350 nm Broadband Tunable Second‐Harmonic Generation in Photonic Crystal Vertical Cavity

Broadband wavelength‐tunable second‐harmonic generation (SHG) is highly desirable for multifunctional photonic integration and nonlinear light sources. However, achieving efficient SHG with wide‐range tunability in a compact device remains challenging. Here, we experimentally demonstrate a lithium‐niobate‐embedded photonic crystal (PhC) vertical cavity that concentrates optical defect‐state resonances within the photonic bandgap and places them within the second‐harmonic wavelength range. By exploiting multiple defect‐mode resonances, which shift spectrally under oblique incidence and overlap with other resonances at normal incidence, the SHG enhancement peaks connect seamlessly across modes, enabling continuous resonance‐assisted tuning over a pump wavelength span of about 350 nm. Our results offer a scalable approach toward broadband nonlinear wavelength conversion and integrated tunable light sources.