# Plasmonic and Dielectric Metasurfaces: Design, Fabrication and Applications
## 2 Plasmonic Metasurfaces
### Graphene Split-Ring Metasurface-Assisted Terahertz Coherent Perfect Absorption [122]
We consider the same split-ring graphene film on a SiO\({}_{2}\) substrate with a refractive index of \(n=1.45\), as shown in Figure 14b. Figure 16a,b presents simulated normalized absorption spectra for single-beam surface-normal illumination from the split-ring graphene side and the SiO\({}_{2}\) side, respectively, under different chemical potentials (Fermi energies). By electrically tuning the chemical potential of the split-ring graphene sheet, one can change the resonance center frequency. For illumination from the split-ring graphene side, the maximum absorption keeps \(\geq\) 49.5% when the center frequency varies from 2.29 to 2.71 THz, as shown in Figure 16a. For illumination from the SiO\({}_{2}\) substrate side, the maximum absorption keeps \(\geq\) 49.8%, as shown in Figure 16b. Figure 17 displays the calculated total absorption spectra of the split-ring graphene under different chemical potentials. An absorption peak of 97.13% at 2.29 THz is seen from Figure 17a for \(\mu_{\text{c}}=0.24\) eV. With the increase of the graphene chemical potential, it shows a blue shift of the resonance peak frequency. When the chemical potential of graphene is tuned to \(\mu_{\text{c}}=0.35\) eV, the absorption at 2.71 THz  
Figure 16: Simulated normalized absorption spectra for single-beam surface-normal illumination from (**a**) the split-ring graphene side; and (**b**) the SiO\({}_{2}\) side under different chemical potentials of graphene. Reproduced with permission from [122], Copyright The Optical Society, 2015. Figure 15: (**a**) Simulated reflection, transmission and absorption of the split-ring graphene film under the illumination of only one beam at normal incidence; (**b**) normalized total absorption under the illumination of two counter-propagating coherent beams with the same intensities and phases; the chemical potential of graphene is assumed to be \(\mu_{\text{c}}=0.3\) eV; (**c**) simulated normalized total absorption as a function of relative phase difference between two counter-propagating coherent beams. Reproduced with permission from [122], Copyright The Optical Society, 2015.  
can still reach 95.6%. As the Fermi level of graphene can be electrically tuned, one can flexibly control the total absorption of the split-ring graphene. The split-ring graphene possesses the minimum absorption when the chemical potential is \(\mu_{\text{c}}\) = 0 eV, as shown in Figure 5b. For the center frequency at 2.5 THz, the maximum and minimum absorption are 97.5% (\(\mu_{\text{c}}\) = 2.9 eV) and 1.25% (\(\mu_{\text{c}}\) = 0 eV), giving a gate-tunable modulation contrast of 19 dB. Fabrication: The proposed graphene split-ring metasurface can be fabricated using the existing fabrication techniques. Figure 18 shows the schematic diagram of the fabrication process of the graphene split-ring metasurface: (1) transferring the graphene with the polymethyl methacrylate (PMMA) layer onto the surface of the glass substrate; (2) using standard electron-beam lithography to form patterns on the PMMA layer; (3) using the O\({}_{2}\) plasma etching technique to remove the undesired parts of graphene; (4) removing the PMMA layers.