  We theoretically investigate high-order harmonic generation (HHG) in graphene
under mid-infrared (MIR) and terahertz (THz) fields based on a quantum master
equation. Numerical simulations show that MIR-induced HHG in graphene can be
enhanced by a factor of 10 for fifth harmonic and a factor of 25 for seventh
harmonic under a THz field with a peak strength of 0.5 MV/cm by optimizing the
relative angle between the MIR and THz fields. To identify the origin of this
enhancement, we compare the fully dynamical calculations with a simple
thermodynamic model and a nonequilibrium population model. The analysis shows
that the enhancement of the high-order harmonics mainly results from a coherent
coupling between MIR- and THz-induced transitions that goes beyond a simple
THz-induced population contribution.
