Enhancing second harmonic generation by merging symmetry-protected BIC and Fabry-Perot BIC

Second harmonic generation (SHG) at nanoscales has significant applications in integrated optical devices, including nanolasers, quantum light sources, and nanosensors. In this work, we numerically demonstrate boosted SHG conversion efficiency by merging symmetry-protected bound state in the continuum (BIC) and Fabry-Perot BIC. We proposed a lithium niobate-based silicon grating metasurface capable of supporting both symmetry-protected BIC and Fabry-Perot BIC and found that these two BICs can be merged through carefully adjusting the height of the silicon grating. The quality factor (Q factor) of the merged BIC maintains a value exceeding 10⁷ over a wide range of asymmetry factor (|α|<±0.2) even when fabrication imperfections are introduced, which is two orders of magnitude higher than that of the isolated BIC, resulting in significantly enhanced SHG. We achieved a 0.11% conversion efficiency of SHG under 1kW/cm² input intensity when α=0.2 based on the merged BICs, which is about four orders of magnitude higher than that of the isolated BIC. This work provides a viable method for boosting SHG conversion efficiency at nanoscales and paves the way for high-efficiency ultra-compact light sources.