Giant Optical Second Harmonic Generation from SnS with Ferroelectric Stacking

In recent years, substantial attention has been directed towards two-dimensional SnS, a member of the group IV monochalcogenides, owing to its exceptional physical properties. Theoretical predictions have emphasized the potential of SnS monolayer to display significant second harmonic generation (SHG) due to its in-plane ferroelectricity. However, the synthesis of monolayers with considerable lateral dimensions remains challenging due to the formidable inter-layer forces between SnS layers. For a large SnS bulk crystal, the layers are typically stacked with an antiferroelectric order (AB stacked), and the ferroelectric property disappears. In this work, we successfully fabricated a sizable flake (approximately 8x8 µm) consisting of a few layers of SnS with ferroelectric stacking (AC stacking), which enables optical measurement feasible. We found giant SHG from AC-SnS, surpassing that of MoS₂ monolayers by 50 times. The SHG susceptibility was experimentally estimated as 85 pm/V, which is much stronger than the value (~2 pm/V) of a typical nonlinear crystal such as BBO and KTP crystals. We have also investigated the angle-resolved SHG patterns and found there is good agreement between experimental and theoretical data. We further developed angle-resolved SHG microscopy as a tool for identifying single or multiple domains in SnS flakes. Orientations of polarization in different domains have been clearly obtained by this technique. From the results of cross-sectional transmission electron microscopy, we found some region that the stacking order of layers is neither AB nor AC stacking. We have also investigated the effect of stacking disorder to the angle-resolved SHG patterns.