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In this article, two anisotropic holographic metasurfaces with ultralow profiles are proposed to produce wideband high-intensity self-bending Bessel-like beams with customized propagation trajectories in the microwave band. The phase distribution for generating Bessel-like beams with arbitrary paths is obtained by appropriately modifying the conical ray pattern of standard Bessel beams. Then, the phase-modulated wavefronts of Bessel-like beams are reconstructed by a holographic impedance surface with anisotropic tensor elements. A monopole is assembled on the holographic impedance surface directly without loading the complex feeding network, resulting in low-cost and low-profile characteristics. Two prototypes for generating Bessel-beams with various preset trajectories are fabricated and measured for demonstration. The measured 10 dB impedance bandwidths of two designs are 15.0–24.0 and 14.6–24.0 GHz, respectively. The longitudinal electric field intensities measured at multiple frequency points verify that the proposed holographic metasurfaces have wideband capabilities, and the resulting beam exhibits properties of high-intensity, self-bending, and self-reconstruction. The transverse electric field intensities along the propagation direction at various positions reveal that both beams achieve circularly symmetrical and apparently invariant main lobes. By extracting the coordinates of the points with the strongest field intensity on the measured reference planes, it is demonstrated that the generated beams propagate along the predetermined curved trajectories. Consequently, the proposed designs have invaluable potential in numerous fields, including secure communication, microwave power transmission, detection, and imaging.

In this article, an enhanced diffractive neural network is proposed for achieving metasurface holograms with high resolution, low noise, and uniform intensity. First, we prove the feasibility of Rayleigh-Sommerfeld diffraction theory on a subwavelength scale. Based on this theory, the fully connected diffraction layer is constructed to build a high-resolution diffractive neural network (HR-DNN). Due to the capability of the diffraction layer in precisely manipulating subwavelength electromagnetic (EM) waves, high-resolution holographic imaging of complex patterns can be realized. In addition, a postprocessing method is particularly designed to separate clean target images from noisy holograms without reference assistance. The metrics, such as imaging efficiency (IE), peak signal-to-noise ratio (PSNR), and structural similarity (SSIM), are defined to estimate the imaging quality of the proposed HR-DNN-based holographic imaging system. Three types of complex patterns (airplane, phrase “WORLD PEACE 0921,” Olympic rings) are performed in the full-wave simulation, as well as the imaging results are highly recognizable with low-noise and uniform-intensity features. Compared with the weighted Gerchberg-Saxton (GS) algorithm, the proposed HR-DNN gains significant improvements in IE (241.6%), PSNR (45.6%), and SSIM (44.0%). Finally, a metasurface with $30\lambda \times 30\lambda $ based on 3-D printing technology is fabricated to image the Olympic rings. The measured results are in good agreement with the simulated and target ones. Therefore, the proposed HR-DNN can provide a pathway for high-resolution metasurface holograms.

In this article, a rasorber with energy-selective passband and ultrawideband absorptive out-of-band is proposed, which combines the advantages of conventional frequency-selective absorber (FSA) and energy-selective surface (ESS). The proposed design consists of an upper lossy layer and a lower active layer. By analyzing the equivalent circuit model (ECM), the lossy layer could generate three transmission zeros, thereby achieving ultrawideband out-of-band absorption independent of the incident power. The active layer embedded with p-i-n diodes exhibits a nonlinear response to the incident intensity of spatial waves, which can be switched between bandpass and bandstop resonances under low and high incident intensities based on ECM. The steady-state and transient-state characteristics of the proposed power-dependent rasorber are analyzed as well. For demonstration, a prototype with $13\times 13$ unit cells is fabricated and measured in free space for a low-power experiment, while two prototypes with $4\times 2$ unit cells and $2\times 1$ unit cells are processed and measured in rectangular waveguides for a high-power experiment. Under low incident power level, a high-selectivity transparent window is successfully realized with simulated (measured) low insertion loss (IL) of 0.9 dB at 3.58 GHz (0.83 dB at 3.44 GHz) and an ultrawide absorption band range of 5.87–17.88 GHz (5.98–18 GHz). As the incident field is higher than 740 V/m, the transmitted power undergoes a rapid reduction in the passband. The maximum shielding effectiveness (SE) reaches 18 dB at 3.58 GHz and the SE is better than 10 dB in the range of 0.5–17.5 GHz. The excellent nonlinear transmission characteristic with ultrawideband out-of-band absorption makes the proposed rasorber have various application prospects in the high-power microwave (HPM) protection and stealth.

