Giant Phonon‐Enhanced Terahertz Electro‐Optic Response in Ferroelectric van der Waals NbOX ₂ (X = Br, I)

ABSTRACT NbOX 2 van der Waals ferroelectric materials exhibit large second‐order nonlinear responses and strong light‐matter interactions, making them promising candidates for ultrafast optoelectronic and quantum photonic applications. However, their terahertz (THz) electro‐optic (EO) properties remain experimentally unexplored. Here, we report the first experimental study of NbOX 2 thin films subjected to intense THz electric fields, combining both the frequency‐dependent electro‐optic (EO) effect and the THz‐field‐induced modulation of second‐harmonic generation (SHG). We observed a pronounced phonon resonance enhanced EO response with effective EO coefficients reaching 332 pm/V at 3.1 THz in NbOI 2 and 192 pm/V at 3.7 THz in NbOBr 2 . Under a 150 kV/cm THz pump field, we further observe a non‐resonant SHG modulation of 4% and a resonant phonon‐enhanced modulation. These results establish NbOX 2 as a class of high‐performance, phase‐matching‐free THz electro‐optic platform for ultrabroadband time‐domain spectroscopy, high‐speed optical switching, and phonon‐enhanced nonlinear optical response in 2D ferroelectrics.