Ultra-high quality-factor enhancement via merged bound states in the continuum for high-performance optical sensing

Symmetry-protected bound states in the continuum (SP-BICs) offer a promising approach for high-quality (Q) metasurface design due to their non-radiative properties. However, conventional isolated SP-BICs typically offer limited enhancement to Q and require extremely small symmetry-breaking perturbations to become excitable, presenting challenges in fabrication and practical deployment. This work proposes a novel approach to enhance the Q-factor of resonances by merging BICs, enabling the structure to sustain ultra-high Q-factor even under relatively large symmetry-breaking conditions. Specifically, a double-layer metasurface composed of two silicon nanopore arrays is designed. By tuning the interlayer distance, SP-BICs are effectively merged with Fabry––Pérot-type BICs in parameter space, modifying the radiation behavior of the original SP-BICs. Results show that the merged BIC structure enhances the Q-factor by three orders of magnitude compared to isolated BICs. In sensing performance, the proposed structure achieves a refractive index sensitivity of 126 nm/RIU and an outstanding figure of merit surpassing 1.4 × 105 RIU−1, which substantially outperforms isolated BICs. Moreover, diverse high-Q modal features are observed in the wavelength-offset parameter space, offering new opportunities for multi-channel sensing and narrowband filtering applications.