  We consider the interaction of gapped graphene in the two-band approximation
using an explicit time-dependent approach. In addition to the full high-order
harmonic generation (HHG) spectrum, we also obtain the perturbative harmonic
response using the time-dependent method at photon energies covering all the
significant features in the responses. The transition from the perturbative to
the fully non-perturbative regime of HHG at these photon energies is studied in
detail.
