In this work, we present an anti-interference direction of arrival (DoA) estimation system based on a spacetime-modulated metasurface (STM-MTS). Composed by a 1-bit STM-MTS and a single receiving antenna, the system can estimate DoA of a linearly frequency modulated (LFM) plane wave in presence of several other LFM interfering waves. The STM-MTS generates a set of scattered harmonics, whose amplitudes are related to the space-time modulation scheme of the metasurface and the specific incident angle. Due to the presence of the interfering signals, we introduce a properly designed separation procedure for the time-domain LFM signals modulated by the STM-MTS, allowing to obtain the intended LFM signal for which the DoA must be estimated via multiple signal classification (MUSIC) algorithm applied to harmonic amplitude distribution. Both simulated and measured results demonstrate the effectiveness of anti-interference DoA estimation. The absolute errors in the DoA estimation are less than 2o  within the incident range of [-50o ,+50o ] , demonstrating the accuracy of the proposed anti-interference DoA estimation system based on space-time-modulated metasurfaces.

An electromagnetic (EM) metasurface is a passive device capable of manipulating the reflected signal of a radar in both spatial and frequency domains. This paper presents a novel analytical design of EM scattering center model guided passive synthetic aperture radar (SAR) deception based on diverse frequency time-modulation. The objective of this design is to disguise real target as an intended deceptive target and conceal its original EM characteristics. To achieve this, we first extract the scattering center model of an intended deceptive target and express its SAR image analytically. The SAR image is determined by the number, amplitude and position of scattering center. Subsequently, we derive the SAR image of a metasurface modulated on fast and slow time scales with diverse frequencies, establishing the relation between modulation parameters and the resulting SAR image properties. By comparing and aligning the SAR images between scattering center model and the modulated metasurface with predetermined similarity threshold, we successfully accomplish scattering center model guided SAR deception. The simulated results demonstrate that the SAR image similarities of both scattering center model and modulated metasurface exceed 0.9, indicating the effectiveness of this method as an electronic countermeasure.

In this letter, we propose a nonlinear reflective metasurface-based frequency mixing system able to perform second-harmonic generation (SHG) and third-harmonic generation (THG). The metasurface consists of a planar bidimensional alignment of unit-cells, each of which encompasses a receiving element at fundamental frequency, and two transmitting elements operating at double and triple the fundamental frequency, respectively. The TX/RX elements within each unit-cell are connected via a microwave network to a RF mixing circuit that performs the conversion, assisted by an RF amplifier that improves the overall conversion efficiency. Efficient switchable SHG and THG frequency conversions are demonstrated by both full-wave simulations and experiments. For the input fundamental frequency of 3 GHz, the conversion efficiencies for SHG and THG are 55.5% and 42.6%, respectively.

 In this paper, we propose a multifunctional space-time modulated metasurface (STM-MTS) able to perform simultaneously a direction of arrival (DoA) estimation and radar cross section (RCS) manipulation in a single system. The metasurface consists of an alignment of unit-cells, whose scattering response can be dynamically controlled to be transparent (full transmissive unit-cell) or opaque (full reflective unit-cell), according to the amplitude of the modulating signal applied to the loading PIN diodes. The DoA estimation is performed behind the metasurface, i.e., in the transmission region, where multi-order harmonics of the incident field whose complex amplitudes are proportional to the DoA is transmitted. The multiple signal classification (MUSIC) algorithm is employed to estimate DoA. Simultaneously, the mono-/bi-static RCSs manipulation is performed in front of the metasurface, i.e. in the reflection region, where the reflected field is properly tailored by controlling the modulation scheme at unit-cell level. An X-band STM-MTS is fabricated to experimentally verify the feasibility of the proposed design. A DoA estimation errors are less than 2° in the incidence angular range of [-50°, +50°] is demonstrated, whereas mono-static and bi-static RCS manipulations of about 17dB and 36dB are achieved at operative frequency.Â

This paper presents a new design of diverse frequency time-modulation for linear frequency modulated (LFM) radar passive false target spoofing. To realize the passive diverse frequency time-modulation, the modulated metasurface is designed accordingly, which is composed by a set of elements, each of which is independently time-modulated to realize specific false targets in the high-resolution range profile (HRRP) of the radar system. Thanks to the control of the modulation scheme in each element and the combined response of the overall surface, we demonstrate the capability to realize several false target patterns consisting of single and multiple targets symmetrically, or asymmetrically, distributed around the actual target. A HRRP general expression of the time modulated diverse frequency metasurface at different observation angles is given to show the relation between the generated false targets and frequency components. Since having more flexible capacity of reflected signal manipulation comparing to false target spoofing only through time modulation with consistent modulation frequency, the proposed method has enhanced performance on control of false targets at different observation angles. Both simulated and measured results demonstrate the validity of false targets manipulation, demonstrating its effectiveness as electromagnetic countermeasure for achieving radar camouflaging and deception.

