Molecule-induced surface second-order nonlinearity in an inversion-symmetric microcavity

Inversion symmetry eliminates second-order nonlinear responses in materials that are commonly used in silicon photonics. The lack of effective methods to induce second-order nonlinearity in silicon photonic materials (silicon, silica, and silicon nitride) significantly hinders their applications in second-order nonlinear integrated photonics. Herein, we demonstrate a surface second-order nonlinear optics approach for boosting the second-harmonic (SH) generation process in a silica microcavity. We theoretically and experimentally investigated whispering gallery mode microcavity-based molecule-induced second-order nonlinearity. A record-high SH efficiency of approximately 6.7%W −1 was achieved in a silica microcavity with a surface asymmetrically aligned molecular monolayer, with an enhancement factor of approximately two orders of magnitude. Our approach is not limited to the specific optical microcavity materials used in the present work, but is also a highly implementable inversion-symmetric material platform. The demonstrated results would open up a promising avenue for integrated microcavity nonlinear photonics and quantum photonics.