Transient absorption spectroscopy of NbOI₂

Recently, NbOI₂ has emerged as a promising layered niobium oxide dihalide that combines semiconducting behavior with in-plane ferroelectricity. Yet, their photocarrier dynamics remain largely unexplored. Here, we report a transient absorption spectroscopy study of NbOI₂. Using femtosecond pump-probe reflectance on hBN-encapsulated NbOI₂ flakes, we observe and time-resolve transient absorption near the 2.34 eV excitonic resonance. The signal originates from pump-induced excitonic resonance shifts and saturation, and its decay reveals a long exciton lifetime of several tens of picoseconds. The dynamics exhibit a density dependence consistent with exciton–exciton annihilation, from which we extract an annihilation coefficient, which is comparable to that in two-dimensional transition metal dichalcogenide monolayers. Angular-dependent measurements with orthogonally polarized pump and probe beams further reveal a pronounced anisotropy following the absorption anisotropy. These results establish key parameters for understanding the optical properties and photocarrier dynamics of NbOI₂ as an anisotropic ferroelectric semiconductor with strong many-body exciton interactions, opening routes toward ultrafast ferroelectric optoelectronics.