Transient Absorption Spectroscopy of NbOI₂.

Abstract

NbOI₂ has recently emerged as a new van der Waals material combining semiconducting behavior with intrinsic in-plane ferroelectricity and pronounced transport and optical anisotropy. However, its photocarrier dynamics remain largely unexplored. Here, we report transient absorption spectroscopy of NbOI₂ using femtosecond pump-probe measurements. A pronounced transient absorption feature is observed near the 2.34 eV excitonic resonance, arising from photocarrier-induced excitonic energy shifts and saturation. The decay dynamics reveal an exciton lifetime of several tens of picoseconds. The observed density-dependent behavior is consistent with exciton-exciton annihilation and defect-assisted Auger recombination, yielding a rate of 0.4 cm² s^-1. Polarization-resolved measurements further reveal a pronounced in-plane anisotropy in the transient response that follows the linear absorption anisotropy. These findings provide fundamental insight into photocarrier dynamics in NbOI₂ and establish key parameters for understanding and exploiting its optoelectronic behavior.