  Graphene, a monolayer of carbon atoms arranged in a hexagonal pattern,
provides a unique two-dimensional (2D) system exhibiting exotic phenomena such
as quantum Hall effects, massless Dirac quasiparticle excitations and universal
absorption & conductivity. The linear energy-momentum dispersion relation in
graphene also offers the opportunity to mimic the physics of far-away
relativistic particles like neutron stars and white dwarfs. In this letter, we
perform a counterintuitive ultrafast pump-probe experiment with high photon
energies to isolate the Drude-like intraband dynamics of photoexcited carriers.
We directly demonstrate the relativistic nature of the photoexcited Dirac
quasiparticles by observing a nonlinear scaling of the response with the
density of photoexcited carriers. This is in striking contrast to the linear
scaling that is usually observed in conventional materials. Our results also
indicate strong electron-phonon coupling in graphene, leading to a sub-100
femtosecond thermalization between high energy photoexcited carriers and
optical phonons.
