Ultrafast relativistic response of Photo-excited Carriers in Graphene*

Understanding the ultrafast non-equilibrium dynamics of photocarriers in graphene's unique relativistic band structure is important for the development of such high-speed, graphene-based photonic devices and also from a fundamental point of view. Here, we directly demonstrate the relativistic nature of a non-equilibrium gas of electrons and holes photogenerated in a graphene monolayer as early as 100 femtoseconds (fs) after photoexcitation. We photoexcited carriers in graphene and then measured the time-resolved, pump-induced change in reflection at various visible probe photon energies. We observe a nonlinear scaling in the Drude-like optical conductivity of the photocarriers with respect to their density, in striking contrast to the linear scaling expected from conventional materials with parabolic dispersion relations.