Second-order nonlinear optical processes lie at the heart of many applications in both classical and quantum regimes^1,2,3. Inversion symmetry, however, rules out the second-order nonlinear electric-dipole response^1,4,5 in materials widely adopted in integrated photonics (for example, SiO₂, Si and Si₃N₄). Here, we report nonlinear optics induced by symmetry breaking^6,7,8,9,10 at the surface of an ultrahigh-Q silica microcavity under a sub-milliwatt continuous-wave pump. By dynamically coordinating the double-resonance phase matching, a second harmonic is achieved with an unprecedented conversion efficiency of 0.049% W⁻¹, 14 orders of magnitude higher than that of the non-enhancement case¹¹. In addition, the nonlinear effect from the intrinsic symmetry breaking at the surface^8,12 can be identified unambiguously, with guided control of the pump polarization and the recognition of the second-harmonic mode distribution. This work not only extends the emission frequency range of silica photonic devices, but also lays the groundwork for applications in ultra-sensitive surface analysis.

二阶非线性光学过程是经典和量子体系^1,2,3的许多应用的核心。但是，反演对称性排除了集成光子学中广泛采用的材料（例如SiO ₂，Si和Si ₃ N ₄）中的二阶非线性电偶极子响应^1,4,5。在这里，我们报道了在亚毫瓦连续波泵的作用下，在超高Q石英微腔表面对称性破坏^6,7,8,9,10引起的非线性光学。通过动态协调双谐振相位匹配，以0.049％W ^-1的空前转换效率实现了二次谐波比非增强情况¹¹高14个数量级。另外，在泵浦极化的引导控制和二次谐波模式分布的识别下，可以明确地识别出在表面^8,12处固有的对称性破坏引起的非线性效应。这项工作不仅扩展了二氧化硅光子器件的发射频率范围，而且为超灵敏表面分析的应用奠定了基础。