Semantic regularization of electromagnetic inverse problems.

Solving ill-posed inverse problems typically requires regularization based on prior knowledge. To date, only prior knowledge that is formulated mathematically (e.g., sparsity of the unknown) or implicitly learned from quantitative data can be used for regularization. Thereby, semantically formulated prior knowledge derived from human reasoning and recognition is excluded. Here, we introduce and demonstrate the concept of semantic regularization based on a pre-trained large language model to overcome this vexing limitation. We study the approach, first, numerically in a prototypical 2D inverse scattering problem, and, second, experimentally in 3D and 4D compressive microwave imaging problems based on programmable metasurfaces. We highlight that semantic regularization enables new forms of highly-sought privacy protection for applications like smart homes, touchless human-machine interaction and security screening: selected subjects in the scene can be concealed, or their actions and postures can be altered in the reconstruction by manipulating the semantic prior with suitable language-based control commands.

A filtering reconfigurable intelligent surface for interference-free wireless communications.

The powerful capability of reconfigurable intelligent surfaces (RISs) in tailoring electromagnetic waves and fields has put them under the spotlight in wireless communications. However, the current designs are criticized due to their poor frequency selectivity, which hinders their applications in real-world scenarios where the spectrum is becoming increasingly congested. Here we propose a filtering RIS to feature sharp frequency-selecting and 2-bit phase-shifting properties. It permits the signals in a narrow bandwidth to transmit but rejects the out-of-band ones; meanwhile, the phase of the transmitted signals can be digitally controlled, enabling flexible manipulations of signal propagations. A prototype is designed, fabricated, and measured, and its high quality factor and phase-shifting characteristics are validated by scattering parameters and beam-steering phenomena. Further, we conduct a wireless communication experiment to illustrate the intriguing functions of the RIS. The filtering behavior enables the RIS to perform wireless signal manipulations with anti-interference ability, thus showing big potential to advance the development of next-generation wireless communications.

Dynamic switching from coherent perfect absorption to parametric amplification in a nonlinear spoof plasmonic waveguide.

Coherent perfect absorption (CPA) and amplification of electromagnetic waves are converse phenomena, where incoming radiations are coherently dissipated or amplified by structured incidences. Realizing such two phenomena simultaneously in a single device may benefit various applications such as biological sensing, photo detection, radar stealth, solar-thermal energy sharing, and wireless communications. However, previous experimental realizations of CPA and amplification generally require precise controls to the loss and gain of a system, making dynamic switching between the absorption and amplification states a challenge. To this end, we propose a nonlinear approach to realize CPA and parametric amplification (PA) simultaneously at the same frequency and demonstrate experimentally dynamic switch from the CPA to PA states in a judiciously designed nonlinear spoof plasmonic waveguide. The measured output signal gain can be continuously tuned from -33 dB to 22 dB in a propagation length of 9.2 wavelengths. Compared to the traditional linear CPA, our approach relaxes the stringent requirements on device dimensions and material losses, opening a new route to actively modulate the electromagnetic waves with giant amplification-to-absorption contrast in a compact platform. The proposed nonlinear plasmonic platform has potential applications in on-chip systems and wireless communications.

Radar Micro-Doppler Signature Generation Based on Time-Domain Digital Coding Metasurface.

Micro-Doppler effect is a vital feature of a target that reflects its oscillatory motions apart from bulk motion and provides an important evidence for target recognition with radars. However, establishing the micro-Doppler database poses a great challenge, since plenty of experiments are required to get the micro-Doppler signatures of different targets for the purpose of analyses and interpretations with radars, which are dramatically limited by high cost and time-consuming. Aiming to overcome these limits, a low-cost and powerful simulation platform of the micro-Doppler effects is proposed based on time-domain digital coding metasurface (TDCM). Owing to the outstanding capabilities of TDCM in generating and manipulating nonlinear harmonics during wave-matter interactions, it enables to supply rich and high-precision electromagnetic signals with multiple micro-Doppler frequencies to describe the micro-motions of different objects, which are especially favored for the training of artificial intelligence algorithms in automatic target recognition and benefit a host of applications like imaging and biosensing.

