Recrystallization-Driven High-Performance Optical Nonlinearity Recovery of Layered van der Waals NbOCl₂

van der Waals crystal NbOCl₂ has attracted considerable interest owing to its spontaneous parametric downconversion properties, but challenges remain due to its degradation and reduced optical nonlinearity under ambient conditions. Here, we show that high-temperature vacuum annealing can recover and even enhance second harmonic generation (SHG) in degraded NbOCl₂ via recrystallization. During degradation, the second-order nonlinearity decreases, accompanied by changes in the crystal structure of NbOCl₂. However, after annealing, SHG is recovered and even improved as a result of recrystallization of the degraded NbOCl₂ nanosheets. This strategy enables the realization of tunable nonlinear optical responses, including harmonic generation and nonlinear absorption. By combining linear absorption spectroscopy, transient absorption, and transmission electron microscopy, we show that recrystallization occurs under specific annealing temperatures, leading to bandgap modulation and altered electronic relaxation. This study offers a new approach to achieving tunable and reversible optical nonlinearity for nonlinear optical device applications.