Transmissive Metasurface Research Articles

Experimental realization of a transmissive microwave metasurface for dual vector vortex beams generation

This work presents a theoretical design and experimental demonstration of a transmissive microwave metasurface for generating dual-vector vortex beams (VVBs). The proposed metasurface consists of an array of pixelated dartboard discretization meta-atoms. By rotating the meta-atoms from 0° to 180°, a Pancharatnam-Barry (P-B) phase covering the full 360° range is achieved, with a transmittance exceeding 90% over the frequency range from 9.7 to 10.2 GHz. The measured results demonstrate that when a linearly polarized microwave normally impinges on the metasurface, the transmitted beams correspond to the dual VVBs with different directions. A good agreement among Poincaré sphere theory, full-wave simulation, and experimental measurement is observed. This proposed transmissive microwave metasurface for VVBs may offer promising applications in communications and radar detection.

Reconfigurable Multifunctional Transmission Metasurface Polarizer Integrated with PIN Diodes Operating at Identical Frequencies

Herein, a reconfigurable multifunctional transmission metasurface polarizer, structured with double Jerusalem crosses and integrated with four PIN diodes, is presented. The bottom Jerusalem cross is rotated by 35∘ with respect to the top cross. Both numerical and experimental observations reveal that a linearly polarized (LP) outgoing wave is transmitted at approximately 4.0 GHz when subjected to a left-handed circularly polarized (LCP) or right-handed circularly polarized (RCP) incident wave. The transmission efficiency reaches -2.5 dB when all elements are in the ON state. Furthermore, active control of switchable PIN diodes operating in various statuses unequivocally demonstrates the ability to convert an incident wave polarized in the x or y direction to a LCP or RCP wave, respectively, within the identical frequency band, spanning from 3.6 GHz to 4.3 GHz. This conversion is achieved with a transmission coefficient of -3.5 dB or -4.2 dB at the peak frequency. The proposed metasurface polarizer presents a potentially dynamic method for simultaneously manipulating various polarization conversions of electromagnetic (EM) waves within a desired frequency band.

Enhancing 5G propagation into vehicles and buildings using optically transparent and polarisation insensitive metasurfaces over wide-incidence angles

This article introduces two transmissive metasurfaces applied to normal windows, aiming to improve the 5G outdoor-to-indoor (O2I) coverage. These windows can be utilized in various settings, such as vehicles or buildings. The proposed unit cells, designed to be wide-incident angle and polarization insensitive, are implemented in both single-glazing and double-glazing glasses, arranged in a periodic structure to form the transmission surfaces. Both metasurfaces maintain optical transparency by incorporating Indium Tin Oxide (ITO) as the conductive element in each unit cell. These engineered transmission surfaces enhance the 5G signal indoor coverage at the 3.5 GHz band across a broad range of incident angles. While multi-layer structures typically exhibit heightened sensitivity to the angle of incidence, the proposed two-layered transmissive surfaces demonstrate substantial angular stability, reaching up to 65 and 75 degrees for double- and single-glazed glass, respectively. To achieve this wide and stable angular response, evolutionary optimization techniques were employed to fine-tune the proposed unit cells. Both designs exhibit a high transmission coefficient across the operating frequency for a variety of incident angles, surpassing those reported in the existing literature. Experimental evaluations of the fabricated prototypes indicate that both metasurfaces hold significant potential for enhancing signal propagation into buildings and vehicles.

Reconfigurable transmissive metasurface with a combination of scissor and rotation actuators for independently controlling beam scanning and polarization conversion

Polarization conversion and beam scanning metasurfaces are commonly used to reduce polarization mismatch and direct electromagnetic waves in a specific direction to improve the strength of a wireless signal. However, identifying suitable active and mechanically reconfigurable metasurfaces for polarization conversion and beam scanning is a considerable challenge, and the reported metasurfaces have narrow scanning ranges, are expensive, and cannot be independently controlled. In this paper, we propose a reconfigurable transmissive metasurface combined with a scissor and rotation actuator for independently controlling beam scanning and polarization conversion functions. The metasurface is constructed with rotatable unit cells (UCs) that can switch the polarization state between right-handed (RHCP) and left-handed circular polarization (LHCP) by flipping the UCs to reverse their phase variation. Moreover, independent beam scanning is achieved using the scissor actuator to linearly change the distance between the UCs. Numerical and experimental results confirm that the proposed metasurface can perform beam scanning in the range of 28° for both the positive and negative regions of a radiation pattern (RHCP and LHCP beams) at an operational frequency of 10.5 GHz.

