  The conversion of light into free electron-hole pairs constitutes the key
process in the fields of photodetection and photovoltaics. The efficiency of
this process depends on the competition of different relaxation pathways and
can be greatly enhanced when photoexcited carriers do not lose energy as heat,
but instead transfer their excess energy into the production of additional
electron-hole pairs via carrier-carrier scattering processes. Here we use
Optical pump - Terahertz probe measurements to show that in graphene
carrier-carrier scattering is unprecedentedly efficient and dominates the
ultrafast energy relaxation of photoexcited carriers, prevailing over optical
phonon emission in a wide range of photon wavelengths. Our results indicate
that this leads to the production of secondary hot electrons, originating from
the conduction band. Since hot electrons in graphene can drive currents,
multiple hot carrier generation makes graphene a promising material for highly
efficient broadband extraction of light energy into electronic degrees of
freedom, enabling high-efficiency optoelectronic applications.
