  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.