Time-varying coding digital double-layered Huygens' metasurface for high-efficiency harmonic frequency conversion

Time-varying metasurfaces offer an efficient means of controlling nonlinear harmonics by manipulating component geometries and modulating signals. This ability renders them valuable across various fields, such as wireless communication, radar sensing, and biological monitoring. However, most of the energy in time-varying metasurfaces is concentrated in the fundamental wave, as well as scattered at various harmonic orders, which reduces the energy efficiency at the desired harmonic. Existing approaches have employed time-varying coding digital metasurfaces to achieve efficient harmonic conversion but are primarily designed for reflection. Reflection-based designs require a feed source to excite the metasurface, which can cause certain shielding effects and limit their application in specific scenarios. Thus, designing transmissive time-varying coding digital metasurfaces for efficient harmonic conversion is currently an urgent problem that needs to be addressed. To solve this problem, this paper develops a time-varying coding digital double-layered Huygens' metasurface, which achieves efficient conversion of the desired transmitted harmonics. The unit structure of the metasurface consists of a pair of reverse-symmetric split rings located on the upper and lower sides of a dielectric substrate, enabling nearly non-reflective Huygens' resonance. Based on a continuous periodic phase modulation strategy, we achieved efficient conversion of transmitted harmonics by loading a time-varying voltage (phase) modulation signal with a 5-bit resolution bit width onto the designed double-layered Huygens' metasurface. This study presents a solution for designing a transmissive time-varying coding digital metasurface to achieve efficient conversion of harmonics, thereby enhancing the application capabilities of time-varying coding digital metasurfaces.

A Broadband Transmissive Metasurface for Non-Diffractive THz OAM Multiplexing and Communication

A Broadband Transmissive Metasurface for Non-Diffractive THz OAM Multiplexing and Communication

Transmissive all-dielectric metasurfaces for reconstructing terahertz holographic images via polarization-multiplexing and polarization-decoupling

Recently, the multifunctional terahertz metasurface holography (meta-holography) has garnered significant attention and sparked wide discussions due to its capacity for carrying a large amount of information. However, traditional multifunctional meta-holography, achieved by integrating metasurfaces with active materials or MEMS technology, have exhibited distinct shortcomings due to their intrinsic properties, such as slow response, complex structure, or low reliability. Herein, we propose a transmissive metasurface platform composed of anisotropic all-dielectric meta-atoms including three parts: the elliptical silicon pillars (Si-pillars), circular Si-pillars, and a quartz substrate sandwiched between them. This platform enables the implementation of dual-channel holographic images by utilizing linear polarization (LP) multiplexing and circular polarization (CP) decoupling. As proof of concept, two transmissive anisotropic metasurfaces (MS-1 and MS-2) are designed and created. When illuminated with two orthogonal LP-polarization waves, the MS-1 can produce holographic images of the letters ‘X’ and ‘Y’ in the corresponding co-polarized channel. Additionally, the MS-2 is illuminated by the left-circularly polarized (LCP) wave, two holographic images with different patterns (‘L’ and ‘R’) are reconstructed in the co-polarized and cross-polarized channels respectively. Therefore, the proposed metasurfaces allowing for polarization-multiplexing or polarization-decoupling can exhibit considerable potential for applications in multifunctional integration and high information capacity.

Generation of Mathieu beam using a single layer transmissive metasurface

Mathieu beams is an important diffraction-free beam. In this letter, we proposed a transmissive metasurface to generate Mathieu beams in microfields. An ultrathin transmissive element with single layer is designed. It consists of two metallic I-shaped structures on both sides of the substrate. By rotating the top layered I-shaped resonator, amplitude adjustment from 0 to 1 with 1-bit phase shift can be achieved when y-polarized incident wave is converted into x-polarized one. A metasurface prototype is designed and fabricated to generate the required Mathieu beam. Both simulation and measurement show that Mathieu beam can be effectively generated and diffraction free feature can be observed in the distance range of 0 36λ (where λ is the wavelength at operating frequency). Compared with traditional devices to generate Mathieu beams, using metasurface reduces the complexity and cost. Our proposed method and metasurface can provide reference for complex field control in 3-dimensional space and may facilitate the uses of Mathieu beams for practical applications.

