Ultrafast reflectance modulation governed by crystallographic domains in multilayer SnS

Layered SnS features diverse stacking polymorphs and pronounced in-plane anisotropy, yet the ultrafast optical response of multi-stacking, multi-domain films remains largely unexplored. In this study, high-quality SnS micro-crystals grown by physical vapor deposition on MgO are probed with polarization-resolved second harmonic generation (SHG) and degenerate pump-probe reflectometry. SHG polarimetry reveals stacking-dependent in-plane symmetry and χ⁽²⁾ variation; a characteristic hump in the polar plots is captured by a two-tensor model, indicating that stacking faults generate distinct effective χ⁽²⁾ tensors. Domain-resolved SHG mapping uncovers the coexistence of centrosymmetric (anti-ferroelectric) and non-centrosymmetric (ferroelectric) domains within single islands. Polarization-resolved transient reflectance uncovers strongly anisotropic relaxation and a reproducible sign reversal of △R/R along the orthogonal crystallographic axes, attributable to photostriction-driven refractive-index modulation. These findings establish a direct link between the crystallographic domain structure and ultrafast reflectance modulation, suggesting a route to engineer thicker 2D ferroelectrics with controllable nonlinear and ultrafast functionalities for integrated photonics.