## 2 Absorber Design  
Figure 1 shows the three-layer metamaterial absorber structure, where the middle layer uses silicon dioxide (SiO\({}_{2}\)) as the dielectric substrate and tungsten (W) as a patch and ground on both sides of the SiO\({}_{2}\). The material properties of SiO\({}_{2}\) and W were considered from [28]. The purpose of using W in the patch and ground of this design was due to the high intrinsic loss and impedance matching feature with free space in optical wavelength applications. Therefore, this leads to an increased absorption behavior by offering low reflection and transmission properties [29]. The SiO\({}_{2}\) was selected as a dielectric substrate due to its lossless property at optical wavelengths and large negative part of permittivity in the whole optical region instated of the large imaginary part. These features of SiO\({}_{2}\) provide a good impedance matching and lead to a high and more comprehensive absorption bandwidth [30]. The proposed DSR metamaterial absorber was designed and simulated using Frequency Integration Technique (FIT)-based Computer Simulation Technology  
(CST) [31]. The overall design parameters are listed in Table 1. Unit cell boundary conditions on the x- and y-axis and open add space on the z-axis were applied for the simulation. The absorption property \(A(\omega)\) was calculated by using Equation (1) [8]. \[A(\omega)=1-\left|S_{11}(\omega)\right|^{2}-\left|S_{21}(\omega)\right|^{2}=1-R-T \tag{1}\]  
where \(S_{11}\) and \(S_{21}\) are the reflection coefficient and transmission coefficient, respectively. \(R\) and \(T\) are the reflectance and transmittance, respectively. Due to the thick back layer, the transmission of electromagnetic waves blocks and the transmission coefficient \(S_{21}\) become zero. Figure 2a presents the absorption properties for the different layers, where no absorption was observed only for the SiO\({}_{2}\) dielectric layer. Due to all incident waves transmitted through the dielectric material, an absorption around 20% was realized by adding a patch on top of the dielectric, but the maximum incident wave still transmitted through the material. After that, the W base ground plane was added to prevent the transmission of the incident wave and a 97.1% average absorption level was attained in the entire visible optical wavelength region. The geometrical design of the patch also played a crucial role in achieving higher and wider absorption levels, which can be understood from the design evolution of the proposed MMA. The absorption property of the proposed MMA of various design evolutions is presented in Figure 2b. The peak absorption reached 99% at 553.63 nm, and an average absorption of 92.46% was achieved for design 1, where only a square resonator was used. In design two, a square slot was made in the patch of design 1, which increased the average absorption level to 94.46%, and the peak absorption shifted toward 495 nm. By adding another square patch inside the square ring in design 2, the average \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Parameters** & **h1** & **h2** & **h3** & **W** & **W1** & **W2** & **W3** \\ \hline Value (nm) & 120 & 62.82 & 15 & 950 & 670 & 350 & 150 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of parameters of the proposed absorber. Figure 1: Design of proposed MMA of visible optical wavelength application. absorption increased up to 96.70% in design 3. Finally, an average absorption of 97.1% was attained by making a square slot at the center of the square plane in design 3. The parametric analysis was also very important for understanding the absorption properties of the proposed MMA. The parameter h1 represents the thickness of W at the ground plane. The absorption property was investigated from 20 to 140 nm with a 20 nm interval for h1 and is presented in Figure 3a. The absorption level increased with h1 to 80 nm and displayed an almost unique absorption curve for higher thicknesses.