# Design of an Optically Transparent Microwave Absorber Based on Coding Metasurface
## 1 Introduction  
The metamaterial is an engineered periodic structure composed of subwavelength units with extraordinary electromagnetic properties. It is used in a variety of applications in the microwave to the terahertz frequency range [1; 2; 3; 4; 5; 6; 7; 8]. Artificially construct metamaterials can exhibit negative refractive indices [9], which can lead to new optical phenomena [10]. The metamaterial microwave absorber is designed by processing and designing the shape, size, and method of combining artificial composite materials for producing efficient absorptance in the very narrow or wide frequency bandwidth. At the moment, metamaterials are used in industrial and military fields [11] such as communication, stealth and mid-infrared and thermal detection devices [12]. Thus, research on metamaterial microwave absorbers is crucial to improve the performance of these applications. In recent years, several microwave absorbers with new properties [13; 14] have been proposed, and the perfect microwave loss characteristics can be achieved by designing the basic unit structure of the metamaterials. Since the characteristics of a metamaterial depend primarily on the geometry, they can be used in a specific electromagnetic spectrum. The perfect loss design of metamaterials overcomes the thickness limitation of traditional absorbers, and the limitation of microwave absorptance efficiency. In addition, depending on the substrate and geometries used, the metasurface can be used for high quality switching between reflection and transmission, phase control and significant resonant behavior within the spectral window of interest [15]. Metamaterial absorbers based on resistive films have thin profiles that are easier to build and shape. Representative resistive materials that can be used for wave-absorbing structures are ITO and ATO (antimony tin oxide), with ITO being more frequently chosen due to its relatively high optical transparency [16]. Optical transparency can be a critical requirement in certain application situations, for example, the plane windows or the display screen glass that needs to shield the microwave yet transmit the visiblelight, and the radome, which needs to transmit both microwave and visible light. The optically transparent absorbers are more suitable for displays, phones, and glass windows of military vehicles [17; 18]. In recent years, RCS reduction technology [19] has become an important research element in electromagnetic stealth topics, which can be divided into two directions: one uses the perfect metamaterial absorber (PMA) proposed by [PERSON] in 2008 [20], which adopts an ultra-thin structure and has a nearly single absorptance rate to achieve high absorptance over a wide frequency band; the other uses the coding metasurface absorber proposed by [PERSON] in 2014 [21], which employs various combinations of artificial magnetic conductors to modulate the absorber surface phase, change its scattering field, and achieve diffuse reflection, thereby reducing RCS. In the field of electromagnetic shielding, new metamaterials and coding techniques have been widely used in the microwave band [22]. The coding metasurface absorber can be applied to the surface of an object to improve its RCS reduction, which is a novel method of modulating microwaves that has great potential in radar stealth, imaging, and broadband communication fields [23; 24]. In this paper, we design an absorber using optically transparent absorbing material (ITO) as the coding metasurface. Based on the APS-PSO 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. It has over 90% absorptance in the 5.06-9.01 GHz band, high transmittance, and a \(-10\) dBsm RCS reduction band from 5.3-8.7 GHz.