Interlayer Resonances and Excitonic Effects in the Optical Conductivity of Twisted Bilayer Graphene

We report quantitative measurements of the optical conductivity ({\sigma}) of twisted bilayer graphene (tBLG) from 1.2 to 5.6 eV for domains with known twist angles ({\theta}). We observe two peaks in the {\sigma} spectrum of tBLG not found in Bernal stacked bilayer graphene, which we ascribe to interlayer coupling. The {\theta}-dependence of the energies of these peaks is smooth and monotonic, and can be modeled based on the band structure of single-layer graphene. In addition, our data suggest that excitonic effects play a vital role in the optical spectra of tBLG, as we find that the inclusion of e-h interactions in first-principles calculations is essential to reproduce the {\sigma} peak shape and to understand the relative heights of the absorption peaks at large {\theta}.