  Graphene is a new material showing high promise in optoelectronics,
photonics, and energy-harvesting applications. However, the underlying physical
mechanism of optoelectronic response has not been established. Here, we report
on the intrinsic optoelectronic response of high-quality dual-gated monolayer
and bilayer graphene p-n junction devices. Local laser excitation at the p-n
interface leads to striking six-fold photovoltage patterns as a function of
bottom- and top-gate voltages. These patterns, together with the measured
spatial and density dependence of the photoresponse, provide strong evidence
that non-local hot-carrier transport, rather than the photovoltaic effect,
dominates the intrinsic photoresponse in graphene. This novel regime, which
features a long-lived and spatially distributed hot carrier population, may
open the doorway for optoelectronic technologies exploiting efficient energy
transport at the nanoscale.
