Nanometer-Scale Structure Property of WS₂ Flakes by Nonlinear Optical Microscopy: Implications for Optical Frequency Converted Signals

Structural irregularities have attracted increasing attention in two-dimensional transition metal dichalcogenides (TMDCs), whose impacts on the electronic structure cannot be neglected. Here, nonlinear optical spectroscopy and microscopy are used to investigate these effects in tungsten disulfide (WS 2 ) flakes based on the spatially resolved second harmonic generation (SHG) and room-temperature two-photon photoluminescence (2PPL). Notably, SHG intensity appears the strongest at these flake edges, which is anticorrelated with the room-temperature 2PPL response in monolayer WS 2 . This work provides a convenient method to probe the second-order susceptibility of TMDCs for the purpose of achieving a high optical frequency converted signal for nonlinear optical applications.