  The unusual electrical and optical properties of graphene make it a promising
candidate for optoelectronic applications. An important, but as yet unexplored
aspect is the role of photo-excited hot carriers in charge and energy transport
at graphene interfaces. Here, we perform time-resolved (~250 fs) scanning
photocurrent microscopy on a tunable graphene pn junction. The ultrafast
pump-probe measurements yield a photocurrent response time of ~1.5 ps at room
temperature increasing to ~4 ps at 20 K. Combined with the negligible
dependence of photocurrent amplitude on environmental temperature this implies
that hot carriers rather than phonons dominate energy transport at high
frequencies. Gate-dependent pump-probe measurements demonstrate that both
thermoelectric and built-in electric field effects contribute to the
photocurrent excited by laser pulses. The relative weight of each contribution
depends on the junction configuration. A single laser beam excitation also
displays multiple polarity-reversals as a function of carrier density, a
signature of impact ionization. Our results enhance the understanding of
non-equilibrium electron dynamics, electron-electron interactions, and
electron-phonon interactions in graphene. They also determine fundamental
limits on ultrafast device operation speeds (~500 GHz) for potential
graphene-based photon detection, sensing, and communication.