Three different deep learning models were designed in this paper, to predict the electric fields of single nanoparticles, dimers, and nanoparticle arrays. For single nanoparticles, the prediction error was 4.4%, respectively. For dimers with strong couplings, a sample self-normalization method was proposed, and the error was reduced by an order of magnitude compared with traditional methods. For nanoparticle arrays, the error was reduced from 28.8% to 5.6% compared with previous work. Numerical tests proved the validity of the proposed deep learning models, which have potential applications in the design of nanostructures.

Planar metasurfaces have been applied in several fields. Near-field coupling is typically neglected in traditional metasurface designs. A numerical modeling method for macrocells that considers near-field couplings between meta-atoms is proposed. A deep neural network (DNN) is constructed to accurately predict the electromagnetic response from different macrocells. Transfer learning is employed to reduce the size of the training datasets. The designed neural network is embedded in the optimization algorithm as an effective surrogate model. Both the deflector and high numerical aperture (NA) metalens are simulated and optimized with our design framework, approximately 30% improvements of efficiencies are achieved.

Theory of characteristic modes (TCM) is applied to the design of a programmable flexible metasurface (PFMS) to achieve its multistate tunable microwave absorption. The designed metasurface consists of two layers. The first layer is a very thin polyimide (PI), on which unit cells consisting of two trapezoidal patches symmetrical along the central bias microstrip are printed. The second layer is grounded polyvinyl chloride (PVC) substrate to maintain the flexibility of the designed structure. The TCM is used to analyze and understand the modal behavior of the unit cell, which provides clear underlying electromagnetic (EM) physics to guide us how to load positive-intrinsic-negative (PIN) diode as a tunable resistance to achieve variable absorption states. The designed PFMS absorber have been fabricated and experimentally verified on the flat and cylindrical foams, respectively. Good agreements can be found for both planar and cylindrical metasurface structures between full-wave simulations and measurements. Less than -10 dB reflectivity is achieved from 3.6 to 10.7 GHz (99%). The mimicry of time-varying scattering cross section (SCS) is further realized through real-time coding PFMS absorber, which can be precisely tuned from -30 dB to 0 dB with a step of 1.5 dB at 5.5 GHz and 10.4 GHz, respectively.

An effective method for identifying inertia characteristics of cone-shaped space target based on deep learning is proposed. The inertia ratio is determined by the time-varying scattering fields from the cone-shaped targets. The multistatic method is introduced to reduce the evaluation time of time-varying scattering fields. The micro-Doppler spectrogram (MDS) dataset is constructed by time-frequency analysis with numerical simulation method, point scattering model, and experimental tests. The compressed dataset is further achieved by truncated singular value decomposition (SVD). Finally, the micro-motion parameter identification model is constructed to identify the inertia ratio for the cone-shaped space target. The interaction loss function and the feed-forward denoising convolutional neural networks (DnCNNs) are employed to improve the identification accuracy. Parameters identification of the precession frequency, precession angle, spin frequency, and inertia ratio with both simulation and experiment datasets demonstrate the validity of the proposed method.

In this paper, we present a metasurface-based Direction of Arrival (DoA) estimation method that exploits the properties of space-time modulated reflecting metasurfaces to estimate in real-time the impinging angle of an illuminating monochromatic plane wave. The approach makes use of the amplitude unbalance of the received fields at broadside at the frequencies of the two first-order harmonics generated by the interaction between the incident plane wave and the modulated metasurface. Here, we first describe analytically how to generate the desired higher-order harmonics in the reflected spectrum and how to realize the breaking of the spatial symmetry of each order harmonic scattering pattern. Then, the one dimensional (1D) omnidirectional incident angle can be analytically computed using +1st and -1st order harmonics. The approach is also extended to 2D DoA estimation by using two orthogonally arranged 1D DoA modulation arrays. The accuracy of 1D DoA estimation is verified through full-wave numerical simulations. Compared to conventional DoA estimation methods, the proposed approach simplifies the computation and hardware complexity, ensuring at the same time estimation accuracy. The proposed method may have potential applications in wireless communications, target recognition, and identification.

Space-time-modulated metasurfaces can manipulate electromagnetic waves in space and frequency domain simultaneously. In this paper, an analytical design of space-time- modulated metasurfaces with modulation elements composed of two paths, In-phase (I) and Quadrature (Q), is proposed. The model is derived analytically, the space/frequency domain manipulations are achieved by designing the dimension and time sequence of I and Q paths. In the specular reflection direction, an objective frequency shift of the reflected first order harmonic can be obtained. While, in other directions, the opposite first order harmonic can be easily controlled by changing the dimension of I/Q paths and the objective first order harmonic remains unchanged. Furthermore, with a small dimension of I/Q paths, the first order harmonic can be used for beam scanning by pre-designing the start time of the modulation element. To realize the space-time-modulated metasurface with the required periodically time-varying responses, 2-bit unit-cells loaded with dynamically switchable pin diodes are employed as I/Q modulation. Both analytical and numerical results demonstrate that space and frequency domain manipulations of the reflected fields by the first order harmonics can be simultaneously obtained. The proposed designs have potential applications in wireless communications, radar camouflaging, and cloaking.