Photo-induced second-harmonic generation in silicon nitride waveguides

Integrated optical waveguides based on stoichiometric silicon nitride (Si3N4) have demonstrated record low loss and a broad spectral transparency range. The third-order nonlinearity and wide transparency has enabled the demonstration of the widest supercontinuum, of nearly 500 THz, generated
on an optical chip [1]. Using Si3N4 waveguides with a large-area core and a 1064-nm, 6.2-ps pump laser, we demonstrate that the third-order nonlinearity can generate, via the photo-galvanic effect, a long-lived second-order
nonlinearity with a spatial grating suitable for quasi-phase matched second harmonic generation. The second harmonic signal is observed for various waveguide cross-sections, and, for an uninitialized waveguide, the signal grows over time before it saturates. This time-dependent initial growth suggests that
the conversion is based on a photo-induced charge relocation, known as the photo-galvanic effect. After the second-order nonlinearity is established, we observe the expected quadratic dependence on input power and relative spectral
narrowing of the second harmonic signal.