Wavelength dependence of polarization-resolved second harmonic generation from ferroelectric SnS few layers

Two-dimensional group-IV monochalcogenides such as GeS, GeSe, SnS, and SnSe were theoretically predicted as multiferroic materials with two or more ferroic properties. However, most of their bulk crystals are stacked layer by layer with an antiferroelectric manner, which lose the macroscopic in-plane ferroelectricity. In this work, we studied SnS in which the layers are stacked in a ferroelectric manner both experimentally and theoretically. We utilized polarization-resolved second harmonic generation (SHG) microscopy to investigate numerous flakes of ferroelectric SnS few layers on mica substrates. We found the SHG polar patterns dramatically varied in the range of 800 nm and 1000 nm due to the frequency-dependent SHG susceptibilities. First-principles calculations have been performed to study the frequency-dependent and layer-dependent SHG susceptibilities in the ferroelectric SnS with AA and AC stacking orders. The variation trend of calculated SHG polar patterns as a function of frequency agrees well with that of the experimental results. Since polarization-resolved SHG is a noncontact and nondestructive technique to determine the crystal orientation, understanding of its properties is important, especially for monitoring the transition of different ferroic phases.