achieving high holographic image quality. By elaborately arranging the geometries of nanoantennas [3] or orientation angles [4,5] of metaatoms, desired abrupt interfacial phase shift at designated points within subwavelength thickness can be realized. Therefore, metasurface has been applied to versatile applications, including directional radiators, [6] thin-film cloaking, [7,8] planar lenses, [9] optical vortex beam generators, [10,11] and digital holograms. [12][13][14][15][16] Compared to the hologram based on traditional optical devices, metasurface hologram improves imaging efficiency, [17] spatial resolution, [18] and robustness against fabrication tolerances. [19] Hence, metasurface holograms have been widely applied in the terahertz, infrared, and visible frequency bands. Nevertheless, the transmittance efficiency of transmission-phaseonly metasurface hologram is still limited by cross-polarization transformation losses.Huygens metasurface can significantly improve the transmittance efficiency and achieve full phase-shift coverage, since it enables to fully control the phase and amplitude of copolarized transmitted wave without polarization conversion losses by integrating designed electric and magnetic dipoles into each metaatom. [20] On account of its superior wave-manipulating ability, various correlative researches have been reported in beam-refracting, [21] focusing, [22,23] beam-shaping, [24] and holographic imaging. [25] Here, we experimentally realize microwave holographic imaging recorded by Huygens metasurfaces with the modulation of focal intensity distribution. By judiciously designing geometrical parameters of electric and magnetic dipoles, 15 metaatoms which cover a 2π phase range with close-to-unity transmission amplitude are extracted for holographic imaging. Moreover, a new holographic algorithm is presented to divert the incident energy to specified focal positions and adjust the energy allocation among focal points. Three parameters are adopted in this work to evaluate the main aspects of image quality, including transmittance efficiency, imaging efficiency, and root-mean-square error (RMSE) of focal intensity ratio. The numerical simulations and experimental results of holographic images carried out in the microwave regime agree well with theoretical predictions, which verify the feasibility and outstanding image quality of the proposed Huygens metasurface holograms, paving the way toward highly efficient microwave holographic imaging.Huygens metasurface, an implementation of Huygens principle with metasurface, shows great potential in the manipulation of electromagnetic wave by elaborately designed subwavelength scale metaatoms with full phase coverage and high transmission amplitude. Here, a multiphase hologram with Huygens metasurface is demonstrated in microwave regime, and a novel algorithm method is proposed to modulate energy distribution among focal points. The proof-of-concept experiments show superior microwave holographic images with 89% transmittance efficiency, 59% imaging...

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Metamaterial absorber (MA) is a hot spot in the research on electromagnetic absorbers. In this paper, a metamaterial based broadband polarization-insensitive absorber is proposed. The absorber is fabricated with FR-4 dielectric substrate foiled with copper. The top layer of the unit cell of the MA is composed of resistors mounted crosswire and gradient split ring resonator (SRR) with a square metal patch (SMP) in it. The overall structure is symmetrical, which makes the MA polarization-insensitive. The gradient SRRs and SMPs resonate at adjacent frequencies resulting in broadband property. The absorption rates of the MA for TE and TM wave are calculated through the simulated S-parameters. The bandwidth is 9.9 GHz, where the absorption rate maintains 60% up to 98.28% in both cases and the relative bandwidth is 57.13%. Both broadband and polarization-insensitivity properties are achieved, which demonstrate promising application prospect of the proposed MA in shielding and stealth technology.

As one of the most critical technology in array signal processing, direction of arrival (DoA) estimation has received a great deal of attention in many areas. Traditional methods perform well when the signal-to-noise ratio (SNR) is high and the receiving array is perfect, which are quite different from the situation in some real applications (e.g., the marine communication scenario). To get satisfying performance of DoA estimation when SNR is low and the array is inaccurate (mutual coupling exist), this paper introduces a scheme consisting of denoising autoencoder (DAE) and deep neural networks (DNN), referred to as DAE-DNN scheme. DAE is used to reconstruct a clean ''repaired'' input from its corrupted version to increase the robustness, and then divide the input into multiple parts in different sub-areas. DNN is used to learn the mapping between the received signals and the refined grids of angle in each sub-areas, then the outputs of each sub-areas are concatenated to perform the final DoA estimation. By simulations in different SNR regimes, we study the performance of DAE-DNN in terms of the different snapshots, batch size, learning rate, and epoch. Our results demonstrate that the proposed DAE-DNN scheme outperforms traditional methods in accuracy and robustness.INDEX TERMS DoA, SNR, denoising autoencoder, deep neural networks, mutual coupling.