The optical conductivity of freely suspended graphene was examined under
non-equilibrium conditions using femtosecond pump-probe spectroscopy. We
observed a conductivity transient that varied strongly with the electronic
temperature, exhibiting a crossover from enhanced to decreased absorbance with
increasing pump fluence. The response arises from a combination of bleaching of
the inter-band transitions by Pauli blocking and induced absorption from the
intra-band transitions of the carriers. The latter dominates at low electronic
temperature, but, despite an increase in Drude scattering rate, is overwhelmed
by the former at high electronic temperature. The time-evolution of the optical
conductivity in all regimes can described in terms of a time-varying electronic
temperature.Comment: 10 pages (4 pages manuscript + Supplemental Info.
