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A novel amplifier‐based transmissive space‐time‐coding metasurface is presented to realize strongly nonlinear controls of electromagnetic (EM) waves in both space and frequency domains, which can manipulate the propagation directions and adjust enhancements of nonlinear harmonic waves and break the Lorenz reciprocity due to the nonreciprocity of unilateral power amplifiers. By cascading the power amplifier between patches placed on two sides of the metasurface, the metasurface can transmit the spatial EM waves in the forward direction while blocking it in the backward direction. Two status of power amplifier biased at the standard working voltage and zero voltage are represented as codes “1” and “0,” respectively. By periodically setting adequate code sequences and proportions in the temporal dimension, according to the space‐time coding strategy, the amplitudes and phases of the harmonic transmission coefficients can be adjusted in a programmable way. A metasurface prototype is fabricated and measured in the microwave frequency to validate the concept and feasibility. The experimental results show good agreement with the theoretical predictions and numerical simulations. The proposed metasurface can achieve controllable harmonic power enhancements for flexibly configuring the power intensities in space, which enlarge and manipulate the quality of transmitting signals. Here the conceptual illustration of the proposed amplifier‐based transmissive space‐time coding metasurface is presented. According to the space‐time coding matrix, the nonreciprocal and enhanced nonlinear manipulation of electromagnetic waves can be obtained by switching the power amplifier integrated with the metasurface.

The integrated modulation of radiation and scattering provides an unprecedented opportunity to reduce the number of electromagnetic (EM) apertures in the platform while simultaneously enhancing communication and stealth performance. Nevertheless, achieving full‐polarization, arbitrary amplitude, and phase modulation of radiation scattering remains a challenge. In this paper, a strategy that realizes space‐time coding of radiation scattering within the same frequency band, which enables the simultaneous and independent modulation of amplitude and phase, is proposed. To address the limitations of the high sideband levels (SBLs) of conventional space‐time‐coding metasurfaces, a strategy comprising nonuniform modulation periods and stochastic coding is proposed. Consequently, beam scanning with ultra‐low sidelobe levels (SLLs) and suppressed SBLs is achieved in the radiation mode (RM). In scattering mode (SM), in‐band low scattering characteristics are achieved within the same operating frequency band as RM. A prototype of a space‐time‐coding radiation‐scattering metasurface (STCRSM) is fabricated and the aforementioned functionalities are validated by measurements. Furthermore, the proposed strategy does not necessitate the utilization of optimization algorithms and exhibits low SLLs and low SBLs, which will make it flourish in RF stealth applications, such as covert communication systems. A novel strategy for integrating radiation and scattering is proposed. The simultaneous and independent modulation of amplitude and phase is achieved by employing space‐time coding of joint amplitude and phase of radiation scattering. Hence, beam scanning with ultra‐low sidelobe and in‐band ultra‐low scattering characteristics are realized. Furthermore, the proposed strategy of stochastic coding and nonuniform modulation enables ultra‐low sideband levels.

Harmonic generation and utilization are significant topics in nonlinear science. Although the progress in the microwave region has been expedited by the development of time‐modulated metasurfaces, one major issue of these devices is the strong entanglement of multiple harmonics, leading to criticism of their use in frequency‐division multiplexing (FDM) applications. Previous studies have attempted to overcome this limitation, but they suffer from designing complexity or insufficient controlling capability. Here a new space‐time‐coding metasurface (STCM) is proposed to independently and precisely synthesize not only the phases but also the amplitudes of various harmonics. This promising feature is successfully demonstrated in wireless space‐ and frequency‐division multiplexing experiments, where modulated and unmodulated signals are simultaneously transmitted via different harmonics using a shared STCM. To illustrate the advantages, binary frequency shift keying (BFSK) and quadrature phase shift keying (QPSK) modulation schemes are respectively implemented. Behind the intriguing functionality, the mechanism of the space‐time coding strategy and the analytical designing method are elaborated, which are validated numerically and experimentally. It is believed that the achievements can potentially propel the time‐vary metasurfaces in the next‐generation wireless applications. This work presents a novel space‐time‐coding metasurface to disentangle the amplitudes and phases of multiple harmonics, providing an effective approach to address the long‐standing challenge of time‐varying metasurfaces. The proposed temporal coding strategy is analytically derived, which makes the designing process simple and efficient. It allows for the maximum exploitation of spatial modulation for space‐ and frequency‐division multiplexing applications.

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. An asynchronous space‐time‐coding digital metasurface (ASTCM) is proposed to generate dynamic wavefronts by modulating the meta‐atoms with various periods. It is the first time to introduce frequency discontinuities to the digital coding metasurfaces. The outstanding abilities of the ASTCM include the automatic spatial scanning and dynamic scattering control, thus benefiting its applications in radars and wireless communications.

A novel method for the direction of arrival (DOA) estimation of coherent signals under a space–time–coding metasurface (STCM) is proposed in this paper. Noticeably, the STCM can replace multi-channel arrays with a single channel, which can be utilized to modulate incident electromagnetic waves and generate harmonics. However, coherent signals are overlapping in the frequency spectrum and cannot achieve DOA estimation through subspace methods. Therefore, the proposed method transforms the angle information in the time domain into amplitude and phase information at harmonics in the frequency domain by modulating incident coherent signals using the STCM and performing a fast Fourier transform (FFT) on these signals. Based on the harmonics in the frequency spectrum of the coherent signals, appropriate harmonics are selected. Finally, the ℓ1 norm singular value decomposition (ℓ1-SVD) algorithm is utilized for achieving high-precision DOA estimation. Simulation experiments are conducted to show the performance of the proposed method under the condition of different incident angles, harmonic numbers, signal-to-noise ratios (SNRs), etc. Compared to the traditional algorithms, the performance of the proposed algorithm can achieve more accurate DOA estimation under a low SNR.

