Optical second-harmonic generation of Janus MoSSe monolayer

The transition metal dichalcogenides (TMD) monolayers have shown strong second-harmonic generation (SHG) owing to their lack of inversion symmetry. These ultrathin layers then serve as the frequency converters that can be intergraded on a chip. Here, taking MoSSe as an example, we report the first detailed experimental study of the SHG of Janus TMD monolayer, in which the transition metal layer is sandwiched by the two distinct chalcogen layers. It is shown that the SHG effectively arises from an in-plane second-harmonic polarization under paraxial focusing and detection. Based on this, the orientation-resolved SHG spectroscopy is realized to readily determine the zigzag and armchair axes of the Janus crystal with an accuracy better than ±0.6°. Moreover, the SHG intensity is wavelength-dependent and can be greatly enhanced (∼ 60 times) when the two-photon transition is resonant with the C-exciton state. Our findings uncover the SHG properties of Janus MoSSe monolayer, therefore lay the basis for its integrated frequency-doubling applications.