  High-order frequency mixing in graphene using a two-color radiation field
consisting of the fundamental and the second harmonic fields of an ultrashort
linearly polarized laser pulse is studied. It is shown that the harmonics
originated from the interband transitions are efficiently generated in the case
of the orthogonally polarized two-color field. In this case, the generated
high-harmonics are stronger than those obtained in the parallel polarization
case by more than two orders of magnitude. This is in sharp contrast with the
atomic and semiconductor systems, where parallel polarization case is more
preferable. The physical origin of this enhancement is also deduced from the
three-step semi-classical electron-hole collision model, extended to graphene
with pseudo-relativistic energy dispersion. In particular, we discuss the
influence of the many particle Coulomb interaction on the HHG process within
dynamical Hartree-Fock approximation. Our analysis shows that in all cases we
have an overall enhancement of the HHG signal compared with the free-charged
carrier model due to the electron-hole attractive interaction.