Time-varying digital coding of induced-magnetism Huygens’ metasurfaces for flexible and continuous control of harmonics

Abstract Time-varying digital coding metasurfaces have garnered significant attention due to their ability to achieve flexible and continuous control over harmonics through the flexible design of coding sequences. These metasurfaces have important applications in wireless communications, radar detection, and biomonitoring. However, the majority of existing time-varying digital coding metasurfaces are reflective in design, leading to an inevitable masking effect at the location of the feed source. Therefore, there is an urgent need to explore the design of transmissive time-varying digital coding metasurfaces that enable flexible and continuous control of harmonics. In this study, we propose a transmissive time-varying digital coding metasurface based on the induced-magnetism Huygens’ principle. We conduct a detailed study of the phase modulation theory for flexible and continuous control of harmonics in the time-varying digital coding induced-magnetism Huygens’ metasurface. By precisely modulating the transmission phase difference of this metasurface and introducing time-delay gradients in each column of metasurface units, we achieve flexible and continuous control over harmonics successfully. Our study presents a solution for the design of transmissive time-varying digital coding metasurfaces, enabling flexible and continuous control over harmonics and expanding the application potential of time-varying metasurfaces.

Intelligent Transmissive Microwave Metasurface with Optical Sensing and Transparency.

Transmissive metasurfaces are essentially conducive to stealth, absorbers, and communications. However, most of the current schemes only allow microwave to transmit and generally adopt multilayer structures or thick dielectric substrates to improve the electromagnetic performance, restricting optical transmission and conformal application. In addition, most metasurfaces still require metal wires and external power suppliers for programmability. Here, we propose and design an intelligent transmissive microwave metasurface with optical sensing and transparency, which provides both microwave and optical channels without redundant optical devices and power suppliers, and the 2 transmission channels are associated with each other. The metasurface is realized by validly integrating photosensitive materials into microwave meta-structures. As a demonstration, we fabricate an ultrathin optically transparent transmissive metasurface based on polyethylene terephthalate substrate and photoresistors, whose thickness is only 0.125 mm. We further construct cross-wavelength transmission links based on the metasurface sample and experimentally validate that the microwave transmissions vary with light intensities under full-polarization and large-angle incidences, and this metasurface possesses high optical transparency. The intelligent transmissive microwave metasurface with optical sensing and transparency has potential applications in optical-microwave hybrid transmission devices and stealth technology.

RFGlass: Optically Transparent Transmissive Metasurface for Coverage Expansion

RFGlass: Optically Transparent Transmissive Metasurface for Coverage Expansion

Multifunctional metasurface for Terahertz wavefront manipulation based on the combination of vanadium dioxide and graphene

In this paper, we propose a multifunctional wavefront control metasurface based on vanadium dioxide (VO2) and graphene. The unit cell structure contains a VO2 substrate, three rectangular metal strips as the bottom layer, orthogonal graphene strips as the top layer, and an intermediate layer consisting of a split ring resonator and circular metal patches. The desired performances can be achieved by changing the parameters of the split ring resonator in the middle layer, as well as controlling the phase transition state of VO2 and the Fermi level of graphene. When VO2 is in the insulating phase and the Fermi level of graphene is 1 eV, a transmissive metasurface is established, which realizes cross-polarization conversion, anomalous refraction, focusing lens, and orbital angular momentum generation at the frequency of 0.63 THz by changing the geometric sizes of the unit cell. When VO2 is in the metal phase and the Fermi level of graphene is 0.01 eV, a reflective metasurface can be achieved, which realizes terahertz beam splitting, radar cross section, and anomalous reflection of a single beam at the frequency of 0.63 THz. This multifunctional wavefront control metasurface treating the combination of VO2 and graphene exhibits significant application prospects in terahertz transmission, imaging, and communication.

