Dynamical symmetry and valley-selective circularly polarized high-harmonic generation in monolayer molybdenum disulfide

Abstract

Circularly polarized high harmonics (CHHs) have been observed in several solid targets under single-color circularly polarized excitation. However, experimental observations show that CHHs cannot be efficiently generated in monolayer molybdenum disulphide (${\text{MoS}}_{2}$) driving by a single-color circularly polarized laser, thus letting its unique valley-selective circular dichroism (VSCD) to remain unexploited in the high harmonic generation process. Here we demonstrate that the efficient generation of CHHs in monolayer ${\text{MoS}}_{2}$ driven by counterrotating bicircular (CRB) fields and the broken inversion symmetry of monolayer ${\text{MoS}}_{2}$ leads to the generation of forbidden $3n$ ($n\ensuremath{\in}\mathbb{N}$) harmonic orders. Interestingly, we find that the VSCD lead to a distinctive valley selection of harmonic orders, while the valley selection caused by the trefoil orientation relative to the lattice is also observed. Dynamical symmetry analyses show that the rotational symmetry of the crystals can be decoded by the combination of a linearly polarized excitation scheme and CRB excitation scheme via the lowest common multiple rule. Additionally, VSCD leaves unique fingerprints in the ellipticity of harmonics.