Transmissive Metasurface Research Articles

Flat double-layer metal-only metasurface with polarization-dependent operation in a C/X dual band

In this communication, a dual-band, orthogonal polarization transmission metasurface antenna operating at 6.5 GHz and 9.5 GHz is proposed. The element of this transmitarray is composed of only two identical layers of metal plates, with air medium filling the space between the upper and lower layers. Each layer has four sets of staggered I-slots, thus ensuring dual-frequency orthogonal polarization operation. Moreover the proposed element overcomes the challenge of achieving full 360-degree phase shift in dual-layer metal-only structures without any additional connecting elements. By altering the length of the slots, it can achieve a continuous full 360-degree phase shift while maintaining a transmission amplitude above −2 dB and exhibits co-polarized transmission in both bands. Then a prototype containing 16 × 16 elements is designed and fabricated to verify the feasibility of the design scheme. The measurement results, which are consistent with the simulation, demonstrate that the maximum gains of 24.85 and 27.64 dBi are obtained at 6.4 GHz and 9.4 GHz, with corresponding aperture efficiencies of 23.18% and 20.4% and a 3-dB gain bandwidth of 15.6% and 22.34%. The good transmission coefficients of the element and the dual-band capability make the design a competitive candidate for satellite and high-power applications.

Wideband Transmissive Programmable Metasurface for Adaptive Millimeter‐Wave Beamforming

AbstractProgrammable metasurfaces provide a promising paradigm for the beamforming of electromagnetic waves in the fifth‐generation (5G) and sixth‐generation (6G) wireless communications with low cost and low complexity. However, most of the existing researches are focused on the sub‐6 GHz spectrum, which limited the gigabits per second (Gb s−1) high‐speed data transmission on future wireless network. Additionally, as a step forward in the development of intelligent metasurfaces, extra peripheral devices are often required to sense environmental variations for self‐adaptive modulations. Here, a wideband millimeter‐wave programmable metasurface is presented for peripheral‐free self‐adaptive signal enhancement in 5G/6G wireless communication. The proposed transmissive programmable metasurface consists of meta‐atoms, exhibiting efficient transmission (1 dB loss) and a stable 1‐bit phase difference from 23.7 to 32.1 GHz, effectively covering the desired 5G millimeter‐wave frequency bands: N257, N258, and N261. By integrating the self‐adaptive algorithm based on received signal intensity into the controller, the programmable metasurface can self‐adaptively establish an optimal or acceptable channel according to the predefined threshold values. To evaluate the performance of the prototype, an experiment on wavefront modulation was first conducted in a microwave chamber, verifying the wideband beamforming performance of the programmable metasurface. Moreover, self‐adaptive signal enhancement and millimeter‐wave wireless communication experiments were conducted in indoor environments, showing a maximum signal enhancement of 21.4 dB and a significant improvement in constellation. The proposed self‐adaptive wideband programmable metasurface demonstrates high potential for application in 5G/6G millimeter‐wave wireless networks.

Generating sub-diffraction microwave needle beam for nondestructive testing by multifunctional transmissive metasurfaces

Sub-diffraction needle beams with high intensity, sub-diffraction focal size, and long depth of focus (DOF) have attracted many researchers’ attention. However, the traditional methods for needle beam generation typically require many devices, such as phase elements, amplitude filters, and lens, which leads to a complex and bulky system and unfavorable for their integration. To address these challenges, we use a single multifunctional transmissive metasurface to convert a linearly polarized plane wave into a needle beam in the microwave range. The guided wave inspired unit cells of the proposed metasurface is designed to simultaneously and independently modulate the polarization and phase of transmitted waves. By imposing the desired polarization and phase distributions on the metasurface, the proposed multifunctional transmissive metasurface can efficiently generate a needle beam with subdiffraction size and extended DOF at 10 GHz when it is illuminated by an x-polarized wave. The proposed metasurface is fabricated, and a sub-diffraction needle beam with good performance is obtained in our measurements. In addition, a proof-of-concept of a high-resolution nondestructive testing experiment based on our designed metasurface is accomplished. Our work is expected to have potential applications in nondestructive testing of materials and structures.

