Strain-Controlled Polar-Ion Displacement Unlocks Reconfigurable Second-Harmonic Generation in van der Waals CuCrP2S6

Abstract

Two-dimensional van der Waals multiferroics offer a low-dimensional platform for tuning nonlinear optical responses through lattice distortion and polar symmetry. Yet translating polar-ion displacement into strong and tunable macroscopic second-harmonic generation (SHG) remains challenging as limited ionic off-centering and interlayer cancellation suppress the effective nonlinear polarization. Here, we show that local strain enables strongly reconfigurable nonlinear optical responses in CuCrP₂S₆ by amplifying polar Cu⁺ displacement. Thickness-dependent measurements reveal nonmonotonic SHG, indicating incomplete interlayer polarization compensation. Strain-enhanced Cu⁺ displacement strengthens SHG, while Au-induced near fields provide additional amplification and reorient the polarization axis. Temperature-dependent SHG exhibits a nonmonotonic evolution across 140–150 K, consistent with Cu⁺-related structural evolution and low-temperature antipolar compensation. Theoretical calculations reveal that strain induces Cu-site reconstruction and charge redistribution, providing microscopic insight into the structural mechanism underlying strain-modulated SHG response. These results establish strain-controlled ionic displacement as a microscopic route to tunable nonlinear optics in multiferroics.