High‐Efficiency Pancharatnam–Berry Metasurface‐Based Surface Plasma Coupler

To excite surface plasmon polaritons (SPPs) in an ultrathin way, metasurface‐based SPP couplers are widely used for converting propagating waves into driven waves bounded on their surfaces. Among them, the Pancharatnam–Berry (PB) metasurface coupler has a strong ability to control spin‐polarized waves, but is still limited by its coupling efficiency. Herein, a PB metasurface‐based SPP coupler that can support efficient circular‐polarization conversion with both the transverse magnetic and transverse electric modes of the SPP is proposed. The coupler is composed of an upper transmissive PB gradient metasurface and a lower eigenmode board to transform a circularly polarized plane wave into left‐handed circularly polarized and right‐handed circularly polarized surface waves that propagate in opposite directions. In the experimental results, it is indicated that the directionality factor of the tested SPP coupler is 28.6. The proposed PB metasurface coupler may pave the way for future integrated optical devices with high efficiency and excellent polarization‐split properties.

Multi-mode vortex beams generation with single-layer transmissive metasurface

Orbital angular momentum (OAM) is a phenomenon of vortex phase distribution in free space, the infinity of whose modes and the orthogonality between modes allow it to possess great potential in enhancing channel capacity. A transmissive metasurface consisting of a single-layer substrate and a two-layer metallic structure is proposed to enable vortex beam generation based on the concept of the Pancharatnam-Berry (PB) phase. The phase response is achieved by continuous phase modulation under circularly polarised (CP) wave incidence due to the simultaneous rotation of the upper and lower metal structures. Besides, through the introduction of both phase compensation on the metasurface and phase superposition of the beams to the array design, multi-beam generation is expected to be realized. A metasurface sample composed of 30 30 elements (450 mm 450 mm) is fabricated and measured in a microwave darkroom on the basis of the simulation results. Both the simulation and measurement results provide firm demonstration that the designed metasurface is capable of simultaneously generating four beams carrying OAM mode numbers of and different beam directions at the same aperture. The designed metasurface in this paper utilized for generating multi-mode vortex multi-beams will be of wide application in multi-channel transmission in wireless communication systems.

Reconfigurable coherent perfect absorption of different polarized electromagnetic fields through vanadium-dioxide based asymmetry transmissive meta-surfaces sandwiched with graphene meta-gratings

We demonstrate coherent perfect absorption (CPA) of different linearly polarized electromagnetic fields using an asymmetry transmissive meta-surface consisting of two outermost VO2-metal layers and symmetrical graphene meta-gratings, as well as an intermediate C-slit metallic layer. Such a five-layered cascaded VO2-metal-graphene meta-surface can perfectly trap either the x- or the y-polarized signal fields when interacting with the x-polarized control wave by imposing specific Fermi level over the graphene together with conducting- or insulating-state VO2 at 3 THz. On the other hand, y-polarized control wave can also interfere with either x- or y-polarized signal fields when the CPA of the electromagnetic wave operates at 3.65 THz. Our design, using asymmetry transmissive meta-surfaces for more advanced coherent control of different linearly polarized electromagnetic fields with the assistance of tunable materials, should pave the way for building up multipolar and multifunctional absorbers.

Polarization-Insensitive Transmissive Metasurfaces Using Pancharatnam–Berry and Resonant Phases in Microwave Band

Most of the existing metasurfaces are effective for the incident wave with the specific circularly polarized (CP) or linearly polarized (LP) state, that is the polarization-sensitive metasurface. This drawback dramatically hinders the practical use of the metasurface. Herein, this paper presents a strategy of polarization-insensitive transmissive microwave metasurfaces to manipulate the incident wave with arbitrary CP and LP states. The metasurface consists of polarization-insensitive unit cells. For the left circularly polarized (LCP) and right circularly polarized (RCP) incident waves, the same abrupt phase covering 0° to 360° can be realized by combining the Pancharatnam–Berry (PB) and resonant phases. As the arbitrary LP wave can decompose into the LCP and RCP waves, metasurfaces consisting of designed unit cells are valid for any polarization states. The polarization-insensitive transmissive microwave metalens and orbital angular momentum multiplexing metasurface working at 23 GHz are devised for verification. Simulation and measurement results verify the availability of the approach. The proposed method is suitable for designing microwave-transmissive metasurfaces capable of polarization insensitivity.

