  Radiation-induced ratchet electric currents have been studied theoretically
in graphene with a periodic noncentrosymmetric lateral potential. The ratchet
current generated under normal incidence is shown to consist of two
contributions, one of them being polarization-independent and proportional to
the energy relaxation time, and another controlled solely by elastic scattering
processes and sensitive to both the linear and circular polarization of
radiation. Two realistic mechanisms of electron scattering in graphene are
considered. For short-range defects, the ratchet current is helicity-dependent
but independent of the direction of linear polarization. For the Coulomb
impurity scattering, the ratchet current is forbidden for the radiation
linearly polarized in the plane perpendicular to the lateral-potential
modulation direction. For comparison, the ratchet currents in a quantum well
with a lateral superlattice are calculated at low temperatures with allowance
for the dependence of the momentum relaxation time on the electron energy.
