Giant Second-Order Nonlinearity and Anisotropy of Large-Sized Few-Layer SnS with Ferroelectric Stacking

The giant second-order nonlinearity of SnS with ferroelectric stacking is reported. Based on theoretical calculations, the susceptibility of second harmonic generation (SHG) from SnS with ferroelectric stacking is up to 1354 pm V⁻¹, which is three orders of magnitude higher than the values of traditional nonlinear crystals such as BBO and KTP. The SHG from ferroelectric SnS few layers is experimentally measured and its intensity is found to be 131 times larger than that of a MoS₂ monolayer under the same experimental conditions, with a photon energy of 1.55 eV. The SHG susceptibility is determined to be on the order of 100 pm V⁻¹. Numerous SnS flakes are systematically investigated using polarization-resolved SHG. Micrometer-sized flakes with a single domain are found, and their SHG anisotropic patterns fit well with the theoretical calculations derived from first-principles methods. The variation in SHG anisotropic patterns, attributed to SHG interference from multiple domains, is investigated both theoretically and experimentally. Additionally, the impact of stacking disorder on the SHG anisotropic pattern is explored. It is demonstrated that polarization-resolved SHG microscopy is a valuable tool for identifying domains in SnS flakes and examining stacking disorder.