Surface-exciton enhanced SHG response in few-layer 2H-TMDC

Abstract

We explore the nonlinear optical properties of few-layer MoS2 by means of polarization and laser-power-dependent measurements as well as ab initio techniques. While for even layer samples a weak second-harmonic (SH) signal can be attributed to the presence of surface defects or interface effects, our measurements resolve a layer-number dependent signal for odd-layer samples. For the excitation energy of 780 nm, we find that the SH intensity decreases steadily with the layer number. Our simulations demonstrate that this effect cannot be purely attributed to modifications of the band structure, but requires the inclusion of excitonic effects and can be explained by the increasing delocalization of excitons with increasing sample thickness.