Tailless Information-Energy Metasurface.

Programmable metasurface technology can achieve flexible manipulations of electromagnetic waves in real time by adjusting the surface structure and material properties and has shown extraordinary potential in many fields such as wireless communications and the Internet of Things. However, most of the programmable metasurfaces have a common feature: a tail (electrical wires and DC powers), which is difficult to supply in some particular application scenarios such as canyons and mountains. To eliminate the limitation of DC power supply, the programmable metasurface and wireless power transfer technology are combined to propose a tailless information-energy metasurface (IEMS). The tailless IEMS platform can dynamically control electromagnetic waves without relying on an external DC power supply; instead, the required DC power is provided internally by the IEMS platform itself. In the tailless IEMS experiments, the concept is demonstrated through the dynamic regulation of wireless channels and the wireless transmission of DC power. This work provides a self-powered method for programmable metasurfaces, expands the application scenarios, facilitates the miniaturization of systems, and makes it easy to integrate with other systems.

Microwave Speech Recognizer Empowered by a Programmable Metasurface.

Speech recognition becomes increasingly important in the modern society, especially for human-machine interactions, but its deployment is still severely thwarted by the struggle of machines to recognize voiced commands in challenging real-life settings: oftentimes, ambient noise drowns the acoustic sound signals, and walls, face masks or other obstacles hide the mouth motion from optical sensors. To address these formidable challenges, an experimental prototype of a microwave speech recognizer empowered by programmable metasurface is presented here that can remotely recognize human voice commands and speaker identities even in noisy environments and if the speaker's mouth is hidden behind a wall or face mask. The programmable metasurface is the pivotal hardware ingredient of the system because its large aperture and huge number of degrees of freedom allows the system to perform a complex sequence of sensing tasks, orchestrated by artificial-intelligence tools. Relying solely on microwave data, the system avoids visual privacy infringements. The developed microwave speech recognizer can enable privacy-respecting voice-commanded human-machine interactions is experimentally demonstrated in many important but to-date inaccessible application scenarios. The presented strategy will unlock new possibilities and have expectations for future smart homes, ambient-assisted health monitoring, as well as intelligent surveillance and security.

Macroscopic model and statistical model to characterize electromagnetic information of a digital coding metasurface.

A digital coding metasurface is a platform connecting the digital space and electromagnetic wave space, and has therefore gained much attention due to its intriguing value in reshaping wireless channels and realizing new communication architectures. Correspondingly, there is an urgent need for electromagnetic information theory that reveals the upper limit of communication capacity and supports the accurate design of metasurface-based communication systems. To this end, we propose a macroscopic model and a statistical model of the digital coding metasurface. The macroscopic model uniformly accommodates both digital and electromagnetic aspects of the meta-atoms and predicts all possible scattered fields of the digital coding metasurface based on a small number of simulations or measurements. Full-wave simulations and experimental results show that the macroscopic model is feasible and accurate. A statistical model is further proposed to correlate the mutual coupling between meta-atoms with covariance and to calculate the entropy of the equivalent currents of digital coding metasurface. These two models can help reconfigurable intelligent surfaces achieve more accurate beamforming and channel estimation, and thus improve signal power and coverage. Moreover, the models will encourage the creation of a precoding codebook in metasurface-based direct digital modulation systems, with the aim of approaching the upper limit of channel capacity. With these two models, the concepts of current space and current entropy, as well as the analysis of information loss from the coding space to wave space, is established for the first time, helping to bridge the gap between the digital world and the physical world, and advancing developments of electromagnetic information theory and new-architecture wireless systems.

Observation of a Higher-Order End Topological Insulator in a Real Projective Lattice.

The modern theory of quantized polarization has recently extended from 1D dipole moment to multipole moment, leading to the development from conventional topological insulators (TIs) to higher-order TIs, i.e., from the bulk polarization as primary topological index, to the fractional corner charge as secondary topological index. The authors here extend this development by theoretically discovering a higher-order end TI (HOETI) in a real projective lattice and experimentally verifying the prediction using topolectric circuits. A HOETI realizes a dipole-symmetry-protected phase in a higher-dimensional space (conventionally in one dimension), which manifests as 0D topologically protected end states and a fractional end charge. The discovered bulk-end correspondence reveals that the fractional end charge, which is proportional to the bulk topological invariant, can serve as a generic bulk probe of higher-order topology. The authors identify the HOETI experimentally by the presence of localized end states and a fractional end charge. The results demonstrate the existence of fractional charges in non-Euclidean manifolds and open new avenues for understanding the interplay between topological obstructions in real and momentum space.

