# An Optically Transparent Metasurface for Microwave Amplitude-Phase Manipulation
###### Abstract  
Current microwave metasurfaces predominantly suffer from the disadvantages of optically opaque and phase-only modulation, which inevitably hinder their application potential. Herein, we have proposed a simple but efficient strategy for designing a multifunctional metasurface that is capable of simultaneously achieving visible transparency and microwave amplitude-phase manipulation. The designed meta-atom consists of a metal-frame-based H-shaped resonator and a metallic mesh layer separated by a transparent dielectric substrate, enabling eight-level phase modulation with a \(\pi/4\) interval and continuous amplitude modulation covering the range of 0-0.9 at 16 GHz. As a proof-of-concept demonstration, a spatially multiplexed complex-amplitude hologram utilizing the designed meta-atom is simulated and experimentally validated. The results show that two distinct holographic images can be reconstructed in different imaging planes, and the measured average optical transmittance attains 63.7% at a wavelength range of 400-800 nm. Our proposed design strategy paves the way to an optically transparent microwave metasurface which is expected to have great potential in application scenarios requiring both visible transparency and microwave wavefront control. metasurface; metal frame; metallic mesh; optical transparency; complex-amplitude holography +
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sidelobe levels and inefficient energy distribution [6; 7; 8]. In contrast, complex-amplitude metasurfaces transcend these boundaries by synergistically tailoring both phase and amplitude profiles of local meta-atoms [9]. Such a paradigm has realized more sophisticated and high-fidelity wavefront control functionalities that are unattainable with phase-only metasurfaces [10; 11; 12; 13; 14; 15; 16]. For example, [PERSON] et al. developed a transmissive metasurface with independent amplitude and phase control, which was utilized to realize a low-side-lobe metalens antenna. Compared to phase-only metalens antennas, the proposed design achieved a 5.7 dB average sidelobe level suppression [17]. Based on an all-dielectric metasurface with independent and complete manipulation of the amplitude and phase, [PERSON] et al. showed that the quality of holographic images was significantly improved by amplitude-phase control, and they successfully created artifact-free two-dimensional holographic images and controlled the surface textures of three-dimensional holographic objects, which are not attainable in phase-only holography [18]. Very recently, [PERSON] et al. proposed the concept of multiplexed coherent pixel metasurfaces that can continuously tune the amplitude and phase under incident plane and orbital angular momentum (OAM) waves. Such a strategy realized the multiple degree-of-freedom control of OAM, significantly increasing the data capacity of metasurfaces [7]. Despite the versatile functionalities realized by the simultaneous modulation of amplitude and phase, the aforementioned metasurfaces fail to realize the compatibility of optical transparency. The pursuit of optically transparent metasurfaces has opened new frontiers in aircraft optical windows, vehicle windshields, display screens, and glass curtain walls of buildings, just to name a few. To date, much effort has been devoted to implementing the convergence of optical transparency and microwave manipulation [19; 20]. For instance, traditional metallic patches fabricated with printed circuit board technology are replaced by transparent conductive oxides such as indium tin oxide (ITO), generating an orbital angular momentum vortex wave while maintaining high optical transparency [21]. However, the inverse relationship between ITO sheet resistance and optical transparency imposes stringent design trade-offs, and existing ITO-based phase gradient metasurfaces usually exhibit relatively large EM losses [22].