Symmetry-breaking-induced nonlinear optics at a microcavity surface

Second-order nonlinear optical processes lie at the heart of many applications in both classical and quantum regimes^1–3. 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^6–10 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.