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

Inversion symmetry eliminates the second-order nonlinear responses in materials commonly used in silicon photonics with electric-dipole approximation. The lack of effective methods to induce the second-order nonlinearity in silicon photonic materials prevents their applications in second-order nonlinear integrated photonics. Here, we experimentally demonstrate a surface second-order nonlinear optics approach for boosting the second harmonic (SH) generation process in a silica microcavity. By leveraging the molecule-induced surface second-order nonlinearity, a record high SH efficiency of about 6.7% W-1 is achieved in a silica microcavity functionalized with a surface asymmetrically-aligned molecular monolayer, which is enhanced of two to four orders of magnitude compared to that before molecule-functionalization. Furthermore, we derive the equations that govern the surface second-order nonlinear process in inversion symmetric microcavities. Our method not only enables high efficiency second-order nonlinear frequency conversions in silica photonics, but also can apply to other inversion symmetric material platforms for integrated photonics.