  Graphene is emerging as a viable alternative to conventional optoelectronic,
plasmonic, and nanophotonic materials. The interaction of light with carriers
creates an out-of-equilibrium distribution, which relaxes on an ultrafast
timescale to a hot Fermi-Dirac distribution, that subsequently cools via phonon
emission. Here we combine pump-probe spectroscopy, featuring extreme temporal
resolution and broad spectral coverage, with a microscopic theory based on the
quantum Boltzmann equation, to investigate electron-electron collisions in
graphene during the very early stages of relaxation. We identify the
fundamental physical mechanisms controlling the ultrafast dynamics in graphene,
in particular the significant role of ultrafast collinear scattering, enabling
Auger processes, including charge multiplication, key to improving photovoltage
generation and photodetectors.