Multi-mode non-diffraction vortex beams enabled by polarization-frequency multiplexing transmissive terahertz metasurfaces

In terahertz (THz) wireless communication systems, non-diffraction vortex beams carrying an orbital angular momentum (OAM) have attracted extensive attention due to their ability to transmit information over long distances with high capacity. However, existing metasurfaces can only generate a single OAM mode non-diffracting vortex beam at reflection space for circular polarization (CP) incidence, limiting practical applications. To address this issue, we propose and design a polarization-frequency multiplexing transmissive THz metasurface to realize multi-mode non-diffracting vortex beams at linear polarization (LP) incidence. The meta-atom of this metasurface is composed of three anisotropic rectangular metallic structures embedded in vanadium dioxide (VO2) square rings, two circular aperture metallic grid layers, and four dielectric layers. By reasonably designing the size of the metal patch and the state of VO2, the designed metasurface can achieve polarization multiplexing and frequency multiplexing for LP incidence. Based on the phase response of the proposed meta-atoms, the transmissive metasurface can implement not only multi-mode non-diffraction vortex beams but also their space separation at two frequency ranges of 0.80–0.90 THz and 1.50–1.80 THz by changing the state of VO2. Therefore, the proposed multiple multiplexing metasurfaces can effectively shape the wavefront of non-diffraction vortex beams, which have broad application prospects in 6G THz communication.

Dual-polarized transmissive metasurfaces for near-unitary-NA analog spatial computing empowered by impedance matching and mismatching.

In recent years, due to the increasing requirement for real-time and massive data processing, optical analog computation has arisen as a promising alternative to digital computation. Optical spatial differentiation plays a fundamentally important role in various emerging technologies, including augmented reality, autonomous driving, and object recognition. However, previous demonstrations encountered several limitations, such as the dependence on polarization states and a typically limited numerical aperture (NA) of about 0.5, especially in the transmission mode. Here, a new, to our knowledge, design strategy based on the evolution between impedance matching and mismatching in a metasurface is proposed to fill this gap, which can perform dual-polarized second-order derivative for image processing. Our scheme enables high transmission under dual polarization over an 85° incident angle range (NA = 0.996), resulting in more than twofold spatial resolution. Our work paves the way for polarization-insensitive high-resolution signal and image processing in the terahertz region.

Transmissive Metasurface Synthesis From Far-Field Masks Using Unsupervised Learning

Transmissive Metasurface Synthesis From Far-Field Masks Using Unsupervised Learning

Nonlinear mid-infrared meta-membranes

Abstract Nanophotonic structures have shown promising routes to controlling and enhancing nonlinear optical processes at the nanoscale. However, most nonlinear nanostructures require a handling substrate, reducing their application scope. Due to the underwhelming heat dissipation, it has been a challenge to evaluate the nonlinear optical properties of free-standing nanostructures. Here, we overcome this challenge by performing shot-controlled fifth harmonic generation (FHG) measurements on a SiC meta-membrane – a free-standing transmission metasurface with pronounced optical resonances in the mid-infrared (λ res ≈ 4,000 nm). Back focal plane imaging of the FHG diffraction orders and rigorous finite-difference time-domain simulations reveal at least two orders of magnitude enhancement of the FHG from the meta-membrane, compared to the unstructured SiC film of the same thickness. Single-shot measurements of the meta-membrane with varying resonance positions reveal an unusual spectral behavior that we explain with Kerr-driven intensity-dependent resonance dynamics. This work paves the way for novel substrate-less nanophotonic architectures.

Bidirectional triple-band truly incident angle insensitive polarization converter using graphene-based transmissive metasurface for terahertz frequency

Abstract The design and response of a triple-band linear-to-circular polarization converter (LTCPC) in the terahertz frequency regime using a graphene-based transmission-type metasurface have been studied numerically and analytically. The unit cell of the converter is constructed using lossy silicon dioxide (SiO2) with a relative permittivity of 3.9 as the substrate. And two similar layers of graphene-based sub-wavelength structures are used at the top and bottom of it. Multiband linear to circular polarization conversion is achieved from 0.74 THz to 0.98 THz, 1.70 THz to 2.33 THz, and 4.06 THz to 5.19 THz, i.e., about 28 %, 31 %, and 24 % fractional bandwidths, respectively. As the same metasurface configuration is used at the top and bottom layers of the transmission-type unit cell, it can be used as a bidirectional polarization converter with identical responses from both sides. The geometrical optimization of the unit cell is done, and an equivalent lumped parameter circuit model is also proposed for the same, considering analogous responses. Tunability over the operating frequency is achieved by varying the chemical potential and relaxation time of graphene. Moreover, due to the ultrathin width and special types of identical configuration of the metasurfaces, the response of the system remains excellent over a wide range of incident angle variations up to 80° angle of incidence.

