Implementing nonlinear optical components in nanoscale photonic devices is challenged by phase-matching conditions requiring thicknesses in the order of hundreds of wavelengths, and is disadvantaged by the short optical interaction depth of nanometre-scale materials and weak photon–photon interactions. Here we report that ferroelectric NbOI₂ nanosheets exhibit giant second-harmonic generation with conversion efficiencies that are orders of magnitude higher than commonly reported nonlinear crystals. The nonlinear response scales with layer thickness and is strain- and electrical-tunable; a record >0.2% absolute SHG conversion efficiency and an effective nonlinear susceptibility \(\chi _{\mathrm{eff}}^{(2)}\) in the order of 10⁻⁹ m V⁻¹ are demonstrated at an average pump intensity of 8 kW cm^–2. Due to the interplay between anisotropic polarization and excitonic resonance in NbOI₂, the spatial profile of the polarized SHG response can be tuned by the excitation wavelength. Our results represent a new paradigm for ultrathin, efficient nonlinear optical components.

在纳米级光子器件中实现非线性光学元件受到相位匹配条件的挑战，需要数百个波长的厚度，并且由于纳米级材料的短光学相互作用深度和弱光子-光子相互作用而处于不利地位。在这里，我们报告铁电 NbOI ₂纳米片表现出巨大的二次谐波产生，其转换效率比通常报道的非线性晶体高几个数量级。非线性响应随层厚成比例，并且是应变和电可调的；记录 >0.2% 的绝对 SHG 转换效率和有效非线性磁化率\(\chi _{\mathrm{eff}}^{(2)}\)，数量级为 10 ^-9  m V ^{-1在 8 kW cm - 2}的平均泵浦强度下得到证明。由于 NbOI ₂中各向异性极化和激子共振之间的相互作用，极化 SHG 响应的空间分布可以通过激发波长进行调整。我们的结果代表了超薄、高效非线性光学元件的新范例。