  Here we present an all-optical plasmon coupling scheme, utilising the
intrinsic nonlinear optical response of graphene. We demonstrate coupling of
free-space, visible light pulses to the surface plasmons in a planar,
un-patterned graphene sheet by using nonlinear wave mixing to match both the
wavevector and energy of the surface wave. By carefully controlling the
phase-matching conditions, we show that one can excite surface plasmons with a
defined wavevector and direction across a large frequency range, with an
estimated photon efficiency in our experiments approaching $10^{-5}$.