A metasurface antenna with beam steering capabilities for radio frequency energy harvesting

Limited control over parameters within the realm of design complexity, it is noted that there are incomplete unit cells displaying slightly lower transmittance in transmissive metasurface antennas. However, traditional methods require time-consuming iterative processes to select unit cells with multiple variables. Therefore, a novel approach is proposed for designing metasurface superstrates on patch antennas operating at 12.5 GHz, focusing on achieving beam steering capabilities and Radio Frequency (RF) energy harvesting functionality. It utilizes a Skeleton-based Attentions Neural Network (SANN) to aid in the complex metasurface unit cell design, enhancing the antenna's flexibility and potential applications. This network is trained to generate patterns for the metasurface unit cell, facilitating accurate and efficient design. The key feature of this antenna is beam steering in multiple directions. Steering is achieved by rotating the metasurface plane in an anti-clockwise direction. Experimental results show that by rotating the metasurface plane at different angles, the beam is directed towards specific directions, such as 10°, 15°, 18°, 21°, and 23°. A load resistance of 8kᾨ and a full wave rectifier with a Positive-Intrinsic-Negative (PIN) diode are utilized to obtain a Direct Current (DC) output voltage of 1.1 V in the proposed rectifier. Copper material is used for the metasurface plane, and the antenna is measured using RT duriod.

A-shaped switchable asymmetric transmission metasurface based on VO2 in the THz band

The asymmetric transmission of terahertz (THz) waves has attracted considerable interest because of its novel applications in isolators and wireless communication. An A-shaped switchable asymmetric transmission metasurface integrating vanadium dioxide (VO2) in the THz band is proposed, which achieved dynamic control of asymmetric transmission by exploiting insulator-to-metal transition characteristics of VO2. When VO2 is in an insulating state, the proposed metadevice realized an asymmetric transmission effect of the asymmetric transmission parameter reaching up to 0.76 and realized linear polarization conversion with a 95% polarization conversion rate. Furthermore, the proposed metadevice prevented asymmetric transmission and polarization conversion as VO2 transforms in the metallic state. This work provides a new ideal for the design of switchable asymmetric transmission metadevices and has potential applications in wireless communication, sensing, and polarization control.

Generation of parabolic beam using an amplitude and phase modulated metasurface

In this work, non-diffraction parabolic beam is generated using a double-layer transmission metasurface in the microwave frequency range. A transmissive unit cell with bilayered substrates and three metallic layers is designed. By controlling the orientation angle and the opening angle of the intermediate ∈-shaped metallic layer, transmission amplitude from 0 to 1 and continuous phase shift from 0° to 360° can be modulated simultaneously. To experimentally verify the proposed concept and design, a transmissive metasurface is fabricated and measured at 20 GHz. The simulated and measured results are in good agreement with the theoretical results, and prove the generated parabolic beam still maintained non-diffraction and self-healing characteristics within the maximum non-diffraction distance of 40λ. The generated microwave parabolic beam with its properties can be applied to improving the distance and efficiency of WPT and near field modulation of complex EM waves.

Ultra-Wideband Transmissive Metasurface With Independent Amplitude–Phase-Polarization Control for Co-Polarized Waves Empowered by Duplexing Meta-Particle

Ultra-Wideband Transmissive Metasurface With Independent Amplitude–Phase-Polarization Control for Co-Polarized Waves Empowered by Duplexing Meta-Particle

Transmissive reconfigurable metasurface enabling independent control of active and passive modules through weak coupling