This paper proposes a novel method for Direction of Arrival (DOA) estimation using a deep unfolded LISTA network in a non-uniform metasurface. Traditional DOA estimation methods often face challenges such as limited accuracy, high computational complexity, and poor adaptability to complex signal environments. To address these issues, we optimize a non-uniform metasurface array to reduce hardware costs and mutual coupling effects while enhancing resolution. Additionally, a deep unfolded Learned Iterative Shrinkage Thresholding Algorithm (LISTA) network is constructed by transforming Iterative Shrinkage Thresholding Algorithm (ISTA) iterative steps into trainable neural network layers, combining model-driven logic with data-driven parameter optimization. Simulation results prove that this method enhances higher precision and reduces computational complexity in comparison with traditional algorithms, especially under low SNR conditions. Furthermore, the method exhibits greater generalization ability, making it a reliable approach for high-precision DOA estimation in practical applications.

Metamaterials have been characterized by effective medium parameters over the past decades due to the subwavelength nature of meta-atoms. Once the metamaterials are fabricated, their functions become fixed or tunable. Recently, the concept of digital metamaterials has been introduced, in which, for instance, the constitutive 1-bit meta-atom is digitalized as “0” or “1” corresponding to two opposite electromagnetic (EM) responses. The digital metamaterials set up a bridge between the physical world and the information world. More interestingly, when the digital meta-atom is programmable, a single metamaterial can be used to realize different functions when programmed with different coding sequences. Moreover, as the states of programmable meta-atoms can be quickly switched, it enables the wave-based information coding and processing on the physical level of metamaterials in real time. For these reasons, we prefer to call digital metamaterials with programmable meta-atoms as “information metamaterials.” In this review article, we introduce two basic principles for information metamaterials: Shannon entropy on metamaterials to measure the information capacity quantitatively and digital convolution on metamaterials to manipulate the beam steering. Afterwards, two proof-of-concept imaging systems based on information metamaterials, i.e. programmable hologram and programmable imager, are presented, showing more powerful abilities than the traditional counterparts. Furthermore, we discuss the time-modulated information metamaterial that enables efficient and accurate manipulations of spectral harmonic distributions and brings new physical phenomena such as frequency cloaking and velocity illusion. As a relevant application of time-modulated information metamaterials, we propose a novel architecture of wireless communication, which simplifies the modern wireless communication system. Finally, the future trends of information metamaterials are predicted.

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. In this work, a new‐architecture sub‐terahertz transmitter for wideband wireless communication applications based on STCM is reported. These experimental findings validate that the STCM is capable of supporting multiple modulation schemes of the digital information and wide‐angle beam scanning of sub‐terahertz waves. STCM‐based communication transmitters can provide a low‐complexity, low‐cost, and power‐efficient approach to future wireless communication systems.

Secure key distribution is fundamental to encrypted communications but remains challenging in indoor wireless scenarios due to high computational overhead and specialized hardware requirements. Here, a Meta Key Distribution (MKD) system based on programmable metasurface to securely distribute cryptographic keys and enable protocol‐independent encrypted communication is proposed. The metasurface embeds synchronized entropy into wireless channel by dynamically modulating the spatiotemporal properties of electromagnetic wave, thus allowing legitimate users to independently extract identical cryptographic keys. A prototype is implemented and experimentally validated in an indoor environment. The results show that the proposed MKD system achieves a key generation rate of 400 bit/s and a bit error rate below 3%, demonstrating reliable key generation performance and strong resistance to passive eavesdropping. In addition, the system can be readily integrated with standard wireless protocols such as WiFi and Bluetooth without requiring significant modifications to existing communication hardware. This metasurface‐assisted approach provides a lightweight, compatible, and secure key distribution solution, suitable for emerging applications in smart homes, the Internet of Things, and healthcare environments. This paper presents a programmable metasurface‐based Meta Key Distribution (MKD) system for secure, protocol‐independent key exchange in indoor wireless settings. By embedding entropy into the wireless channel, it enables lightweight, compatible cryptographic key generation. Experiments confirm reliable key agreement, low error rates, and strong eavesdropping resistance, supporting applications like IoT and smart healthcare.

Space-time and time-varying metastructures have attracted a lot of research interest in recent years. On the other hand, digital programmable metasurfaces have also gained great attention owing to their powerful capabilities in controlling electromagnetic (EM) fields and waves in real time, which is very suitable for implementing spatiotemporal modulations in a digital manner. Accordingly, space-time-coding (STC) digital metasurfaces have recently been proposed to realize advanced manipulations of EM wavefronts and digital information, allowing simultaneous control of propagation directions in the space domain and harmonic distributions in the frequency domain. However, their instantaneous responses and the connection between the time- and frequency-domain characteristics have not yet been fully revealed. Here, we present a joint time-frequency analysis method to revisit STC digital metasurfaces, in which the time-domain instantaneous scattering patterns and frequency-domain equivalent excitations are investigated to analyze the spatial-spectral distributions of the modulated waves. This joint time-frequency analysis method helps to better explain the basic working principle of STC digital metasurfaces and is expected to facilitate more applications in wireless communications, radar, imaging, and beamforming.