Giant Second‐Harmonic Generation Enhancement in Ferroelectric Crystal via Three‐Dimensional Strain Engineering

ABSTRACT

Ferroelectric materials exhibit great potential for nonlinear optical applications due to their robust second‐order nonlinear optical responses. Strain engineering of such materials has demonstrated significant enhancement of second‐harmonic generation (SHG) responses. However, achieving order‐of‐magnitude amplification in bulk crystals and understanding anisotropic strain‐property relationships remain challenging. Here, we report strong SHG enhancement in anisotropically strained ferroelectric NbOCl ₂ crystals via uniaxial stretching and wrinkle‐induced strain engineering. Both polar b‐axis and nonpolar c‐axis strain enhance SHG, while b‐axis strain exhibits higher strain‐normalized efficiency. Wrinkle‐induced strain produces SHG amplification tens of times larger than that of conventional stretching, and the maximum absolute enhancement observed here is a 50‐fold increase for a ∼2% c‐axis wrinkle. This large enhancement is likely driven by 3D strain‐modified intrinsic nonlinear susceptibility and Peierls distortion, and further reinforced by wrinkle‐induced optical and geometric effects. Our work establishes strain engineering as a powerful tool for unlocking giant nonlinear optical responses in van der Waals ferroelectrics, paving the way for dynamically tunable photonic devices.