Efficient photoinduced second-harmonic generation in silicon nitride photonics

Silicon photonics lacks a second-order nonlinear optical (chi((2))) response in general, because the typical constituent materials are centrosymmetric and lack inversion symmetry, which prohibits chi((2)) nonlinear processes such as second-harmonic generation (SHG). Here, we realize high SHG efficiency in silicon photonics by combining a photoinduced effective chi((2)) nonlinearity with resonant enhancement and perfect phase matching. We show a conversion efficiency of (2,500 +/- 100)% W-1 that is two to four orders of magnitude larger than previous field-induced SHG works. In particular, our devices realize milliwatt-level SHG output powers with up to (22 +/- 1)% power conversion efficiency. This demonstration is a breakthrough in realizing efficient chi((2)) processes in silicon photonics, and paves the way for further integration of self-referenced frequency combs and optical frequency references.