Wideband 1-bit reconfigurable transmission metasurface unit cell design in Ka-band with polarization hold and conversion

In this paper, a wideband transmission unit cell is proposed for programmable metasurfaces operating in the Ka-band. The unit cell features a compact period of only 2.91 mm, corresponding to 0.34 λ0 at the center frequency of 35 GHz. A receiving layer, consisting of a patch loaded with two PIN diodes, is utilized to achieve 1-bit phase modulation, while a U-shaped patch serves as the transmitting layer to enable selection of linear polarization hold or conversion. Based on the multi-resonance principle, the proposed unit cell exhibits broadband behavior, as demonstrated by simulation results under periodic boundary conditions, which indicate a 3 dB transmission bandwidth of 29.4–40 GHz (30.5%). Two unit cells were fabricated and tested in a standard waveguide, with the minimum insertion loss of the two states tested being 1.2 dB and 3 dB bandwidths of 30.1–31.2 GHz and 33.5–38.5 GHz, respectively. The maximum 180° phase error is 10°, indicating the high quality of the proposed unit cell.

Inverse design of metasurfaces with customized transmission characteristics of frequency band based on generative adversarial networks.

In recent years, deep learning (DL) has demonstrated significant potential in the inverse design of metasurfaces, and the generation of metasurfaces with customized transmission characteristics of frequency band remains a challenging and underexplored area. In this study, we propose a DL-assisted method for the inverse design of transmissive metasurfaces. The method consists of a generative adversarial network (GAN)-based graph generator, an electromagnetic response predictor, and a genetic algorithm optimizer. By integrating these components, we can obtain customized metasurfaces with desired transmission characteristics of frequency band. We demonstrate the effectiveness of the proposed method through examples of inverse-designed three-layer cascaded transmissive metasurfaces with wideband, dual-band, and stopband responses in the 8∼12 GHz frequency range. Specifically, we realize three different types of dual-band metasurfaces, namely double-wide, front-wide and rear-narrow, and front-narrow and rear-wide configurations. Additionally, we analyze the accuracy and reliability of the inverse design method by employing data from the training dataset, self-defined objectives, and bandwidth-reduced target responses scaled from the wideband type as design inputs. Quantitative evaluation is performed using metrics such as mean absolute error and average precision. The proposed method successfully achieves the desired effect as intended.

Dual-mode vortex beam transmission metasurface antenna based on linear-to-circular polarization converter.

The generation of multi-mode vortex beams at the same aperture is currently emerging as a research hotspot. In this paper, a method based on a linearly polarized-circularly polarized translational transmission metasurface (TM) is proposed to enable a dual-circularly polarized dual-mode vortex beam generation. Through the judicious implementation of an additional rotational phase and the combination of the initial transmission phase, the phases of the left-hand circularly polarized (LHCP) and right-hand circularly polarized (RHCP) waves can be manipulated arbitrarily and independently. Meanwhile, the design of the array phase is utilized for the dual-mode dual-circularly polarized beam generation. Simulation and sample measurements provide validation data for the feasibility of this method, in which the measurement results are in excellent consistency with the simulation ones. This proposed method paves the way toward the enhancement of the channel capacity of mobile communication.

Terahertz vector beams generated by rectangular multilayer transmission metasurface

We propose a multilayer unit structure with an “H” shape to construct a vector beam metasurface generator. Based on the principle that half-wave plate can modulate and change the polarization state of linearly polarized incident waves, we have designed a metasurface composed of rectangular units, which can convert linearly polarized beams into azimuthally or radially polarized vector beams at 0.1 THz. The theoretical transmission efficiency is about 60.92 %.We also have prepared a sample of the vector beam generating metasurface by printed circuit board (PCB) technology, and tested the intensity distribution and polarization state of the transmitted beam on the self-built terahertz testbed. The vector beam metasurface realizes the miniaturization and integration characteristics of the device, which has potential applications in microscopic imaging, biophotonics, quartentum information, laser acceleration, micromachining, and other fields.