Metasurfaces have demonstrated rich electromagnetic control capabilities and degrees of freedom in past years. As is well known, for passive metasurfaces, their functionalities cannot be further expanded accordingly once prototypes are established. Therefore, reconfigurable metasurfaces, utilizing active devices to replace geometric changes in passive structures, have received widespread attention, especially with the development of wireless communication recently. In reconfigurable metasurfaces, artificial meta-atoms are composed of active devices and passive structures combined together. However, these two modules are usually utilized as a whole due to the tight coupling of the active devices and the passive structures, which results in passive structures not receiving sufficient attention and being utilized as independent degrees of freedom. In this article, we propose the concept of weakly coupled reconfigurable metasurfaces in transmissive systems, enabling independent control of active and passive modules through weak coupling. As the proof of concept, a simple weakly coupled system is proposed, which can realize the transmission wavefront engineering through the geometric changes of meta-structures in passive mode, while achieving switching between transmission and reflection states in active mode, respectively. Our exploration lies in making use of the physical structure, which is easily neglected in traditional reconfigurable metasurface design, emphasizing the collaborative work of active and passive modules, exploring more available variables within the same aperture, and providing a potential solution for balancing functionality and resource consumption in practical applications.

Broadband achromatic transmission stealth cloak based on all dielectric metasurfaces

The emergence of metasurface technology has brought innovation to the design of optical devices, and many optical devices based on metasurface have been proposed. However, the inherent dispersion of metasurface structures limits their ability to operate at wide frequencies. By changing the geometric parameters of the metasurface unit structure, the unit structure has achromatic properties on the basis of satisfying the phase gradient. Numerical simulation results demonstrate that the proposed multilayer transmissive metasurface stealth cloak has broadband stealth function in the frequency range of 0.7 to 0.77 THz. Achromatic broadband metasurface stealth devices provide new ideas for the design of broadband metasurface stealth devices, and also promote the research of other broadband metasurface optical devices.

Terahertz vortex beam generation based on reflective and transmissive graphene metasurfaces

Graphene metasurfaces have substantial potential for generating vortex electromagnetic waves carrying orbital angular momentum (OAM) at terahertz frequencies. In this paper, a deformed H-shaped graphene unit cell is utilized to design a half-space reflective metasurface (with a metal backplane, operating in the reflection mode) and a full-space metasurface (operating in both the reflection and transmission modes) to generate OAM beams. The two structures are intertransformable at the level of physical composition, where one configuration enables efficient anomalous reflection of electromagnetic waves within the 4.6–8.9 THz band, while the other enables uniform modulation efficiency of reflective and transmissive waves within the 5.5–7.0 THz band. Equivalent circuit models have been provided to establish a convenient closed-form mathematical description of the metasurface’s electromagnetic behavior. By performing the near- and far-field simulations, a series of THz vortex generators based on the proposed graphene metasurface are demonstrated. In particular, high-purity reflective and transmissive OAM waves with distinct topological charges are observed. Our study provides a path towards the practical realization of active THz OAM metadevices.

RCS reduction of wide-band, high-gain antenna array based on asymmetric transmission metasurfaces

In this paper, an ultra-wideband stealth antenna with high gain on the basis of asymmetric transmission metasurface (ATMS) is proposed. ATMS can convert an incident y-polarised sphere wave into an x-polarised plane wave at the front side and controls the scattering of the incident y-polarised wave to the back side. Excitation of ATMS via a horn antenna, a low radar cross-section (RCS) and wideband antenna system is designed. Furthermore, through design of the meta-atoms and optimization of the macrosequencing, broadband RCS reduction is achieved. The experimental data indicated the reduction of the RCS of the antenna system by up to 10 dB and more than 20 dB in the frequency range of 10.1 GHz to 18 GHz (relative bandwidth is 56.2%) and 13.9 GHz to 18 GHz (relative bandwidth is 25.7%), respectively. In addition, a 3 dB gain relative bandwidth of 57.4% is achieved between 10 and 18 GHz, with a peak gain of 28.2 dB. It is noteworthy that the high gain and low scattering performance of the antenna are achieved in the same spectral range (10–18 GHz), and there is no interference between the scattering performance and radiation performance of the antenna, which could be controlled separately.

Unidirectional Transmission Metasurfaces with Topological Continuity Generated from High-dimensional Design Space.

