  Silicon nitride (SiN) photonics platform has attributes of ultra-low linear
and nonlinear propagation losses and CMOS-compatible fabrication process,
promising large-scale multifunctional photonic circuits. However, the
centrosymmetric nature of SiN inhibits second-order nonlinear optical responses
in its photonics platform, which is desirable for developing efficient
nonlinear active devices. Here, we demonstrate high-efficiency second-order
nonlinear processes in SiN photonics platform by integrating a few-layer GaSe
flake on a SiN microring resonator. With the pump of microwatts continuous-wave
lasers, second-harmonic generation and sum-frequency generation with the
conversion efficiencies of 849%/W and 123%/W, respectively, are achieved, which
benefit from the ultrahigh second-order nonlinear susceptibility of GaSe,
resonance enhanced GaSe-light interaction, and phase-matching condition
satisfied by the mode engineering. Combining with the easy integration, the
GaSe-assisted high-efficiency second-order nonlinear processes offer a new
route to enriching already strong functionality of SiN photonics platform in
nonlinear optics.
