Milliwatt Second Harmonic Generation in a Photonic Crystal Microcavity via Harmonic Wavelength Resonance.

Second harmonic generation (SHG) in out-of-plane nanophotonic platforms such as photonic crystals and metasurfaces offers compact and efficient routes for free-space nonlinear light sources. However, the output power typically remains at microwatt levels, limited by thermo-optic and Kerr-induced resonance shifts in strongly confined fundamental-frequency (FF) modes. Here, we demonstrate milliwatt-level second harmonic (SH) emission from a photonic crystal microcavity that is resonant at the SH wavelength. By suppressing FF resonance and instead enhancing SH radiation through harmonic-wavelength resonance, the onset of resonance detuning and saturation is substantially delayed, allowing for higher pump powers. The designed cavity, consisting of a lithium niobate layer sandwiched between distributed Bragg reflectors, delivers an over 1 mW SH output and a saturation threshold 6.4 times higher than its FF-resonant counterpart. These results establish SH-wavelength resonance as an effective strategy for scalable, high-power nonlinear light sources in integrated and free-space photonics.