Digital Metasurface Research Articles

Two-dimensional direction-of-arrival estimation based on time-domain-coding digital metasurface

Direction-of-arrival (DOA) estimation is one of the most critical technologies of radar, remote sensing, and wireless communications. The traditional DOA estimation is closely related to phased array antennas, which require complicated and expensive hardware and high-power consumption. Metasurface can manipulate electromagnetic waves without using massive transceivers, which makes it possible to realize antenna arrays in a cost-effective way. Here, we propose a strategy of DOA estimations by using a time-domain-coding digital metasurface and a single receiver. Specifically, the incident wave on the metasurface is modulated by the time-domain orthogonal codes impressed on the meta-atoms, and their amplitude and phase distributions are precisely retrieved from the signals detected by the receiver antenna. The effectiveness and accuracy of the proposed strategy are verified by experiments on a two-dimensional metasurface with individually addressable meta-atoms. The strategy features low cost and high flexibility and will facilitate various wireless applications.

Space-frequency-polarization-division multiplexed wireless communication system using anisotropic space-time-coding digital metasurface.

In the past few years, wireless communications based on digital coding metasurfaces have gained research interest owing to their simplified architectures and low cost. However, in most of the metasurface-based wireless systems, a single-polarization scenario is used, limiting the channel capacities. To solve the problem, multiplexing methods have been adopted, but the system complexity is inevitably increased. Here, a space-frequency-polarization-division multiplexed wireless communication system is proposed using an anisotropic space-time-coding digital metasurface. By separately designing time-varying control voltage sequences for differently oriented varactor diodes integrated on the metasurface, we achieve frequency-polarization-division multiplexed modulations. By further introducing different time-delay gradients to the control voltage sequences in two polarization directions, we successfully obtain space-frequency-polarization-division multiplexed modulations to realize a wireless communication system with a new architecture. The new communication system is designed with compact dual-polarized meta-elements, and can improve channel capacity and space utilization. Experimental results demonstrate the high-performance and real-time transmission capability of the proposed communication system, confirming its potential application in multiple-user collaborative wireless communications.

A programmable digital metasurface structure designed using ANN technique

ABSTRACT In this paper, an 8-bit programmable digital metasurface is designed in the operating frequency range from 4 to 7 GHz. A monopole antenna operating at 5 GHz is characterised by using a metasurface structure as the ground plane. Various combinations of the metasurface structure are examined by altering the state of each unit-cell between ‘ON (1)’ and ‘OFF (0)’ in the coding matrix. The purpose of the 8-bit programmable digital metasurface is to control the electromagnetic wave effectively in the frequency range of interest. Dynamically controllable metasurface structure is utilised to change the unit cells’ configurations. In addition, the proposed adjustable metasurface is capable to control the monopole antenna directivity, gain and main lobe magnitude, efficiently. According to various via conditions, different metasurface-based antenna parameters such as return loss (S11), radiation pattern, directivity and surface current distribution are investigated by means of the commercial numerical simulation software, CST microwave studio. Employing the simulation results of the metasurface-based antennas, the artificial neural network (ANN) data set is obtained. 128 activation conditions are trained and tested by Levenberg Marquart learning algorithm. During the ANN procedure, MATLAB is used to obtain accurate results by changing the rate of test and training data set.

Reconfigurable Intelligent Surfaces: Simplified-Architecture Transmitters—From Theory to Implementations

Reconfigurable intelligent surfaces (RISs) offer an entirely new route to alter the propagation properties of electromagnetic waves and thus control their reflection, refraction, and scattering features in arbitrary manners. Such physical attributes are perceived to bring about fundamental influence on the modern wireless communication system due to the possibilities to establish artificial and controllable propagation environments for radio signals, which no longer rely on the complex encoding, decoding, and other signal processing techniques. Recent studies reveal that the wave manipulation is not the only skill of the RISs. With the rapid developments of space–time digital metasurface and information metasurface, there has been increasing attention focused on the information manipulation via these artificial surfaces. In this article, we provide an overview of the theoretical models of the space–time digital metasurface and information metasurface, the mechanisms of wavefront shaping, and the signal modulations in space and time domains during the wave–matter interactions. We will also address some practical issues during implementations of the reconfigurable intelligent metasurfaces and the associated hardware architectures at microwave frequencies to realize simplified radio frequency transmitters. Several modulation schemes and the corresponding demonstration systems are introduced to illustrate the powerful abilities of the reconfigurable intelligent metasurfaces. Potential research directions of this technique are briefly discussed for their potential applications in future wireless networks.

