# Electrically Tunable Perfect Terahertz Absorber Using Embedded Combine Graphene Layer
## 4 Simulation Results
It is mainly because more surfaces of the combline graphene layer are subjected to the incident wave that means more electron density on the graphene surfaces, and absorption intensity is enhanced. Also, shifting absorption peak over frequency is due to altering the surface plasmons in size manipulation. As a notable point, it is worth mentioning that the ribbons play a determining role in the frequency assignment of plasmonic modes over the absorption band and function as a frequency selective surface (FSS). In addition, a nano strip on the top of the insulator can present a tunability potential for plasmonic resonance due to coupling that directly affects the plasma frequency. Figure 5: The influence of designed ribbons number of the graphene layer on ACS of perfect absorber. For more study, the size influence of the g-distance of the designed configuration on the absorbance factor is assessed. In this regard, the g parameter is scaled to 10 nm, 20 nm, 30 nm, 40 nm, 50 nm while the stripline width is fixed at 25 nm, and then the corresponding absorption characteristic is simulated numerically. As shown in Figure 6 and Table 6, it is well-recognized that g-distance can adjust the amplitude and frequency of absorption on the spectral response of the designed perfect absorber. By size increase in the gap of g, the activated combine surface of the graphene layer becomes smaller, which means dropping in absorption intensity. Moreover, a blueshift is detected over the spectra presented in Figure 6. As an overall result, manipulating the developed model's geometric specification allows the perfect absorber's plasmon peak position and absorption amplitude to be tunned on the absorption band. Outstandingly, one of the unique properties of graphene layers is that they can be controlled via Fermi energies change. As previously mentioned, the Fermi levels of graphene layers can be tunned by an external DC source. In the developed structure, two layers of graphene with different Fermi levels, including \(E_{f}\) = 0.7 eV for combine layer and \(E_{f}\) = 0.43 eV for nano stripline are embedded. Since the coupling effect of the upper graphene layer in interaction with light was negligible due to its small width in the initial \begin{table}
\begin{tabular}{c c c} \hline \hline
**g-Distance (nm)** & **Plasmonic Resonant Frequency (THz)** & **Absorption Intensity (\(\times\)10\({}^{5}\) nm\({}^{2}\))** \\ \hline
10 & 8.477 & 2.929 \\
20 & 10.45 & 2.56 \\
25 & 11.348 & 2.366 \\
30 & 11.89 & 2.122 \\
40 & 12.891 & 1.899 \\
50 & 13.66 & 1.822 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Variations of absorption characteristics under change of the distance two ribbons in two sides of the structure (g-distance). Figure 6: The influence of g variations on ACS of proposed perfect absorber. \begin{table}
\begin{tabular}{c c c} \hline \hline
**The Number of Ribbon Teeth** & **Plasmonic Resonant Frequency (THz)** & **Absorption Intensity (\(\times\)10\({}^{5}\) nm\({}^{2}\))** \\ \hline N = 17 & 9.246 & 1.784 \\ N = 23 & 11.346 & 2.366 \\ N = 26 & 12.556 & 2.823 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Variations of absorption characteristics under change of teeth number of combline structure. structure. The results from Figure 3 show that the combine graphene layer was introduced as an active surface for the stimulation of plasmon modes; thus, we are intended to evaluate the impact of Fermi potential change on the absorbance rate of the model in the absorption band. It is important to note that wider graphene stripline exhibits an influential role, and changes in its Fermi level alter the absorption response. Nevertheless, the Fermi potential of the combine graphene layer is tuned between 0.4 eV and 0.9 eV. Interestingly, the simulation results indicate that the resonant peak of the absorber linearly follows an increasing trend on the absorption spectrum, and a blue shift occurs, as shown in Figure 7. Also, the absorption intensity of the designed absorber is regularly enhanced with the increase in Fermi energy.