# Design of an Optically Transparent Microwave Absorber Based on Coding Metasurface
## 2 Design and Simulation  
The structure of a rectangular patch and a circular patch unit is shown in Figure 1. The structure of a rectangular patch unit is shown in Figure 1a, where the top layer of the unit consists of four 13 mm \(\times\) 13 mm rectangular ITO film patches with a square resistance of 110 \(\Omega\)/sq. The structure of a circular patch unit is shown in Figure 1b, where the top layer of the unit is made up of four circular ITO film patches with a radius of 3 mm and a square resistance of 110 \(\Omega\)/sq. The ITO rectangular patch structures and ITO circular patch structures are symmetrical. Figure 1: Unit structure. (**a**) Top view of rectangular patch. (**b**) Top view of circular patch. (**c**) Side view of the unit. As shown in Figure 1c, both rectangular and circular unit structures have a sandwich structure consisting of five layers. The upper surface is made up of a simple patterned ITO thin film resistive patch (\(R_{\text{s1}}=110\)\(\Omega/\text{sq}\)). The ground plane is made of a low-resistance ITO thin film (\(R_{\text{s2}}=8\)\(\Omega/\text{sq}\)), which completely reflects the incident wave and ensures that the microwave cannot pass through the absorber. The ITO films are printed on both the upper surface and the ground plane on two flexible polyethylene terephthalate (PET) sheets (relative permittivity of 3.0, dielectric loss tangent of 0.06, and thickness of \(h_{\text{s}}\)). PMMA (relative permittivity of 2.25, dielectric loss tangent of 0.01, thickness of \(h_{\text{p}}\)) is in the middle of two PET layers. Therefore, the PMMA layer is the middle layer of the sandwich structure. To ensure the high light transmission of the proposed metamaterial absorber, all materials used in the design have high visible light transmission. In this paper, we use 110 \(\Omega/\text{sq}\) ITO film to form a patterned resistive metasurface to efficiently dissipate the incident energy. The resistance of ITO films can be fine-tuned by precisely controlling the film thickness. The detail of the parameters of the proposed structure, shown in Figure 1, is given in Table 1.  
As the frequency increases, the reflection coefficient phase of the proposed unit structure changes from \(+180^{\circ}\) to \(-180^{\circ}\) (Figure 2a). At the resonant frequency of 3.4 GHz, the reflection phase of the circular patch reaches \(0^{\circ}\) and the reflection coefficient at the resonant frequency is \(-0.57\) dB (Figure 2b). At the resonant frequency of 3.22 GHz, the reflection phase of the rectangular patch reaches \(0^{\circ}\) and the reflection coefficient at the resonant frequency is \(-8.82\) dB, which is attributed to the dielectric and radiation losses. Because of the \(180^{\circ}\) reflection phase difference between them, they can be used as \(-0^{\circ}\) and \(-1^{\circ}\) in the coding sequence to increase the frequency bandwidth of the RCS reduction. Different coding sequences yield different electromagnetic properties, so the optimization of the coding sequences is particularly important to enhance the performance of the absorber and the RCS reduction. Based on the particle swarm algorithm, the spatial arrangement of the encoded basic unit is designed by regulating the scattering field of the absorber so that the incident electromagnetic wave is diffusely reflected and the RCS reduction is achieved. Combining the integrated algorithm of PSO and APS, the phase of the coding metasurface can be optimized quickly to obtain the best performance of the coding metasurface absorber. The PSO module evaluates the fitness in each iteration, keeps track of the particle velocities and population positions, and then shares the information with the APS module using the APS-PSO algorithm to encode the above two basic encoding units in an arrangement. The algorithm is implemented using Matlab, and the population