Nonlinear response of graphene to a few-cycle terahertz laser pulse: Role of doping and disorder

The nonlinear response of graphene to a THz laser pulse is studied by solving the time-dependent Dirac equation and the time-dependent Schr\"odinger equation within a tight-binding approximation applied to finite-sized structures. We compare predictions of these two approximations for the harmonic spectrum with the recent experiment by P. Bowlan et al. [Phys. Rev. B 89, 041408(R) (2014)]. We highlight the influence of short-range and long-range disorder which can be accounted for within the tight-binding description on a microscopic level. We find good agreement with the experiment. Most notably, the intensity of the second harmonic offers a quantitative indicator for the amount of short-range disorder.