## 2 Design and Simulation  
The structure of the unit cell of the transparent metamaterial absorber is depicted in Figure 1a. The unit cell is composed of three layers of indium tin oxide (ITO), two layers of polyvinyl butyl (PVB) films and three layers of dielectric substrates. In order to improve the transparency of the absorber, we used ultra-white glass as the substrate. The ultra-white glass was low-iron glass with high light transmittance (>91.5%). The relative permittivity of ultra-white glass is \(\varepsilon_{r}\) = 7.0, and the dielectric loss is negligible. The thicknesses of the three layers of ultra-white glass substrate are \(d_{1}\), \(d_{2}\) and \(d_{3}\), respectively. The sheet resistances of the three layers of ITO films we utilized were SR\({}_{1}\), SR\({}_{2}\) and SR\({}_{3}\), respectively. The first layer of ITO was printed on the first substrate to serve as the ground plane. The second layer of ITO on the second substrate was designed as a closed ring with period p\({}_{2}\), outer diameter R\({}_{2}\) and line width w\({}_{2}\). The third layer of ITO on the third substrate was designed as a closed ring with period p\({}_{3}\), outer radius R\({}_{3}\) and line width w\({}_{3}\). The two layers of PVB films were used to bond the three substrates. The relative permittivity of PVB is 2.22. The thickness of the PVB is t = 0.76 mm (selected from commercial standards). Figure 1: The structure of the absorber and the CST MWS simulation model. (**a**) The structure, (**b**) the CST MWS simulation model. We used full-wave analysis software CST MWS (computer simulation technology microwave studio) to simulate the performance of the absorber. The CST MWS simulation model is shown in Figure 1b. As shown in Figure 1b, the unit cell boundary was used to simulate the infinite periodic element of the transparent absorber. The Floquet ports were used to simulate the incident wave. In order to obtain good microwave absorption performance, we optimized the geometric and physical parameters of the absorber. At the same time, the actual processing conditions were fully considered. The optimized structural parameters of the transparent absorber are shown in Table 1.  
The absorptance of the transparent absorber can be calculated by \(A(\omega)=1-T(\omega)-R(\omega)\), where \(T(\omega)=|S_{21}|^{2}\) is the transmittance, and \(R(\omega)=|S_{11}|^{2}\) is the reflectance. The simulated reflectance, transmittance and absorptance curves of the proposed absorber under normal incidence of transverse electric (TE) wave, obtained by CST MWS, are presented in Figure 2. As shown in Figure 2, the absorber obtained wideband absorption with an efficiency above 85% over the frequency range from 6.0 to 16.5 GHz, in which the bandwidth with an absorptance higher than 90% ranged from 6.4 to 16.0 GHz, and the relative absorption bandwidth was about 91.4%. In addition, the microwave transmittance of the proposed absorber was close to zero. Thus, it was omitted and the calculation of the absorptivity simplified into the formula: \(A(\omega)=1-R(\omega)\). There were two absorption peaks at 7.3 GHz and 13.9 GHz, and the peak absorptance in each case was 94.2% and 96.7%, respectively. For insight into the physical mechanism of the proposed absorber, surface current distributions of the absorber under different frequencies were studied in Figure 3. By comparing the surface current distribution of three ITO-resistive metasurfaces at different frequencies, it was found that the intensity of surface currents on the first and second ITO film was stronger than the third ITO film at 7.3 GHz. This indicates that the absorption property is mainly attributed to the first and second ITO film. Layer 1 was used to avoid the transmission of electromagnetic waves, so as to achieve better absorption