Realization of highly tunable second-order nonlinear optical responses, e.g., second-harmonic generation and bulk photovoltaic effect, is critical for developing modern optical and optoelectronic devices. Recently, the van der Waals niobium oxide dihalides are discovered to exhibit unusually large second-harmonic generation. However, the physical origin and possible tunability of nonlinear optical responses in these materials remain to be unclear. In this article, we reveal that the large second-harmonic generation in NbOX₂ (X = Cl, Br, and I) may be partially contributed by the large band nesting effect in different Brillouin zone. Interestingly, the NbOCl₂ can exhibit dramatically different strain-dependent bulk photovoltaic effect under different polarized light, originating from the light-polarization-dependent orbital transitions. Importantly, we achieve a reversible ferroelectric-to-antiferroelectric phase transition in NbOCl₂ and a reversible ferroelectric-to-paraelectric phase transition in NbOI₂ under a certain region of external pressure, accompanied by the greatly tunable nonlinear optical responses but with different microscopic mechanisms. Our study establishes the interesting external-field tunability of NbOX₂ for nonlinear optical device applications.

实现高度可调的二阶非线性光学响应，例如二次谐波产生和体光伏效应，对于开发现代光学和光电器件至关重要。最近，发现范德华氧化铌二卤化物表现出异常大的二次谐波产生。然而，这些材料中非线性光学响应的​​物理起源和可能的可调性仍不清楚。在本文中，我们揭示了 NbO X ₂ ( X = Cl、Br 和 I)中大量二次谐波的产生 可能部分是由不同布里渊区的大能带嵌套效应造成的。有趣的是，NbOCl ₂在不同偏振光下可以表现出截然不同的应变依赖性体光伏效应，这源于光偏振依赖性轨道跃迁。_{重要的是，我们在一定范围的外部压力下实现了NbOCl 2}中的可逆铁电到反铁电相变和NbOI ₂中的可逆铁电到顺电相变，并伴随着可大幅可调的非线性光学响应，但具有不同的微观机制。我们的研究确立了 NbO X ₂在非线性光学器件应用中有趣的外场可调性。