1-Bit dual-polarized ultrathin lens antennas based on Huygens’ metasurface

In this paper 1-bit dual-polarized ultra-thin lens antennas are presented based on Huygens’ principle. The unit cell provides two-state transmission phase compensation for dual-polarized waves. By tuning state 0 away from resonance and state 1 near Huygens’ resonance, the 180° transmission phase difference between the two states is achieved. In the frequency range of 27.5–28.5 GHz, the transmission phase difference between the two states of the unit cell is 180° ± 20°, and the transmission amplitude is greater than −2 dB. Using the proposed unit cell, three ultrathin 1-bit Huygens’ metasurface antennas comprising 33 × 33 unit cells with single beams pointing separately at 0°, 15°, and 30° are designed, fabricated and measured. Simulated and measured results show that the proposed 1-bit transmitarray antenna can achieve single-beam patterns, which is useful for the development of reconfigurable transmission and digital metasurface antennas in the future.

Asynchronous Space-Time-Coding Digital Metasurface.

Recent progress in space‐time‐coding digital metasurface (STCM) manifests itself a powerful tool to engineer the properties of electromagnetic (EM) waves in both space and time domains, and greatly expands its capabilities from the physical manipulation to information processing. However, the current studies on STCM are focused under the synchrony frame, namely, all meta‐atoms follow the same variation frequency. Here, an asynchronous STCM is proposed, where the meta‐atoms are modulated by different time‐coding periods. In the proposed asynchronous STCM, the phase discontinuities on traditional metasurface are replaced with the frequency discontinuities. It is shown that dynamic wavefronts can be automatically realized for both fundamental and high‐order harmonics by elaborately arranging the spatial distribution of meta‐atoms with various time‐coding periods. The physics insight is due to the accumulated rapidly changing phase difference with time, which offers an additional degree of freedom during the wave‐matter interactions. As a proof‐of‐principle example, an asynchronous STCM for automatic spatial scanning and dynamic scattering control is investigated. From the theory, numerical simulations, and experiments, it can be found that the proposed STCM exhibits significant potentials for applications in radars and wireless communications.

Programmable Manipulations of Terahertz Beams by Graphene-Based Metasurface With Both Amplitude and Phase Modulations

Terahertz metasurface with digital, programmable control recently gained considerable attention for its potential applications in high-speed imaging, nondestructive sensing, and wireless communication. With elaborate design, the metasurface can perform amplitude, phase, and polarization modulations of electromagnetic waves. Most digital programmable metasurfaces focus on only one of the three dimensions. Here, we propose a graphene-based THz metasurface with both phase and amplitude modulations, which consists of an artificially constructed metal-insulator-metal structure and two-dimensional graphene material. Each meta-atom of the metasurface is divided into two sub-atoms, and the two sub-atoms can reflect terahertz waves with a phase difference of 180°. Meanwhile, the amplitude of the sub-atom can be effectively modulated or even switched off by applying different gate voltages to the graphene. By independently controlling the amplitude response of the two sub-atoms, the whole meta-atom can dynamically control both amplitude and phase responses of the cross-polarization waves. By carefully designing the coding patterns, the digital metasurface can control both beam direction and intensity, which may lead to various advanced applications in beam shaping, radar detection systems, and high-quality holography.

Digital Polarization Programmable Metasurface for Continuous Polarization Angle Rotation and Radar Applications

Polarization angle manipulation has been a vital technic in radar applications. This paper proposes and demonstrates a digital polarization programmable metasurface for continuous polarization angle rotation and radar applications. By coding “0” and “1” elements with the two orthogonally polarized waves having 180° phase difference, the polarization angle of electromagnetic (EM) waves can be continually and arbitrarily manipulated. The designed metasurface adopts a patch-transmission and line-patch structure and integrates two polarization channels to carry out 1-bit coding. By rotating the azimuth angle of the designed metasurface mechanically, a continuous rotation of the polarization angle of the transmitted wave can be achieved. Moreover, the transmission around 9.4 GHz can reach higher than 95%. The metasurface sample with optimized structure parameters has been fabricated and tested, where the measurement agrees well with the simulation results. In addition, a radar detection experiment was implemented with an anisotropic target, demonstrating the practical use of the proposed metasurface for polarimetric radar.