Independent Manipulations of Transmitting and Receiving Channels by Nonreciprocal Programmable Metasurface.

The electromagnetic (EM) beam manipulations such as spatial scanning have always been the focus in information science and technology. Generally, the transmitting and receiving (T/R) beams of the same aperture should be coincident due to the reciprocal theory, and hence, more flexible controls of the spatial information are limited accordingly. Here, we propose a new approach to achieve independent controls of beam scanning in spatial T/R channels based on one aperture made by a nonreciprocal programmable metasurface. The meta-atom is designed to have independent propagation chains for T/R waves by introducing dual-direction power amplifiers (PAs) as the isolators for one-way transparency. A programmable phase shifter with a 360° coverage is loaded with the PA device in the transmitting or receiving chain to realize independent beam scanning in the T/R channels. A prototype of the proposed metasurface is fabricated, and independent beam scanning in the T/R channels is directly acquired with good performance in our measurements. In addition, a proof of concept of integrated sensing and auxiliary communications is accomplished to verify the validity of the presented method.

Simultaneously Intelligent Sensing and Beamforming Based on an Adaptive Information Metasurface.

Due to its ability to adapt to a variety of electromagnetic (EM) environments, the sensing-enabled metasurface has garnered significant attention. However, large-scale EM-field sensing to obtain more information is still very challenging. Here, an adaptive information metasurface is proposed to enable intelligent sensing and wave manipulating simultaneously or more specifically, to realize intelligent target localization and beam tracking adaptively. The metasurface is composed of an array of meta-atoms, and each is loaded with two PIN diodes and a sensing-channel structure, for polarization-insensitive and programmable beamforming and sensing. By controlling the state of the PIN diode, the proposed meta-atom has 1-bit phase response in the designed frequency band, while the sensing loss keeps higher than -10 dB for both "ON" and "OFF" states. Hence there is nearly no interaction between the beamforming and sensing modes. Experiments are conducted to show multiple functions of the metasurface, including intelligent target sensing and self-adaptive beamforming, and the measured results are in good agreement with the numerical simulations and theoretical calculations.

Computer-Vision Based Gesture-Metasurface Interaction System for Beam Manipulation and Wireless Communication.

Hand gesture plays an important role in many circumstances, which is one of the most common interactive methods in daily life, especially for disabled people. Human-machine interaction is another popular research topic to realize direct and efficient control, making machines intelligent and maneuverable. Here, a special human-machine interaction system is proposed and namedas computer-vision (CV) based gesture-metasurface interaction (GMI) system, which can be used for both direct beam manipulations and real-time wireless communications. The GMI system first needs to select its working mode according to the gesture command to determine whether to perform beam manipulations or wireless communications, and then validate the permission for further operation by recognizing unlocking gesture to ensure security. Both beam manipulation and wireless communication functions are validated experimentally, which show that the GMI system can not only realize real-time switching and remote control of different beams through gesture command, but also communicate with a remote computer in real time by translating the gesture language to text message. The proposed non-contact GMI system has the advantages of good interactivity, high flexibility, and multiple functions, which can find potential applications in community security, gesture-command smart home, barrier-free communications, and so on.

High-performance cost efficient simultaneous wireless information and power transfers deploying jointly modulated amplifying programmable metasurface.

