  We report an optical study of charge transport in graphene. Diffusion of hot
carriers in epitaxial graphene and reduced graphene oxide samples are studied
using an ultrafast pump-probe technique with a high spatial resolution.
Spatiotemporal dynamics of hot carriers after a point-like excitation are
monitored. Carrier diffusion coefficients of 11,000 and 5,500 squared
centimeters per second are measured in epitaxial graphene and reduced graphene
oxide samples, respectively, with a carrier temperature on the order of 3,600
K. The demonstrated optical techniques can be used for non-contact and
non-invasive in-situ detection of transport properties of graphene.
