Unit Cell Of Metasurface Research Articles

Ultra-broadband terahertz bandpass filter with dynamically tunable attenuation based on a graphene-metal hybrid metasurface.

We propose an ultra-broadband terahertz bandpass filter with dynamically tunable attenuation based on a graphene-metal hybrid metasurface. The metasurface unit cell is composed of two metal stripes enclosed with a graphene rectangular ring. Results show that when the metasurface is normally illuminated by a terahertz wave polarized along the metal stripes, it can act as an ultra-broadband bandpass filter over the spectral range from 1.49 THz to 4.05 THz, corresponding to a fractional bandwidth of 92%. Remarkably, high transmittance above 90% covering the range from 1.98 THz to 3.95 THz can be achieved. By changing the Fermi level of graphene, we find that the attenuation within the passband can be dynamically tuned from 2% to 66%. We expect that the proposed ultra-broadband terahertz bandpass filter with tunable attenuation will find applications in terahertz communication and detection and sensing systems.

Optomechanical Self-Stability of Freestanding Photonic Metasurfaces

We develop an analytical framework for self-stabilizing optical manipulation of freestanding metasurfaces in three dimensions. Our framework reveals that the challenging problem of stabilization against translational and rotational perturbations in three dimensions is reduced to a simpler scattering analysis of the metasurface unit cell in two dimensions. We derive universal analytical stiffness coefficients applicable to arbitrary three-dimensional radial metasurfaces and radial beam-intensity profiles. The analytical nature of our framework facilitates highly efficient discovery of optimal optomechanical metasurfaces. Such use of metasurfaces for mechanical stabilization enables macroscale and long-range control in collimated, but otherwise unfocused light beams, and could open up avenues for manipulation beyond traditional optical tweezing and transport.

A metasurface-enabled Fabry–Pérot (FP) circularly polarized antenna with continuous wideband polarization conversion purity

Abstract In this paper, a wideband continuous pure right hand circularly polarized (RHCP), high gain, and low-radar cross-section (RCS) array antenna is proposed. A linear-to-circularly polarization conversion (LCPC) Metasurface (MS) is employed as the superstrate of the Fabry–Pérot (FP) resonator antenna, consisting of two oblique slits etched patches located at top and bottom, respectively, and a metal ring with corner-cutting patch inside, that ensure a wideband transmission and reflection LCPC frequencies ranging from 9 to 22 GHz, and 7–13.5 GHz, respectively. While, a pure RHCP LCPC frequency band of 9–12 GHz is produced by adopt the proposed MS that is benefit from the design of etched oblique slits and corner-cutting patch surrounded by the metal ring, where the magnitude and phase difference can be kept in the variation of ±3 dB and 10°, respectively. Then, a rectangle patch-fed MS FP antenna is designed by an arrangement of 5 × 5 MS unit cells. Following this, the sequence rotated technique is utilized to arrange the array antenna by 2 × 2 units, ensuring a wide band RCS frequency band. The proposed array antenna is fabricated and measured, which indicated the correctness of this design for performance of high gain, low RCS, and wideband pure RHCP. Compared with recent reported MS-based FP works, a wideband LCPC frequencies purity is obtained, and a good radiation and scattering performance is obtained in the design.

Design of a transmissive metasurface antenna using deep neural networks

This article presents design methods for a transmissive metasurface antenna composed of four layers of meta-structures based on the deep neural network (DNN). Owing to the structural complexity as well as side effects such as couplings among the adjacent meta-structures, the conventional design of metasurface unit cell strongly relies on the researcher’s intuition as well as time-consuming iterative simulations. A design method for a metasurface antenna unit cell with a size of a quarter wavelength operating at a frequency of 5.8GHz is presented. We describe two unique implementations for designing the target metasurfaces: 1) utilizing the inverse network 2) data augmentation by the forward network and a random search algorithm. With the usage of the two DNNs, the average transmittance of the unit cells is improved by about 0.024 than that of the unit cells designed by the conventional approach. This research invokes the application of DNN in designing antennas and other structures operating at radio frequency.

Radiation pattern direction control of THz antenna with applying planar graphene metasurface

In this paper an antenna with dynamically controllable radiation pattern direction, in the low THz frequency band, is designed and investigated. The radiating element is composed of a rectangular patch antenna on a grounded dielectric while, radiation pattern control is made possible through applying a superstrate, inspired by metasurface structures, with gap of 25μm from the antenna surface. The radiating patch as well as the ground plane are designed with gold while the metasurface consists of graphene loops and patches which are placed on the either sides of a dielectric with the same material of the antenna substrate. Tunable radiation pattern direction is provided through activating appropriate columns of the metasurface unit cells. With applying an electrostatic bias voltage the graphene surface conductivity increases and the corresponding unit cells resemble properties of metals and impose parasitic effects on the antenna radiation pattern, which through wise selection of the metasurface unit cells the antenna radiation pattern is tilted in the desired direction. It is shown that the antenna radiation pattern can be dynamically controlled in the wide range of ±35degrees, symmetrically, in the H-plane.

