Remarkable Phonon‐Assisted SHG Splitting in Ferroelectric NbOI 2 Associated With Anisotropic Electron‐Phonon Coupling

ABSTRACT Layered NbOI 2 has attracted significant interest due to its strong nonlinear optical response and notable electron‐phonon coupling. Here, we observe an unexpected splitting of second‐harmonic generation (SHG) peaks in ferroelectric NbOI 2 flakes. We propose that the additional SHG peak arises from a phonon‐assisted electronic transition, differing from the conventional dipole‐driven SHG mechanism. This interpretation is supported by the pronounced anisotropy of the split peak intensity, which is consistent with direction‐dependent electron‐phonon coupling modulated by anisotropic polar optical phonons in the low‐symmetry lattice. Temperature‐dependent SHG and Raman measurements further support the close correlation between the SHG splitting and a specific Raman‐active phonon mode. These interpretations are further supported by theoretical analysis. Polarization‐resolved transient reflectivity spectroscopy reveals anisotropic carrier relaxation dynamics, which are correlated with the polarization dependence of the phonon‐assisted SHG peak. Overall, our findings reveal a fundamental link between phonon symmetry, anisotropic electron‐phonon interactions, and nonlinear optical behavior in NbOI 2 , offering new strategies for tuning nonlinear optical properties in low‐dimensional materials through phonon engineering.