Efficient photo-induced second harmonic generation in silicon nitride photonics

Silicon photonics lacks a second-order nonlinear optical (${\chi }^{\left(2\right)}$) response in general because the typical constituent materials are centro-symmetric and lack inversion symmetry, which prohibits ${\chi }^{\left(2\right)}$ nonlinear processes such as second harmonic generation (SHG). Here, we realize record-high SHG efficiency in silicon photonics by combining a photo-induced effective ${\chi }^{\left(2\right)}$ nonlinearity with resonant enhancement and perfect-phase matching. We show a conversion efficiency of (2,500 ± 100) %/W, which is 2 to 4 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 major breakthrough in realizing efficient ${\chi }^{\left(2\right)}$ processes in silicon photonics, and paves the way for further integration of self-referenced frequency combs and optical frequency references.