## 2 Design and Simulation
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline P1 & 50 mm & d3 & 0.9 mm \\ P2 & 10 mm & w2 & 3 mm \\ P3 & 50 mm & w3 & 1 mm \\ R2 & 4 mm & t & 0.76 mm \\ R3 & 7 mm & SR1 & 3 f/sq \\ d1 & 0.9 mm & SR2 & 45 f/sq \\ d2 & 1.1 mm & SR3 & 200 f/sq \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The optimized structural parameters of the absorber. Figure 2: The characteristics of the proposed absorber. performance, so the lower resistance was selected. Note that at 13.9 GHz, the surface currents of layer 2 and layer 3 were higher than that of layer 1. In addition, it can be observed that the current flowed in opposite directions on layer 1 and layer 2. Such anti-parallel current flows imply the occurrence of magnetic resonances between layer 1 and layer 2. However, the direction of the current flow between layer 2 and layer 3 was parallel. Such parallel current flows indicate the occurrence of electric resonances between layer 2 and layer 3. Thus, the electric and magnetic resonances occured simultaneously, resulting in a strong broadband absorption. Furthermore, the power loss, \(P_{D}\), in the unit cell can be calculated by Equation (1). \[P_{D}=SR\iint\limits_{unitcell}\left|J(x,y)\right|^{2}dxdy \tag{1}\]  
where \(J(x,y)\) is the surface current, and \(SR\) is the sheet resistance. Thus, the surface current in the three ITO layers, combined with the ohmic loss from the ITO film, further contribute to the excellent absorption properties. Therefore, the absorption bandwidth and absorptance of the absorber can be precisely adjusted by the geometry and sheet resistance of the ITO metasurface. In order to further explore this mechanism, the effects of each parameter of the transparent absorber on the absorption property were analyzed. As shown in Figure 4, the absorption response of the proposed absorber under different sheet resistance values of each layer was studied. Note that when the sheet resistance of one layer changed, the sheet resistance of the other two layers remained unchanged. It can be observed from Figure 4a that as the resistance SR\({}_{1}\) increased, the first absorption peak intensity slightly decreased, while the second absorption peak intensity stayed pretty much the same. Therefore, changes in SR\({}_{1}\) had little impact on absorption performance. The smaller SR\({}_{1}\) is, the lower the optical transparency of the absorber is, and the better the shielding effectiveness is. Thus, considering the optical transparency and shielding effect, SR\({}_{1}\) = 3 \(\Omega\)/sq was selected. It can be seen from Figure 4b that with the increase in resistance SR\({}_{2}\), the first absorption peak intensity first decreased and then increased, while the second absorption peak intensity decreased. At the same time, the first and second absorption peaks shifted toward each other. Therefore, the absorption bandwidth of the absorber decreased with the increase in SR\({}_{2}\). Thus, it is clear that there is an optimal sheet resistance value (SR\({}_{2}\) = 45 \(\Omega\)/sq) for the widest absorption bandwidth (greater than 85%). In this case,  
Figure 3: The distributions of surface currents under normally incident TE waves at 7.3 GHz and 13.9 GHz. the two absorption peaks overlap and together form a continuous broadband absorption of above 85%, ranging from 6.0 to 16.5 GHz. As shown in Figure 4c, the first and second absorption peak intensities slightly increased with the increase in resistance SR3. At the same time, the change in SR3 had little influence on the absorption bandwidth. Considering that ITO is difficult to prepare when its sheet resistance is very high, we chose SR3 = 200 \(\Omega\)/sq. In conclusion, the absorption performance of the absorber was not sensitive to SR1 and SR3, but sensitive to SR2. Therefore, in preparation, SR2 needs to be strictly controlled, while SR1 and SR3 can be appropriately relaxed.