Anisotropy of Second-Harmonic Generation in SnSe Flakes with Ferroelectric Stacking

The second-harmonic generation (SHG) susceptibilities of few-layer SnSe with ferroelectric stacking are investigated using both experimental and theoretical approaches. Theoretical calculations predict a maximum bulk SHG susceptibility of 2444 pm V⁻¹ at 1.2 eV, which is three orders of magnitude larger than that of typical nonlinear crystals. Experimentally, a maximum value of 1424 pm V⁻¹ at 1.19 eV in close agreement with the theoretical prediction is measured. The anisotropic SHG patterns observed experimentally align with theoretical predictions based on the material's point group symmetry. The photon-energy dependence of SHG patterns is also measured within the range of 1.19 to 1.55 eV to explore the relative strengths of various SHG susceptibilities. Notably, the measured ${\chi }_{yyy}^{\left(2\right)}$ is significantly larger than the theoretical value of bulk AC-SnSe, likely due to the strain effects and the mixing of ferroelectric and antiferroelectric stacking configurations in the practical SnSe few-layer samples.