Multi-functional high-efficiency reflective polarization converter based on an ultra-thin graphene metasurface in the THz band

In this study, an ultra-thin reflective metasurface is proposed for polarization conversion in the terahertz band. Each unit cell of metasurface is composed of graphene ribbons lying diagonally on silicon substrate. A reflective metal is also placed at the bottom of the structure. Our polarization converter works as a linear polarization converter (LPC) and linear to circular polarization converter (LTC-PC) by variation of the chemical potential of graphene, which can actively be changed by chemical doping or electrical bias of the graphene. The working bandwidth of LPC changes by adjusting the chemical potential of the graphene. The LPC structure has more than 99% polarization conversion ratio in the frequency range of 0.83-0.92 THz, even by changing the angle of incident wave up to 45°, the results are still acceptable. The LTC-PC has less than 3dB axial ratio (AR) in the frequency range of 0.6-0.67 THz for left-handed circularly polarized (LHCP) waves and 0.72-0.97 THz for right-handed circularly polarized (RHCP) waves. To verify the simulation results, an equivalent circuit model based on the structure performance is proposed. Equivalent circuit model results agree very well with the simulation results. Due to the fabrication feasibility, ultra-thin thickness, incident angle insensitive, and high efficiency, our structure has great potential in state-of-the-art technologies such as imaging, sensing, communication, and other optical applications.

Multifunctional near field focusing transmission metasurface based on polarization sensitivity

AbstractIn this paper, a near field focusing transmission metasurface (MS) based on polarization sensitivity is developed at microwave frequency. The MS unit cell has only three metal layers, which is less than the published dual‐polarized transmission structure. And the phase shifting range can cover 320° for x‐ and y‐polarized waves. The biggest bright spot of the focusing MS made of the proposed units is its changeable focus point by rotating the horn 0°–90° round its central axis: focusing switching between one and two focusing points. In addition, the developed MS has the following advantages: multifunctional, low sidelobe, high focusing efficiency, small size, wideband, and less layer number. The focusing relative bandwidth is 9.7%, and the maximum focusing efficiency is 30% at center frequency 15.5 GHz. The efficiency is higher than that of published results in transmission orthogonal polarization mode. The proposed MS has potential applications in medical treatment, noncontact sensing system.

Equivalent Circuit Models for Metasurfaces Using Floquet Modal Expansion of Surface Current Distributions

This work presents a fully analytical method for obtaining equivalent circuit models (ECMs) that predict the magnitude and phase of scattering parameters for reflectarray (RA) and/or metasurface (MTS) unit cells. The proposed ECM is derived from a Floquet modal expansion of surface currents and does not rely on full-wave simulations. Analytical functions are used to represent the current distribution on the unit cell, and the proposed ECM is validated against full-wave simulations for several RA/MTS element types including loaded loops, dogbone and symmetric cross unit cells, and is shown to be accurate under normal and oblique incidence conditions, thus providing a fast and efficient design tool for deriving the scattering parameters of patch-like unit cells in the initial design process. As this approach allows for modeling unit cells loaded using lumped components such as varactor diodes, it can also be used to derive the dynamic response of tunable RA and MTS unit cells.

Air-bridged Schottky diodes for dynamically tunable millimeter-wave metamaterial phase shifters

A low loss metamaterial unit cell is presented with an integrated GaAs air-bridged Schottky diode to produce a dynamically tunable reflective phase shifter that is capable of up to 250° phase shift with an experimentally measured average loss of 6.2 dB at V-band. The air-bridged Schottky diode provides a tuneable capacitance in the range between 30 and 50 fF under an applied reverse voltage bias. This can be used to alter the resonant frequency and phase response of a split patch unit cell of a periodic metasurface. The air-bridged diode die, which is flip-chip soldered to the patch, has ultra-low parasitic capacitance and resistance. Simulated and measured results are presented which verify the potential for the attainment of diode switching speeds with acceptable losses at mmWave frequencies. Furthermore the study shows that this diode-based unit cell can be integrated into metamaterial components, which have potential applications in future mmWave antenna beam-steering, intelligent reflecting surfaces for 6G communications, reflect-arrays, transmit-arrays or holographic antennas.

