A study of the nonlinear optical response of the plain graphene and gapped graphene monolayers beyond the Dirac approximation

In this work, we present numerical results for the second and third order conductivities of the plain graphene and gapped graphene monolayers associated with the second and third harmonic generation, the optical rectification and the optical Kerr effect. The frequencies considered here range from the microwave to the ultraviolet portion of the spectrum, the latter end of which had not yet been studied. These calculations are performed in the velocity gauge and directly address the components of the conductivity tensor. In the velocity gauge, the radiation field is represented by a power series in the vector potential, and we discuss a very efficient way of calculating its coefficients in the context of tight-binding models.