Sliding ferroelectricity and tunable linear and nonlinear optical properties in bilayer SnP₂Se₆

Layer stacking of van der Waals bilayers provides an effective means of engineering their physical properties. Here, based on first-principles calculations, we systematically investigate the stacking configurations, electronic structure, and linear and nonlinear optical properties of bilayer SnP₂Se₆. Our results show that the AB and AC stackings are energetically degenerate polar states connected by interlayer sliding, with a low switching barrier and opposite out-of-plane polarizations. Interlayer sliding markedly modifies the electronic structure and optical response, and reverses the orbital character of the lowest conduction-band states, thereby enabling electric-field control of the spatial distribution of photogenerated electrons and the second-harmonic generation (SHG) response. These findings reveal an intrinsic coupling among stacking patterns, ferroelectric polarization, and nonlinear optical activity, and identify bilayer SnP₂Se₆ as a promising platform for reconfigurable ferroelectric and optoelectronic devices.