Mechanisms of Thickness-Dependent Nonlinear Optical Processes and Ultrafast Dynamics in NbOCl₂

NbOCl2 has garnered interest owing to its second-order nonlinear optical properties, but the influence of thickness on its third-harmonic generation (THG) and excited-state dynamics has not been fully investigated. Here, mechanically exfoliated NbOCl2 flakes were found to produce THG and second-harmonic generation (SHG), simultaneously, and to show a nonmonotonic dependence on sample thickness. The increase in thickness leads to a decrease in the bandgap. The transient absorption spectrum of NbOCl2 reveals an enhancement of the long-lived excited-state absorption (ESA) at 2.62 eV and a red-shift of the band-edge absorption (BEA). The ground-state bleaching (GSB) exhibits an accelerated relaxation. Additionally, THG of NbOCl2 arising from three-photon resonant absorption is competing with SHG, while their optimal excitation band varies with thickness. This study improves our understanding of the mechanisms of the thickness of NbOCl2 and the development of ultrafast photonics and all-optical switching.