With the growing demand for nanodevices, there is a concerted effort to improve the design flexibility of nanostructures, thereby expanding the capabilities of nanophotonic devices. In this work, a Laplacian-weighted binary search (LBS) algorithm is proposed to generate a unidirectional transmission metasurface from a high-dimensional design space, offering an increased degree of design freedom. The LBS algorithm incorporates topological continuity based on the Laplacian, effectively circumventing the common issue of high structural complexity in designing high-dimensional nanostructures. As a result, metasurfaces developed using the LBS algorithm in a high-dimensional design space exhibit reduced complexity, which is advantageous for experimental fabrication. An all-dielectric metasurface with unidirectional transmission, designed from the high-dimensional space using the LBS method, demonstrated the successful application of these design principles in experiments. The metasurface exhibits high optical performance on unidirectional transmission in measurements by a high-resolution angle-resolved micro-spectra system, achieving forward transmissivity above 90% (400-700 nm) and back transmissivity below 20% (400-500 nm) within the targeted wavelength range. This work provides a feasible approach for advancing high-dimensional metasurface applications, as the LBS design method takes into account topological continuity during experimental processing. Compared to traditional direct binary search (DBS) methods, the LBS method not only improves information processing efficiency but also maintains the topological continuity of structures. Beyond unidirectional transmission, the LBS-based design method has generality and flexibility to accommodate almost all physical scenarios in metasurface design, enabling a multitude of complex functions and applications.

High-Efficiency Transmissive Tunable Metasurfaces for Binary Cascaded Diffractive Layers

High-Efficiency Transmissive Tunable Metasurfaces for Binary Cascaded Diffractive Layers

Ultrafast modulable 2DEG Huygens metasurface

Huygens metasurfaces have demonstrated remarkable potential in perfect transmission and precise wavefront modulation through the synergistic integration of electric resonance and magnetic resonance. However, prevailing active or reconfigurable Huygens metasurfaces, based on all-optical systems, encounter formidable challenges associated with the intricate control of bulk dielectric using laser equipment and the presence of residual thermal effects, leading to limitations in continuous modulation speeds. Here, we present an ultrafast electrically driven terahertz Huygens metasurface that comprises an artificial microstructure layer featuring a two-dimensional electron gas (2DEG) provided by an AlGaN/GaN heterojunction, as well as a passive microstructure layer. Through precise manipulation of the carrier concentration within the 2DEG layer, we effectively govern the current distribution on the metasurfaces, inducing variations in electromagnetic resonance modes to modulate terahertz waves. This modulation mechanism achieves high efficiency and contrast for terahertz wave manipulation. Experimental investigations demonstrate continuous modulation capabilities of up to 6 GHz, a modulation efficiency of 90%, a transmission of 91%, and a remarkable relative operating bandwidth of 55.5%. These significant advancements substantially enhance the performance of terahertz metasurface modulators. Importantly, our work not only enables efficient amplitude modulation but also introduces an approach for the development of high-speed and efficient intelligent transmissive metasurfaces.

Nonvolatile Phase-Only Transmissive Spatial Light Modulator with Electrical Addressability of Individual Pixels.

Active metasurfaces with tunable subwavelength-scale nanoscatterers are promising platforms for high-performance spatial light modulators (SLMs). Among the tuning methods, phase-change materials (PCMs) are attractive because of their nonvolatile, threshold-driven, and drastic optical modulation, rendering zero-static power, crosstalk immunity, and compact pixels. However, current electrically controlled PCM-based metasurfaces are limited to global amplitude modulation, which is insufficient for SLMs. Here, an individual-pixel addressable, transmissive metasurface is experimentally demonstrated using the low-loss PCM Sb2Se3 and doped silicon nanowire heaters. The nanowires simultaneously form a diatomic metasurface, supporting a high-quality-factor (∼406) quasi-bound-state-in-the-continuum mode. A global phase-only modulation of ∼0.25π (∼0.2π) in simulation (experiment) is achieved, showing ten times enhancement. A 2π phase shift is further obtained using a guided-mode resonance with enhanced light-Sb2Se3 interaction. Finally, individual-pixel addressability and SLM functionality are demonstrated through deterministic multilevel switching (ten levels) and tunable far-field beam shaping. Our work presents zero-static power transmissive phase-only SLMs, enabled by electrically controlled low-loss PCMs and individual meta-molecule addressable metasurfaces.