Nonlinear optical response of graphene in time domain

We study nonlinear optical response of electron dynamics in graphene to an intense light pulse within the model of massless Dirac fermions. The time-dependent Dirac equation can be cast into a physically transparent form of extended optical Bloch equations that consistently describe the coupling of light-field-induced intraband dynamics and interband transitions. We show that the nonlinear optical response is not sufficiently described neither by pure intraband nor by pure interband dynamics but their interplay has to be taken into account. When the component of the instantaneous momentum parallel to the field changes its sign, the interband transition is strongly enhanced and considerably influences the intraband dynamics. This counteracts anharmonic response expected from purely intraband dynamics and relaxes nonlinearly. Nevertheless, graphene is still expected to exhibit nonlinear optical response in the terahertz regime such as harmonic generation.