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Role of Anisotropy in Nonlinear Harmonic Generation Across TMD Monolayers

Recent techniques have allowed transition metal dichalcogenides (TMD) monolayers to be grown and adequately characterised. Of particular interest, their nonlinear optical response presents many promising opportunities for future nanophotonic devices and technology. The dispersion of the carriers is trigonally-warped, leading to an anisotropic Fermi surface for low-lying states. In this paper, the effects of such a deformation on the nonlinear harmonic generation are studied by considering a tight-binding model expanded up to third order in $\mathbf{k} \cdot \mathbf{p}$. By solving exactly the free-carrier dynamics of the carriers when interacting with intense and ultrashort pulses of light, we predict the photogenerated current in a nonperturbative way and study its harmonic composition. We find frequency and amplitude modulation of the nonlinear current in quadratic and cubic models. Furthermore, we demonstrate anisotropy-induced modulation of the intensity of higher-order harmonics and the existence of harmonic crossovers, depending on the incident light polarisation. The methodology presented in this paper may be applied to any general effective two-band model and offer a pathway to identify signatures of electronic features in optical output.