  Interband optical transitions in graphene are subject to pseudospin selection
rules. Impulsive excitation with linearly polarized light generates an
anisotropic photocarrier occupation in momentum space that evolves at
timescales shorter than 100fs. Here, we investigate the evolution of
non-equilibrium charges towards an isotropic distribution by means of
fluence-dependent ultrafast spectroscopy and develop an analytical model able
to quantify the isotropization process. In contrast to conventional
semiconductors, the isotropization is governed by optical phonon emission,
rather than electron-electron scattering, which nevertheless contributes in
shaping the anisotropic photocarrier occupation within the first few fs.

Mesoscale and Nanoscale Physics