Linear and nonlinear optical susceptibilities of bilayer graphene

The linear and nonlinear optical susceptibilities of bilayer pristine graphene (BLPG) and H2S single molecule adsorbed at three different sites on a single graphene sheet of BLPG are calculated to obtain further insight into the electronic properties. Calculations show that the adsorption of H2S on the bridge and top sites open a gap around the Fermi level, while adsorption of H2S on the hollow site closes the energy gap, resulting in significant changes in the linear and nonlinear optical susceptibilities. This is attributed to the fact that the adsorbed H2S onto a single graphene sheet of BLPG cause significant changes in the electronic structure. The calculated linear optical susceptibilities show a huge anisotropy confirming that the graphene has unusual and interesting optical properties. We find that the absorption spectrum of graphene is quite flat extending from 300-2500 nm with an absorption peak in the UV region (similar to 270 nm), which is in excellent agreement with the experimental data. The pristine graphene shows a strong saturable absorption because of a large absorption and Pauli blocking. We have calculated the nonlinear optical susceptibilities of BLPG and the three configurations and found that they possess a huge second harmonic generation. We have also calculated the microscopic hyperpolarizability, beta(ijk), for BLPG.