Abstract
Silicon photonics lacks a second-order nonlinear optical (χ(2)χ(2)<math><mrow><msup><mi>χ</mi><mrow><mo>(</mo><mn>2</mn><mo>)</mo></mrow></msup></mrow></math>) response in general because the typical constituent materials are centro-symmetric and lack inversion symmetry, which prohibits χ(2)χ(2)<math><mrow><msup><mi>χ</mi><mrow><mo>(</mo><mn>2</mn><mo>)</mo></mrow></msup></mrow></math> nonlinear processes such as second harmonic generation (SHG). Here, we realize record-high SHG efficiency in silicon photonics by combining a photo-induced effective χ(2)χ(2)<math><mrow><msup><mi>χ</mi><mrow><mo>(</mo><mn>2</mn><mo>)</mo></mrow></msup></mrow></math> 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 χ(2)χ(2)<math><mrow><msup><mi>χ</mi><mrow><mo>(</mo><mn>2</mn><mo>)</mo></mrow></msup></mrow></math> processes in silicon photonics, and paves the way for further integration of self-referenced frequency combs and optical frequency references.