Reconfigurable metasurface provides real-time modulation of electromagnetic (EM) waves in a digital manner, which has gained significant attention in antenna engineering and wireless communications. However, it requires an external feed source with a significant distance to the aperture, resulting in a relatively high profile. Herein, an innovative design of a radiation-type slot coupled polarization converter array with low-profile and wide-angle beam scanning capabilities is proposed in this letter. Based on the framework of a Fabry–Pérot-like cavity, the proposed unit cell is composed of a coupling slot layer, an anisotropic resonator layer, and the upper grating structure, which perfectly combines the functions of 1-bit phase shifter and efficient power radiator. A feed network based on true-time-delay lines is used to excite the radiation element and introduce an initial phase gradient for pencil beam scanning. Due to the direct-radiation characteristic, the proposed array can acquire the low profile of 0.41λ at 10 GHz. A 10 × 10-element array prototype is designed, fabricated, and measured. The measured results show the maximum gain is 18.3 dBi at 10 GHz, and it has wide-angle 2-D beam scanning capability in the range of 60°.

A wideband energy capturing adapter based on 3-D meta-structure is presented in this article for wireless power transmission (WPT) system. Compared with conventional planar resonant structure, the proposed 3-D meta-structure for energy collection is designed first. After analyzing the magnetic field magnitude and power loss density distribution of the 3-D meta-structure unit cell, it is found that the induced current lengths are different under ${x}$ -polarization and ${y}$ -polarization, resulting in different equivalent circuits and operating bandwidths. The collecting efficiency of the 3-D meta-structure unit cell under oblique incidence is investigated as well, demonstrating the stable wide-angle reception. Then, a wideband combining network is proposed to collect the alternating current (ac) power from an $8\times8~3$ -D meta-structure array based on multistage T-junction power dividers and impedance transformers. The operating bandwidth of the proposed combining network can cover that of the 3-D meta-structure array. After energy collection, a rectifier circuit based on a multichannel parallel connection is presented for wideband ac-to-direct current (dc) conversion. Due to the nonlinear characteristics of the proposed rectifier circuit, the effects of input power level, load resistance, and operating frequency on rectification efficiency are studied in detail. Finally, a prototype is fabricated and tested for demonstration. Measured results show that the proposed design can achieve over 30% system conversion efficiency $\eta $ at 3.5–8.5 GHz (6.2–6.7 GHz) under ${x}$ -( ${y}$ -) polarization. Simulated and measured results are in good agreement.

Manipulating the wavefront in complex scattering environments has become imperative for various applications. Here, we propose a time-reversal interpolation metasurface to achieve electromagnetic focusing in such intricate environments, with a significant emphasis on the efficiency achieved through low sampling of virtual source information. Moreover, it avoids the complex channel estimation software algorithms in traditional communications and directly implements channel feedback in hardware. By introducing cubic spline interpolation into time reversal, the collection and processing of prior information are greatly simplified. To verify the feasibility of the time-reversal interpolation metasurface theory, we conducted simulations and experiments. Ultimately, high-precision electromagnetic focusing is achieved under low sampling of pilot information, ensuring a focusing efficiency ranging from 36% to 59%, with the lowest sampling rate being 12.25%. This paves the way for the utilization of metasurfaces in complex environments, encompassing electromagnetic communications, energy transfer, and other related fields.