Boosting second harmonic generation by merging bound states in the continuum

Ultracompact devices for second harmonic generation (SHG) have broad applications in various fields. In this study, we numerically demonstrate efficient SHG from a merged bound state in the continuum (BIC) based on a lithium niobate (LN) freestanding metasurface. Such merged BIC efficiently reduces the radiative loss and strongly enhances the quality factor (Q factor). We achieved 1% conversion efficiency under the ${5}^{\ensuremath{\circ}}$ incident angle with 2 $\mathrm{kW}/{\mathrm{cm}}^{2}$ input intensity, which surpasses the performances of other nanostructures supported by traditional isolated BICs. Our work paves a promising way to enhance the interactions between light and matters at nanoscales, and provide a feasible idea in designing ultracompact SHG light sources.