Switchable digital metasurface based on phase change material in the terahertz region

We report a new approach for making a reconfigurable terahertz digital metasurface that is created with vanadium dioxide (VO2) integrated metasurface unit cells. Such a metasurface achieves terahertz wave beam splitting and switching functionalities for the terahertz wave normal incidence by inducing VO2 conductivity change under a different external temperature. The three-dimensional (3D) far-field scattering patterns and normalized electric field distribution obtained by using full-wave numerical simulations verify the behavior of the terahertz waves in each of the cases and illustrate our general theoretical predictions. This scheme provides a new effective method for the design of terahertz multifunctional devices.

Dynamically reversible and strong circular dichroism based on Babinet-invertible chiral metasurfaces.

We propose a Babinet-invertible chiral metasurface for achieving dynamically reversible and strong circular dichroism (CD). The proposed metasurface is composed of a VO2-metal hybrid structure, and when VO2 transits between the dielectric state and the metallic state, the metasurface unit cell switches between complementary structures that are designed according to Babinet's principle. This leads to a large and reversible CD tuning range between ±0.5 at 0.97 THz, which is larger than the one found in the literature. We attribute the CD effect to extrinsic chirality of the proposed metasurface. We envision that the Babinet-invertible chiral metasurface proposed here will advance the engineering of active and tunable chiro-optical devices and promote their applications.

Scattering Analysis and Efficiency Optimization of Dielectric Pancharatnam-Berry-Phase Metasurfaces.

In this study, the phase modulation ability of a dielectric Pancharatnam–Berry (PB) phase metasurface, consisting of nanofins, is theoretically analyzed. It is generally considered that the optical thickness of the unit cell of a PB-phase metasurface is λ/2, i.e., a half-waveplate for polarization conversion. It is found that the λ/2 is not essential for achieving a full 2π modulation. Nevertheless, a λ/2 thickness is still needed for a high polarization conversion efficiency. Moreover, a gradient phase metasurface is designed. With the help of the particle swarm optimization (PSO) method, the wavefront errors of the gradient phase metasurface are reduced by fine-tuning the rotation angle of the nanofins. The diffraction efficiency of the gradient phase metasurface is thus improved from 73.4% to 87.3%. This design rule can be utilized to optimize the efficiency of phase-type meta-devices, such as meta-deflectors and metalenses.

Design of a Single-Layer Frequency Selective Surface for 5G Shielding

In this letter, a single-layer bandpass frequency selective surface (FSS) working at 2.6 GHz is proposed to decrease the interference to communication systems and the cost of processing based on a multimode resonator. A metasurface unit cell is adopted to suppress the grating lobe and achieve miniaturization. Therefore, the performance of the proposed FSS can remain stable under a large oblique incidence. The surface current distribution and equivalent circuit model at each design step are extracted to explain its working principle. Good agreement between the experimental results and the simulated response verifies the feasibility of the proposed FSS.

Realization and validation of continuous tunable metasurface for high resolution beam steering reflector at K‐band frequency

The realization and validation of a new reconfigurable metasurface high-resolution steering reflector for K-band are presented. The continuous tuning is achieved by integrating a varactor diode with each metasurface unit cell. This approach is shown in many studies, but not above Ku-band, because of implementation difficulties. This letter presents design considerations, reflection simulations, and reflection measurements of a new resonant unit cell at K-band, used for reconfigurable metasurface steering reflector. The reflection steering method and the reflection phase calibration are presented. Finally, the experimental far-field results are provided to validate the continuous beam-steering reflector performances in various frequencies at K-band, with large-angle steering, high resolution, and wide-incident-angular-range abilities.

Bilayer terahertz chiral metasurfaces with different dihedral symmetries

In this paper, we discuss the isotropic chiroptical response of two-layered metasurfaces with n -fold dihedral symmetries of resonators ( D n ) in the terahertz (THz) range. The analysis of numerical calculations is based on the homogenization model through effective polarizability approximation. We reveal the impact of resonator symmetry on circular dichroism and polarization anisotropy of the metasurface. In particular, it is shown that an increase in the symmetry order of a metasurface unit cell leads to a proportional resonance red shift in circular dichroism spectra. This research has both fundamental and applied relevance, and may enable the realization of THz devices capable of polarization manipulation or possessing polarization sensitive features.

Full-Stokes Polarization Perfect Absorption with Diatomic Metasurfaces.

Metamaterial-based perfect absorbers provide efficient ways for selective absorption of light with both linear or circular polarizations. Perfect absorption for an arbitrary polarization requires the development of subwavelength structures absorbing efficiently elliptically polarized light, but they remain largely unexplored. Here, we design and realize experimentally novel plasmonic metasurfaces for full-Stokes polarization perfect absorption in the mid-infrared. The metasurface unit cell consists of coupled anisotropic meta-atoms forming a diatomic metamolecule. The designed plasmonic metastructures provide a strong field enhancement by at least 1 order of magnitude higher than conventional perfect absorbers. In experiment, our plasmonic metasurfaces demonstrate sharp differentiation of spectral responses for an arbitrary pair of orthogonal polarization states (linear, circular, or elliptical) providing perfect absorption for one polarization with strong reflection for its counterpart. Our results suggest a novel route for efficient control of light polarization in metasurfaces offering numerous potential applications ranging from thermal imaging to chiral molecule detection.

