# Electrically Tunable Perfect Terahertz Absorber Using Embedded Combine Graphene Layer
## 3 System Design  
The schematic demonstration and geometric parameters of the designed model are illustrated in Figure 1. Three cube structures are stacked on each other, and from the top of the configuration, there is an insulator with a thickness of 50 nm (h1) and permittivity of 1.96 [22; 37]. Beneath it, another insulator with a depth of 1.5 \(\upmu\)m (h2) with identical permittivity has been placed. In the third layer, a structure of gold with a thickness of 3 \(\upmu\)m (h3) is positioned, which functions as a mirror substrate for THz waves. Two additional layers of graphene were added to the structure. One of these layers is located above the insulator and has a nano-stripline, while the other one is embedded between the first and the second insulator one and has a comb design. The combline patch on the intermediate substrate is initially dimensioned according to Table 1. The length and width of the square shape of the cell unit are equivalent to 400 nm. In simulations, the Drude model is also employed for the gold substrate. On this model, the permittivity of bulk gold is determined via \(\epsilon_{\infty}\) = 1, the plasma frequency \(\omega_{p}\) = 1.37 \(\times\) 10\({}^{16}\) s\({}^{-1}\) and damping constant \(\omega_{r}\) = 1.23 \(\times\) 10\({}^{14}\) s\({}^{-1}\)[27].  
In the case of the graphene layer, the conductive surface has opted for analysis of construction, and the graphene layer's thickness is 1 nm in the model [38; 39]. For extraction of the desired conductivity concerning graphene by Equation (2), we consider the temperature and relaxation time as 300 K and 1 ps, respectively [28; 40]. Considering the appropriate data related to temperature and relaxation time is essential for the structure design because these parameters can change the conductivity and optical properties of the graphene sheet. As mentioned earlier, the initial geometric values of the combline graphene layer are demonstrated in Table 1. Accordingly, the number of ribbons of the graphene layer in the initial design is 23 on each side.  
In order to evaluate the proposed structures in terms of functionality, the absorption cross-section (ACS) parameter as a measure of the absorption process is obtained, and its relationship with the absorption efficiency is defined by [41]:  
\[Q_{abs}=\sigma_{abs}/A \tag{7}\]  
Where \(\sigma_{abs}\), \(Q_{abs}\) and \(A\) are the absorption cross-section (ACS) normalized, absorption efficiency, and cross-sectional area, and the absorption efficiency corresponds to the cross-section normalized to the geometrical area of the structure. The simulations are carried out by numerical computations based on finite integration technique (FIT) in CST Microwave Studios. We employ a time domain solver of CAD to extract the spectral response of the introduced perfect absorber in the absorption band. Further, the graphene used in the proposed combine structure is defined as a thin layer, and boundary conditions are considered open and space in different directions of the CAD environment around the implemented configuration. We utilize the default option of CAD in the meshing process of the simulation structure.