Programmable metasurfaces present significant capabilities in manipulating electromagnetic waves, making them a promising candidate for simultaneous wireless information and power transfer (SWIPT), which has the potential to enable sustainable wireless communication in complex electromagnetic environments. However, challenges remain in terms of maximum power transmission distance and stable phase manipulation with high-power scattered waves. Additionally, waveform limitations restrict average scattered power and rectifier conversion efficiency, affecting data transmission rates and energy transmission distance. Here we show an amplifying programmable metasurface (APM) and a joint modulation method to address these challenges. The APM mitigates the peak-to-average power ratio and improves maximum power, phase response stability, average output power, and rectifier conversion efficiency. Through experimental validation, we demonstrate the feasibility of the SWIPT system, showcasing simultaneous LED array powering and movie video transmission. This innovative SWIPT system holds promise for diverse applications, including 6 G wireless communications, IoT, implanted devices, and cognitive radio networks.

Manipulations of multi-frequency waves and signals via multi-partition asynchronous space-time-coding digital metasurface.

Manipulations of multiple carrier frequencies are especially important in a variety of fields like radar detection and wireless communications. In conventional radio-frequency architecture, the multi-frequency control is implemented by microwave circuits, which are hard to integrate with antenna apertures, thus bringing the problems of expensive system and high power consumption. Previous studies demonstrate the possibility to jointly control the multiple harmonics using space-time-coding digital metasurface, but suffer from the drawback of inherent harmonic entanglement. To overcome the difficulties, we propose a multi-partition asynchronous space-time-coding digital metasurface (ASTCM) to generate and manipulate multiple frequencies with more flexibility. We further establish an ASTCM-based transmitter to realize wireless communications with frequency-division multiplexing, where the metasurface is responsible for carrier-wave generations and signal modulations. The direct multi-frequency controls with ASTCM provides a new avenue to simplify the traditional wireless systems with reduced costs and low power consumption.

Flexible and fast estimation method of far-field patterns for digital-coding metasurfaces.

We propose a flexible and fast estimation method to calculate the far-field patterns of digital-coding metasurfaces (DCMs) by performing chirp Z-transform (CZT), called the DCM-CZT method. Because of the expression form of convolution, CZT can be accelerated by fast Fourier transform. Compared with the traditional discrete Fourier transform (DFT) method, the DCM-CZT method can accurately estimate the far-field patterns with arbitrary element periods. More importantly, the DCM-CZT method can calculate partial far-field patterns for some specific orientations, instead of the global far-field patterns like DFT does. We show that the DCM-CZT method can efficiently improve the partial space-resolution to avoid the calculation error caused by the fence effect under acceptable computing time. We present six representative examples to demonstrate the capabilities of the proposed method. Results show that the far-field patterns calculated by the DCM-CZT method have good agreements with full-wave simulations and experimental measurements. However, the results of main-lobes calculated by the DFT method have obvious deviations when the element period is about 0.2 wavelengths. We believe that the DCM-CZT method has potential applications in wireless communications and radar detections.

A universal metasurface antenna to manipulate all fundamental characteristics of electromagnetic waves.

Metasurfaces have promising potential to revolutionize a variety of photonic and electronic device technologies. However, metasurfaces that can simultaneously and independently control all electromagnetics (EM) waves' properties, including amplitude, phase, frequency, polarization, and momentum, with high integrability and programmability, are challenging and have not been successfully attempted. Here, we propose and demonstrate a microwave universal metasurface antenna (UMA) capable of dynamically, simultaneously, independently, and precisely manipulating all the constitutive properties of EM waves in a software-defined manner. Our UMA further facilitates the spatial- and time-varying wave properties, leading to more complicated waveform generation, beamforming, and direct information manipulations. In particular, the UMA can directly generate the modulated waveforms carrying digital information that can fundamentally simplify the architecture of information transmitter systems. The proposed UMA with unparalleled EM wave and information manipulation capabilities will spark a surge of applications from next-generation wireless systems, cognitive sensing, and imaging to quantum optics and quantum information science.

Toward Sub-Terahertz: Space-Time Coding Metasurface Transmitter for Wideband Wireless Communications.