Ultra-high extinction-ratio light modulation by electrically tunable metasurface using dual epsilon-near-zero resonances

The lossy nature of indium tin oxide (ITO) at epsilon-near-zero (ENZ) wavelength is used to design an electrically tunable metasurface absorber. The metasurface unit cell is constructed of a circular resonator comprising two ITO discs and a high dielectric constant perovskite barium strontium titanate (BST) film. The ENZ wavelength in the accumulation and depletion layers of ITO discs is controlled by applying a single bias voltage. The coupling of magnetic dipole resonance with the ENZ wavelength inside the accumulation layer of ITO film causes total absorption of reflected light. The reflection amplitude can achieve ~84 dB or ~99.99% modulation depth in the operation wavelength of 820 nm at a bias voltage of −2.5 V. Moreover, the metasurface is insensitive to the polarization of the incident light due to the circular design of resonators and the symmetrical design of bias connections.

Экспериментальное исследование перестраиваемого акустического фазовращателя

The paper presents the results of a numerical and experimental study of an acoustic phase-shifter based on a chain of rectangular Helmholtz resonators. Phase regulation of the transmitted acoustic wave is made by simultaneously changing the resonators volumes with the stepper motor. As a result of the experiment it was found that the developed acoustic phase-shifter allows to manipulate the wave phase within the range of 0 - 2π in the frequency range of 2000 - 2500 Hz. The transmittance factor is not less than 0.7. The phase-shifter can be used as a unit cell of tunable acoustic metasurface.

A novel low-RCS antenna array based on integration of electromagnetic metasurface and conventional antenna

Aiming at obtaining low scattering antenna array, in this paper a novel method of integrating electromagnetic metasurface with conventional antenna is proposed. The theoretical analysis and practical implementation of this method are presented. Using this method, a novel antenna array is obtained by connecting partial unit cells of metasurface with transmission line and adopting coaxial excitations. In the radiation mode, the metasurface is excited and radiates effectively. Besides, the array has almost the same performance as the conventional array. In the scattering mode, this array demonstrates low in-band RCS due to the scattering cancellation of middle metasurface and other surrounding structures. Using this method, a 2 × 1 array, as an example, is designed and numerically analyzed. The results show that the array has the well-behaved radiation performance and low RCS property. The working principle of the proposed array is illustrated by investigating the current and resultant field. Further analysis also reveals the effecting law of metasurface unit cells in antenna's radiation and scattering performance. Therefore, flexible designs can be obtained to fit different requirements. Finally, experiments are conducted. And the good agreement between computations and measurements further verifies the validity of the proposed design. Moreover, the proposed method also features easy implementation and high integrity and can be extended to the designing of large scale array antennas.

Wideband and high efficiency orbital angular momentum generator based on bi-layer metasurface

A broadband and high-efficieny bi-layer metasurface is proposed in this paper. The unit cell of the metasurface is formed by symmetrically etching two cross-type metal patches on both sides of a dielectric plate. Furthermore, the two metal patches have a displacement of half a period along the <i>y</i>-axis. By employing the displacement, the transmission bandwidth of the bi-layer metasurface is significantly expanded. In order to obtain a physical insight into bandwidth broadening, a π-type equivalent circuit that presents the electromagnetic coupling between within the bi-layer metasurfaces is successfully extracted to investigate the influence of electromagnetic coupling on transmission performance. The results show that by shifting the metal patches along the <i>y</i>-axis by half a period, the coupling impedance (<i>Z</i><sub>12</sub> or <i>Z</i><sub>21</sub>) of bi-layer metasurface can be significantly modified, which further changes the electromagnetic coupling of the bi-layer metasurface. Correspondingly, the impedances <i>Z</i><sub>p</sub> and <i>Z</i><sub>s</sub> in the π-type circuit are changed to approximately meet the resonant condition of circuit in broadband, resulting in the bandwidth expansion of the proposed device. By using Pancharatnam-Berry phase theory, we redesign the proposed metasurface unit cell into a broadband orbital angular momentum generator. The simulation and measurement results verify that the bi-layer metasurface can convert a left-hand circularly polarized wave into a right-hand circularly polarized wave carrying orbital angular momentum in a frequency range between 11 GHz and 12.8 GHz, demonstrating the performance of device.