Simultaneous in situ Direction Finding and Field Manipulation Based on Space-Time-Coding Digital Metasurface

The smart radio environment has become an eye-catching topic and is believed to own great potential in future wireless networks. Its essential demand lies in sensing and manipulating the electromagnetic (EM) fields accurately and simultaneously with the same aperture, which is hard to achieve using conventional methods due to the difficulty in integrating different systems. We propose a millimeter-wave space–time-coding digital metasurface (STCM) to realize simultaneous <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> direction finding and field manipulation. An analytic method of direction finding is first developed using the STCM modulated by a space–time-coding (STC) matrix while maintaining its original capability of EM field manipulations. In addition, three STC matrices corresponding to different EM functions are applied to the STCM to verify the feasibility and robustness of direction findings experimentally. Finally, a proof-of-concept prototype is built to exhibit different EM functions according to the incident angle detected simultaneously. The proposed concept sets the stage for the integrated implementation of the perception and response of the radio environment, which may find promising applications in future intelligent wireless networks and self-adaptive systems.

Digital metasurface for selective information distribution in the spatial domain in the THz region.

This paper presents a new, to the best of our knowledge, technique to control the flow of electron packets employing a switchable digital metasurface in the terahertz region. The proposed structure can act as a device that converts an electromagnetic (EM) wave into electron packets that can be represented in the form of digital bit sequences at the output. The top layer of the structure controls the intensity of the electron packets while the bottom layer involves the spatial distribution of the information with the help of electrical switching. The electrical switching is achieved with the application of suitable biasing across a vanadium dioxide (VO2) patch. This approach to the distribution of information helps feed the circuitry of an electronic system as per the requirement using electrical tuning, which is not possible in a conventional antenna system.

Near-infrared plasmonic sensing and digital metasurface via double Fano resonances.

Plasmonic sensing that enables the detection of minute events, when the incident light field interacts with the nanostructure interface, has been widely applied to optical and biological detection. Implementation of the controllable plasmonic double Fano resonances (DFRs) offers a flexible and efficient way for plasmonic sensing. However, plasmonic sensing and digital metasurface induced by tailorable plasmonic DFRs require further study. In this work, we numerically and theoretically investigate the near-infrared plasmonic DFRs for plasmonic sensing and digital metasurface in a hybrid metasurface with concentric ϕ-shaped-hole and circular-ring-aperture unit cells. We show that a plasmonic Fano resonance, resulting from the interaction between a narrow and a wide effective dipolar modes, can be realized in the ϕ-shaped hybrid metasurface. In particular, we demonstrate that the tailoring plasmonic DFRs with distinct mechanisms of actions can be accomplished in three different ϕ-shaped hybrid metasurfaces. Moreover, the resonance mode-broadening and mode-shifting plasmonic sensing can be fulfilled by modulating the polarization orientation and the related geometric parameters of the unit cells in the near-infrared waveband, respectively. In addition, the plasmonic switch with a high ON/OFF ratio can not only be achieved but also be exploited to establish a single-bit digital metasurface, even empower to implement two- and three-bit digital metasurface characterized by the plasmonic DFRs in the telecom L-band. Our results offer a new perspective toward realizing polarization-sensitive optical sensing, passive optical switches, and programmable metasurface devices, which also broaden the landscape of subwavelength nanostructures for biosensors and optical communications.

