Directional selectivity of strain-enhanced second-harmonic generation in 2D ferroelectric SnS

Strain engineering provides a direct way to tune the optoelectronic properties of van der Waals materials. Here, we investigate the nonlinear optical response of two-dimensional ferroelectric SnS crystals under uniaxial tensile strain. When the strain direction is aligned with the crystal axes, strain applied along the polar armchair axis leads to a strong increase in the second-harmonic generation (SHG) intensity. At a strain of 0.5%, the SHG intensity increases by over 200%. Conversely, strain along the non-polar zigzag axis shows almost no effect on the SHG response. This pronounced anisotropy originates from the preferential coupling between mechanical deformation and the intrinsic in-plane ferroelectric polarization of the puckered SnS lattice, which selectively modulates the dominant second-order nonlinear susceptibility tensor components. Our results show the microscopic relation between lattice strain and the nonlinear optical response in group-IV monochalcogenides and they offer a simple, symmetry-guided approach for mechanically tunable nonlinear photonic devices.