High-harmonic generation in graphene: Interband response and the harmonic cutoff

We numerically examine the high-harmonic generation in graphene caused by an intense few-cycle terahertz laser field by solving the time-dependent Dirac equation. The observed spectra feature a complex interplay of interband and intraband electron dynamics. At high harmonic frequencies, we observe a plateau region with a cut-off frequency linearly proportional to the laser field. The linear dependence of the cutoff on the field resembles the behavior for electrons driven in bulk crystals but differs from the case of atoms or molecules. The unique features of the graphene band structure allow for a transparent decomposition of interband and intraband contributions and for an analytic estimate of the harmonic cut-off frequency in excellent agreement with the simulations.