Highly Optically Transparent Absorption And Diffusion Metamaterials Based On Metallic Mesh

With the increasing development of wireless communication technology,the exploration of detection technology and stealth technology should also keep pace with the times.Optically transparent precision instruments,cockpits of aircraft and ships,and mobile communication equipment with visualization requirements are all obvious radar detection targets due to their large Radar Cross Section（RCS）.Stealth materials with transparent characteristics that can effectively achieve RCS reduction are urgent.Metamaterials are artificial composite structures based on subwavelength scales that can flexibly control the phase,polarization,and other states of electromagnetic waves,and have been widely used in many fields.Metamaterials with optically transparent properties are a worthwhile research topic in achieving RCS reduction for radar targets.In this thesis,based on the metallic mesh process,the metasurfaces with high transmittance that can realize the RCS reduction of the target are studied,including the diffusion metasurface with high shielding effectiveness and the frequency selective rasorber（FSR）with low transmission loss.The former can realize the scattering suppression in the whole operating frequency band.The latter can realize the electromagnetic signal transmission within the passband and the electromagnetic energy suppression of the partial frequency band out of the passband.The two metasurfaces have different characteristics and can correspond to different application scenarios.（1）The design of the diffusion metasurface with high transparency and high shielding effectiveness based on metallic mesh is studied.Starting from the bottom layer of the metasurface,the optically transparent electromagnetic shielding film,the structural characteristics,light transmittance,and electromagnetic properties of the electromagnetic shielding films with different mesh structures are analyzed.Additionally,the indium tin oxide（ITO）-made EMI shielding film are listed for comparison.The shielding film with a regular hexagonal grid structure with a shielding effectiveness of 30d B and an optical transmittance greater than 90%is used as the floor of the diffusion metasurface.The quasi-Minkowski rings with different scale are used as the two coding units to form the1-bit diffusion metasurface.The two units have the same amplitude and opposite phase in the 10.5-19GHz frequency band（the difference is within 180°±30°）,and theoretically optical transmittance can reach more than 93%.The optimization algorithm is used to design a reasonable phase arrangement for the two elements,to achieve a more random scattering pattern like diffusion.In the case of ensuring the diffusion scattering effect,the proposed design solves the problem that the diffusion metasurface cannot realize high optical transmittance and high shielding effectiveness simultaneously.The simulation and experimental measurement results are in reasonable agreement,thus demonstrating the effectiveness and application potential of the proposed design method.（2）The design of a transparent and low-loss frequency-selective wave absorber based on metallic mesh is studied.Based on the equivalent circuit model,a comparison is made between the metallic mesh frequency selective surface（FSS）and ITO-made FSS about the structural characteristics.In this thesis,a low-cost method is proposed to solve the problem of high transmittance,low loss in the transmission band,and high loss in the absorption band cannot be realized at the same time.The patterns composed of extremely thin metal wires with different structures to realize the loss layer,so that while maintaining high optical transmittance,it can achieve high-Q parallel resonance corresponding to the FSS layer and low-Q series resonance in different frequency bands.The loss layer is composed of a relatively dense T-shaped grid structure and a square spiral ring in the center,and the FSS layer is metallic mesh with a regular hexagonal structure.Because the FSR is asymmetric structure,it has the characteristics of insensitivity to TE and TM polarization.The simulation results show that the insertion loss is only 1d B at the center frequency of the pass band of 8.3GHz,and the frequency band with S₁₁less than-10d B is 2.7-8.9GHz.The overall structure can achieve a theoretical transmittance of 96.3%.The simulation and experimental measurement results are in reasonable agreement,thus illustrating the broad application potential of the proposed design method.