Room‐Temperature Linear Electro‐Optic Response in van der Waals Ferroelectric NbOI 2

ABSTRACT Two‐dimensional (2D) van der Waals (vdW) ferroelectrics offer a versatile platform for integrated electro‐optics due to their switchable polar order and strong in‐plane optical anisotropy. Despite theoretical predictions of large electro‐optic (EO) activity in Niobium oxide diiodide (NbOI 2 ), experimental evidence has remained elusive. Here, we report the experimental observation of the linear electro‐optic (LEO) effect in vdW ferroelectric NbOI 2 at the telecom wavelength of 1550 nm. Variable‐temperature Raman and second‐harmonic generation (SHG) measurements confirm that the non‐centrosymmetric polar phase remains stable up to ∼175°C. We extract an effective EO coefficient, r eff = 8.5 pm/V, which remains stable up to 10 kHz. First‐principles calculations reveal that the LEO response is intrinsically driven by the field‐induced perturbation of electron localization within Nb‐Nb dimers under a Peierls‐type distortion. These results validate NbOI 2 as a promising candidate for compact vdW optical modulators.