasurface is reported, which is prepared through a facile and peculiar Fraunhofer diffraction technique. The detector achieves an excellent photo‐to‐dark‐current ratio of 2.4×108 and maintains an extremely low dark current on the order of pA magnitude. The intrinsic response peak and resonance absorption phenomenon in solar‐blind UV are evident from the optical properties. Moreover, the surface potential distribution intuitively reveals that plasmon resonance enhances the UV scattering and absorption cross sections, thus facilitating the photogeneration of carriers. Meanwhile, it expressly demonstrates that multi‐level dipole interaction in the MCA structure accelerates the separation and extraction of electron–hole pairs through enhancing the localized electric field intensity. In addition, a wide range of variable temperature measurements further prove the device possesses capacity for working in extreme environments. This photodetection technology by virtue of a delicate and viable diffraction metasurface is anticipated to provide a leap forward for future applications in wide‐band‐gap semiconductor UV detectors.

Multi-resonances terahertz refractive index sensor based on a ring with embedded split rings metasurface array

Terahertz biosensors have gained significant attention due to their low energy and high penetrability, which enables them to interact with analytes without causing ionization damage. However, the weak interaction between terahertz waves and natural materials often results in poor sensing performance. To address this issue, various metasurface structures have been designed to enhance the light-matter interaction between the incident electromagnetic waves and the metasurface structure, thereby improving sensing performance. In this paper, we propose a metal–insulator–metal metasurface structure with a top metal consisting of a metasurface array that includes an outer ring with two embedded split rings. This structure generates three resonances at 0.732 THz, 1.916 THz, and 2.602 THz. Our simulation results show that the resonance formed by the mode coupling between the outer ring and the embedded split rings at 1.916 THz achieves the best sensing performance, with a sensitivity of 572 GHz/RIU, a quality factor of 22.8, and a figure of merit of 6.81. Our findings suggest that designing high-performance refractive index sensing with resonances generated from mode coupling is preferred. The remarkable sensing performance of the proposed metasurface structure enables the measurement of trace amounts of analytes in the terahertz region.

Bending Sensing Based on Quasi Bound States in the Continuum in Flexible Terahertz Metasurface

AbstractBound states in the continuum (BIC) promote strong interaction between light and matter, which has been introduced into the metasurfaces to achieve ultra‐high Q resonance and provide a powerful tool for high‐sensitivity sensing. At present, BIC‐based sensors have been applied in biological and chemical sensing. Here, their application in bending sensing is investigated. BIC is employed to design the terahertz metallic metasurfaces and propose a feasible method for the high‐Q sensitivity detection of micro‐bending. It is demonstrated theoretically and experimentally that BIC can transform into an observable quasi‐BIC by tuning the structural parameters. More importantly, it is the first report to investigate the bending sensing performance of BIC metallic metasurfaces. The experimental results verify that both metasurfaces supporting symmetry‐protected BIC and parameter‐tuned BIC are sensitive to changes in curvatures. It indicates that the designed BIC metasurfaces can realize the bending deformation detection of some precision instruments. These results expand the application of BIC‐based sensors in the sensing field. It is expected that the design can find potential applications in wearable photonic devices, intelligent robots, and bionic machines.

Highly sensitive and polarization-insensitive terahertz microfluidic biosensor based on gap plasmon model

The gap plasmon resonance can generate a local enhanced electric field and make the electromagnetic field energy mostly confined in the microfluidic, thereby increasing the interaction between the detection sample and terahertz wave. In this paper, a five-layer microfluidic terahertz biosensor based on gap plasmon resonance is presented. The simulation results show that the designed biosensor can realize a resonant absorption peak with near-perfect absorption at the frequency of 0.655 THz and high refractive index sensitivity of 191.3 GHz/RIU. In addition, the biosensor possesses the characteristics of polarization insensitivity and 50° incidence angle insensitivity due to the symmetry of the metal metasurface, which make the biosensor can be widely used in the biological label-free detection, medical trace element detection.

Dual-Band Terahertz Metamaterial Absorber and Its Refractive-Index-Sensing Application for Material Films

Metamaterial absorbers are an emerging and rapidly growing technology with potential applications in optoelectronics, such as refractive index sensing and energy harvesting. Herein, a dual-band terahertz metamaterial absorber is investigated by exploiting a patch of circular loops and cross-shaped structures. The designed absorber composes of a periodic metallic metasurface and a continuous reflector layer, which are separated by a benzocyclobutene dielectric film. The perfect absorptivity properties of the absorber, revealed at frequencies of 0.6 and 1.2 THz, can be flexibly designed by manipulating the geometrical dimensions. A sample of the designed absorber prototype is manufactured under a microfabrication process and measured through a terahertz time-domain spectrometer in free space. In addition, the potential application of the proposed terahertz absorber in refractive index sensing is analyzed and experimentally verified, which has an excellent unit sensitivity of 0.13 THz/RIU at 0.6 THz and 0.27 THz/RIU at 1.2 THz, respectively. The practicability in sensing applications is verified by coating polyimide and poly(dimethylsiloxane) dielectric films on the surface of the absorber, and the absorption frequency shifts at 1.2 THz are practically coincident with the simulated ones. This proposed prototype is an effective manner for terahertz metamaterial absorbers with potential applications in the sensor field.

Ultrasensitive optical modulation in hybrid metal-perovskite and metal-graphene metasurface THz devices.

Implementation of efficient terahertz (THz) wave control is essential for THz technology development for applications including sixth-generation communications and THz sensing. Therefore, realization of tunable THz devices with large-scale intensity modulation capabilities is highly desirable. By integrating perovskite and graphene with a metallic asymmetric metasurface, two ultrasensitive devices for dynamic THz wave manipulation through low-power optical excitation are demonstrated experimentally here. The perovskite-based hybrid metadevice offers ultrasensitive modulation with a maximum modulation depth for the transmission amplitude reaching 190.2% at the low optical pump power of 5.90 mW/cm2. Additionally, a maximum modulation depth of 227.11% is achieved in the graphene-based hybrid metadevice at a power density of 18.87 mW/cm2. This work paves the way toward design and development of ultrasensitive devices for optical modulation of THz waves.