Orientation dependence of high-harmonic generation in monolayer transition metal dichalcogenides

We theoretically investigate the orientation dependence of high-harmonic generation (HHG) in monolayer transition metal dichalcogenides (TMDCs). We find that, unlike conventional solid-state and atomic layered materials such as graphene, both parallel and perpendicular emissions with respect to the incident electric field exist in TMDCs. Interestingly, the parallel (perpendicular) emissions principally contain only odd-(even-) order harmonics. Both harmonics show the same periodicity in the crystallographic orientations but opposite phases. These peculiar behaviors can be understood on the basis of the dipole moments in TMDCs, which reflect the symmetries of both atomic orbitals and lattice structures. Our findings are qualitatively consistent with recent experi- mental results and provide a possibility for high-harmonic spectroscopy of solid-state materials.