Photon absorption in twisted bilayer graphene

We investigate one- and two-photon absorption in twisted bilayer graphene (TBLG) by examining the effects of tuning the twist angle theta and the excitation energy El on its absorption coefficients alpha i = 1 , 2 . We find that alpha 1 as a function of El for TBLG exhibits distinct peaks corresponding to its van Hove singularities. For small twist angles, such as theta similar to 1.8 degrees, the magnitude of the resonant peak for alpha 1 is roughly twice that of bilayer graphene (BLG). This enhanced response, compared to BLG, can be attributed to the increased density of states in the twisted structure. However, as the twist angle increases, the magnitude of the resonant peak approaches that of two decoupled single-layer graphene (SLG) sheets. On the other hand, the two-photon absorption coefficient alpha 2 for TBLG at low twist angles displays an enhancement of about one order of magnitude compared to SLG at the energies corresponding to the resonant peak, as well as a small but notable increase relative to BLG. As the twist angle decreases from 8 degrees to 2.5 degrees, the resonant peak intensifies by three orders of magnitude. Interestingly, as theta increases, the resonant features exhibited by alpha i = 1 , 2 vs El shift progressively from the infrared to the visible. On doping TBLG, both alpha 1 and alpha 2 vs El remain essentially unchanged, but with a slight redshift in their resonant peaks. Additionally, we explore various polarization configurations for two-photon absorption and determine the conditions under which alpha 2 becomes extremal.