Multi-Channel Near-Field Terahertz Communications Using Reprogrammable Graphene-Based Digital Metasurface

Digital metasurfaces have opened unprecedented ways to accomplish novel electromagnetic devices thanks to their simple manipulation of electromagnetic waves. However, the metasurfaces leveraging phase-only or amplitude-only modulation restricted the full-functionality control of the devices. Herein, a digital graphene-based metasurfaces engineering wavefront amplitude and phase are proposed for the first time to tackle this challenge in the terahertz (THz) band. The concept and its significance are verified using reprogrammable multi-focal meta-lens based on a 2/2-bit digital unit cell with independent control of 2-bit states of amplitude and phase individually. Moreover, we introduce a novel method to directly transmit digital information over multiple channels via the reprogrammable digital metasurface. Since these metasurfaces are composed of digital building blocks, the digital information can be directly modulated to the metasurface by selecting specific digital sequences and sent them to predetermined receivers distributed in the focal points. Following that, a multi-channel THz high-speed communication system and its application to build three-dimensional wireless agile interconnection are demonstrated. The presented method provides a new architecture for wireless communications without using complicated components of conventional systems. This work motivates versatile meta-devices in many applications envisioned for the THz frequencies, which will play a vital role in modern communications.

Programmable Manipulations of Terahertz Beams by Transmissive Digital Coding Metasurfaces Based on Liquid Crystals

AbstractDigital coding metasurfaces have attracted considerable interest, owing to their ability to manipulate electromagnetic waves and implement various functionalities with programmable controls. These metasurfaces offer high‐resolution and high‐speed responses; thus, they are implemented in high‐speed imaging, nondestructive sensing, and wireless communication. However, traditional active metasurfaces are typically based on semiconductor components, which are difficult to realize at terahertz frequencies. Additionally, non‐magnetic transmissive metasurfaces cannot realize a large phase‐tuning range without increasing the number of metasurface layers. Herein, a 1‐bit transmissive digital coding metasurface based on liquid crystals is proposed to achieve programmable terahertz beam manipulations with electric control. The Fano resonance that is excited in the asymmetrical metasurface element enables a larger phase variation while maintaining the transmittance. The proposed digital metasurface can effectively achieve many functionalities, such as dual beam steering, multiple beams, and orbital angular momentum beams, via altered coding patterns; an example of dual beam steering is experimentally verified. This study is expected to provide a promising method to manipulate transmissive terahertz beams using a digitally programmable metasurface, and it has potential applications in imaging, sensing, and wireless communications.

Phase modulation of metasurfaces for polarization conversion and RCS reduction

Metasurface (MS) can manipulate the transmission of electromagnetic (EM) wave with more degrees of freedom than other conventional radar stealth material, enabling numerous potential applications in both military and civilian fields. In this paper, we propose two kinds of polarization converters to reveal the relation between phase gradient and linear/circular polarization conversion (LPC/CPC). The proposed split square-ring (SSR) and split circle-ring (SCR) converters both can realize the CPC with different mechanisms. Further, four kinds of digital MSs are designed to explore the energy scattering characteristics. We find that the efficient radar cross section (RCS) reduction of the 01/01-typed SSR converter is due to the 180° phase gradient of reflected x - and y -polarized waves; the RCS reduction of the 01/10-typed SCR converter depends on the phase difference of u - and v -polarized waves and the amplitudes of x - and y -polarized waves. The calculation and the simulation results verify the relevance between RCS reduction and reflection amplitude, the reason of insufficient RCS reduction is fully explained. The experiments match well with the simulations. This work can be useful for the exploration of polarization conversion MSs and build the connection between reflection loss and RCS reduction. • The relationship between phase modulation and linear/circular polarization conversion. • The method to achieve circular polarization conversion with different mechanisms. • Optimizing the defects of RCS reduction. • Associating the relation between reflection loss and RCS reduction based on phase manipulation.

Spatiotemporal Binary Acoustic Metasurfaces

Over recent years, digital acoustic metasurfaces have been developed rapidly as a highly active research area owing to their unique and flexible manipulation of acoustic wavefronts. Nevertheless, nearly all recent attention in the acoustic community has been concentrated on space-encoded architectures, leaving the room free for benefiting from the unique features of spatiotemporally modulated metasurfaces. By entering the world of time, here, we propose a space-time-coding acoustic digital metasurface with exotic ability to dynamically transfer the energy of the carrier acoustic signal to a series of harmonic components, with equivalent magnitudes and phases that can be precisely and independently engineered. The transmission phases of the contributing elements are dynamically controlled in time by exploiting an elaborately designed electromechanical controller system. The study is supported by theoretical, numerical, and experimental verifications which are in excellent agreement. The results demonstrate that by distributing the coding sequences in both space and time dimensions, diverse scattering functionalities can be elaborately acquired for one or multiple harmonic frequencies in a programmable way. This paradigm of acoustic metasurfaces, without resorting to high-cost nonlinear components, opens up unprecedented potential for efficient harmonic control used in adaptive beamforming and acoustic imaging systems.

