Nonlinear optical imaging of in-plane anisotropy in two-dimensional SnS

TL;DR

This paper demonstrates a nonlinear optical imaging technique using polarization-resolved second harmonic generation to map in-plane anisotropy and crystallographic orientation in 2D SnS crystals with high resolution.

Contribution

It introduces a method to quantitatively determine the in-plane crystallographic directions of 2D SnS using P-SHG imaging and nonlinear optics modeling.

Findings

P-SHG intensity patterns correlate with crystallographic axes.

The method provides high-resolution, quantitative in-plane anisotropy maps.

It enables characterization of 2D SnS for optoelectronic applications.

Abstract

Two-dimensional (2D) tin(II) sulfide (SnS) crystals belong to a class of orthorhombic semiconducting materials that are lately attracting significant interest, given their remarkable properties, such as in-plane anisotropic optical and electronic response, multiferroic nature and lack of inversion symmetry. 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. Therefore, calculating the AC/ZZ directions is important in characterizing the 2D SnS crystals. In this work, we take advantage of the lack of inversion symmetry of the 2D SnS crystal, that produces second harmonic generation (SHG), to perform polarization-resolved SHG (P-SHG) nonlinear imaging of the in-plane anisotropy. We fit the P-SHG experimental data with a nonlinear optics model, that allows us 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 belonging 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 second order nonlinear susceptibility tensor in different directions. Therefore, our method provides quantitative information of the optical in-plane anisotropy of orthorhombic 2D crystals, offering great promise for performance characterization during device operation in the emerging optoelectronic applications of such crystals.