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.