Imaging the crystal orientation of 2D transition metal dichalcogenides using polarization-resolved second-harmonic generation

We use laser-scanning nonlinear imaging microscopy in atomically thin transition metal dichalcogenides (TMDs) to reveal information on the crystalline orientation distribution, within the 2D lattice. In particular, we perform polarization-resolved second-harmonic generation (PSHG) imaging in a stationary, raster-scanned chemical vapor deposition (CVD)-grown WS2 flake, in order to obtain with high precision a spatially resolved map of the orientation of its main crystallographic axis (armchair orientation). By fitting the experimental PSHG images of sub-micron resolution into a generalized nonlinear model, we are able to determine the armchair orientation for every pixel of the image of the 2D material, with further improved resolution. This pixel-wise mapping of the armchair orientation of 2D WS2 allows us to distinguish between different domains, reveal fine structure, and estimate the crystal orientation variability, which can be used as a unique crystal quality marker over large areas. The necessity and superiority of a polarization-resolved analysis over intensity-only measurements is experimentally demonstrated, while the advantages of PSHG over other techniques are analysed and discussed.