High-energy optical conductivity of graphene determined by reflection contrast spectroscopy

Optical conductivity G(omega) of graphene, which is famous for its universality close to the Dirac point, is determined from the visible to near-ultraviolet region by using reflection contrast spectroscopy. We find that the universality does not survive beyond the visible region. Instead, G(omega) increases rapidly with the energy, which is originated from the flat dispersion relation at the M point corresponding to an extraordinarily large density of states (DOS). The first-principles-calculated band structure and DOS spectrum explain well the high-energy behaviors of G(omega). Our work presents a more comprehensive map of graphene's optical conductivity and electronic structure.