Developing an optimized metasurface for light trapping in thin-film solar cells using a deep neural network and a genetic algorithm

In this paper, using a deep neural network and a genetic algorithm, an optimized digital metasurface is designed to trap sunlight in thin-film solar cells. The deep neural network is trained using full-wave numerical simulation results as the training dataset, and it is designed to predict the electromagnetic response of thin-film solar cells whose active layers are shaped as a digital metasurface. The developed neural network can predict the results much faster than full-wave solvers and therefore can be used for optimization purposes. Using the results generated by the trained neural network, an evolutionary procedure based on the genetic algorithm is developed to find the optimum structure for the digital metasurface, which provides the highest short circuit current inside the thin-film solar cell. The performance of the resultant optimum design is validated using full-wave numerical simulation illustrating a short circuit current of 15.39 m A / c m 2 and 13.30 m A / c m 2 for TE and TM polarization of the incident light, respectively. The resultant short circuit current is 2.47 and 2.13 times higher than a simple thin-film solar cell with the same amount of silicon inside, for TE and TM polarization of the incident light, respectively. To have a more comprehensive comparison, the designed optimum structure is compared with several standard shapes for the metasurface, such as star and plus sign. This comparison showed that the optimum structure provides a short circuit current which is much higher than the current achieved by standard shapes.

Self-adaptive metasurface platform based on computer vision.

Programmable metasurfaces allow real-time electromagnetic (EM) manipulation in a digital manner, showing great potential to construct advanced multifunctional EM devices. However, the current programmable metasurfaces typically need human participation to achieve various EM functions. In this Letter, we propose, design, and construct a self-adaptive metasurface platform that can achieve beam control automatically based on image recognition. Such a platform is composed of a metasurface with 36×36 active units, a single-board computer, and two independent cameras that can detect the position of the objects. The single-board computer, Raspberry Pi, is used to calculate the information of the objects and generate the coding sequences to control the digital metasurface based on a low complexity binocular localization algorithm. Such a smart metasurface platform can generate different beams according to the location of the receiver and can be used as intelligent antennas in future communications and radars.

Polarization Multiplexing Hologram Realized by Anisotropic Digital Metasurface

AbstractMetasurface holograms have the potential to be applied in a variety of practical fields. Here an anisotropic digital metasurface is proposed that can project polarization multiplexing patterns at the microwave frequency, in which a modified Gerchberg–Saxton algorithm is presented to calculate the phase masks. A 3‐bit coding metamaterial unit is also proposed, which is capable of independently manipulating the transmission phases of orthogonal linearly polarized electromagnetic waves. The influence of wave‐front has been analyzed by comparing two holographic results with distinct source optimization methods. A prototype of the anisotropic digital coding metasurface optimized with horn wave‐front is designed, manufactured, and tested. The experimental measurements present the expected holographic fields and have a good match with the simulation results.

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.

A wireless communication scheme based on space- and frequency-division multiplexing using digital metasurfaces

Digitally programmable metasurfaces are of potential use in wireless multiplexing techniques because they can encode and transmit information without using traditional radio-frequency components such as antennas or mixers. Space–time-coding digital metasurfaces can, in particular, manipulate the propagation direction and harmonic power distribution of electromagnetic waves, making them suitable for space- and frequency-division multiplexing. However, while digital metasurfaces have been used for wireless communication, these systems could implement signal modulation only in the time domain. Here, we report a wireless communication scheme that uses space–time-coding digital metasurfaces to implement both space- and frequency-division multiplexing. By encoding space–time-coding matrices through multiple channels, digital messages can be directly transmitted to different users at different locations simultaneously, without the need for digital-to-analogue conversion and mixing processes. To illustrate this approach, we have built a dual-channel wireless communication system based on a two-bit space–time-coding digital metasurface and use it to transmit two different pictures to two users simultaneously in real time. Space–time-coding digital metasurfaces can be used to implement secure and low-cost space- and frequency-division multiplexing in a dual-channel wireless communication system.