A space-time coding metasurface (STCM) operating in the sub-terahertz band to construct new-architecture wireless communication systems is proposed. Specifically, a programmable STCM is designed with varactor-diode-tuned metasurface elements, enabling precise regulation of harmonic amplitudes and phases by adjusting the time delay and duty cycle of square-wave modulation signal loaded on the varactor diodes. Independent electromagnetic (EM) regulations in the space and time domains are achieved by STCM to realize flexible beam manipulations and information modulations. Based on these features, a sub-terahertz wireless communication link is constructed by employing STCM as a transmitter. Experimental results demonstrate that the STCM supports multiple modulation schemes including frequency-shift keying, phase-shift keying, and quadrature amplitude modulations in a wide frequency band. It is also shown that the STCM is capable of realizing wide-angle beam scanning in the range of ±45o , which offers an opportunity for user tracking during the communication. Thus, the STCM transmitter with high device density and low power consumption can provide low-complexity, low-cost, low-power, and low-heat solutions for building the next-generation wireless communication systems in the sub-terahertz frequency and even terahertz band.

Intelligent indoor metasurface robotics.

Intelligent indoor robotics is expected to rapidly gain importance in crucial areas of our modern society such as at-home health care and factories. Yet, existing mobile robots are limited in their ability to perceive and respond to dynamically evolving complex indoor environments because of their inherently limited sensing and computing resources that are, moreover, traded off against their cruise time and payload. To address these formidable challenges, here we propose intelligent indoor metasurface robotics (I2MR), where all sensing and computing are relegated to a centralized robotic brain endowed with microwave perception; and I2MR's limbs (motorized vehicles, airborne drones, etc.) merely execute the wirelessly received instructions from the brain. The key aspect of our concept is the centralized use of a computation-enabled programmable metasurface that can flexibly mold microwave propagation in the indoor wireless environment, including a sensing and localization modality based on configurational diversity and a communication modality to establish a preferential high-capacity wireless link between the I2MR's brain and limbs. The metasurface-enhanced microwave perception is capable of realizing low-latency and high-resolution three-dimensional imaging of humans, even around corners and behind thick concrete walls, which is the basis for action decisions of the I2MR's brain. I2MR is thus endowed with real-time and full-context awareness of its operating indoor environment. We implement, experimentally, a proof-of-principle demonstration at ∼2.4 GHz, in which I2MR provides health-care assistance to a human inhabitant. The presented strategy opens a new avenue for the conception of smart and wirelessly networked indoor robotics.

Ka-band beam-scanning leaky-wave antenna fed by reconfigurable spoof surface plasmon polaritons.

A leaky-wave antenna (LWA) based on reconfigurable spoof surface plasmon polaritons (SSPP) is proposed for beam scanning in the Ka band, which consists of a reconfigurable SSPP waveguide and a periodic array of metal rectangular split rings. Both numerical simulations and experimental measurements show that the reconfigurable SSPP-fed LWA has good performance in the frequency range from 25 to 30 GHz. Specifically, as the bias voltage changes from 0 to 15 V, we can achieve the maximum sweep range of 24° at a single frequency and 59° at multiple frequency points, respectively. Owing to the wide-angle beam-steering feature, as well as the field confinement and wavelength compression properties derived from the SSPP architecture, the proposed SSPP-fed LWA possesses great potential applications in the compact and miniaturized devices and systems of the Ka band.

Optoelectronic Metasurface for Free-Space Optical-Microwave Interactions.

Photon-electron interactions are essential for many areas such as energy conversion, signal processing, and emerging quantum science. However, the current demonstrations are typically targeted to fiber and on-chip applications and lack of study in wave space. Here, we introduce a concept of optoelectronic metasurface that is capable of realizing direct and efficient optical-microwave interactions in free space. The optoelectronic metasurface is realized via a hybrid integration of microwave resonant meta-structures with a photoresponsive material. As a proof of concept, we construct an ultrathin optoelectronic metasurface using photodiodes that is bias free, which is modeled and analyzed theoretically by using the light-driven electronic excitation principle and microwave network theory. The incident laser and microwave from the free space will interact with the photodiode-based metasurface simultaneously and generate strong laser-microwave coupling, where the phase of output microwave depends on the input laser intensity. We experimentally verify that the reflected microwave phase of the optoelectronic metasurface decreases as the incident laser power becomes large, providing a distinct strategy to control the vector fields by the power intensity. Our results offer fundamentally new understanding of the metasurface capabilities and the wave-matter interactions in hybrid materials.