Nonlinear Optical Imaging of In-Plane Anisotropy in two-dimensional SnS

Two-dimensional (2D) tin (II) sulfide (SnS) crystals belong to a class of orthorhombic semiconducting materials with remarkable properties, such as in-plane anisotropic optical and electronic response, and multiferroic nature. The 2D-SnS crystals exhibit anisotropic response along the in-plane armchair (AC) and zigzag (ZZ) crystallographic directions, offering an additional degree of freedom in manipulating their behavior. Here we take advantage of the lack of inversion symmetry of the 2D-SnS crystal to perform polarization-resolved SHG (P-SHG) nonlinear imaging of the in-plane anisotropy. The P-SHG data are fitted with a nonlinear model, allowing to calculate the AC/ZZ orientation from every point of the 2D crystal and to map with high-resolution the AC/ZZ direction of several 2D-SnS flakes in the same field of view. It is found that the P-SHG intensity polar patterns are associated with the crystallographic axes of the flakes and with the relative strength of the 2^ndorder nonlinear susceptibility tensor in different directions. Οur method provides quantitative information of the in-plane anisotropy of orthorhombic 2D-crystals, offering great promise for performance characterization during device operation in the emerging optoelectronic applications of such crystals.