High-order harmonic generation from gapped graphene: perturbative   response and transition to non-perturbative regime
  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. 
Darko Dimitrovski, Lars Bojer Madsen, and Thomas Garm Pedersen
cond-mat.mtrl-sci cond-mat.mes-hall physics.optics