Multimode-coupled all-dielectric metasurface based on bound states in the continuum for refractive index sensing

Achieving high sensitivity and a high figure of merit (FOM) in refractive index sensors requires two key factors. The first is a high quality (Q) factor, and the second is significant local field enhancement. The bound states in the continuum (BIC) offer a straightforward approach to designing metasurfaces with high Q factor. However, traditional design methodologies require high-precision machining to achieve high Q factor, which complicates manufacturing and limits practical applications. Moreover, conventional metasurfaces exhibit a limited local field enhancement area, hindering the attainment of large local field enhancement. Therefore, these reasons limit the ability of refractive index sensors to achieve both high sensitivity and high FOM. Here, we introduce an all-dielectric metasurface structure composed of asymmetric Si blocks. Initially, Friedrich-Wintgen BIC (FW-BIC) is realized by adjusting the gap between the two Si blocks. Subsequently, a symmetry-protected BIC (SP-BIC) is introduced by breaking the symmetry of one of the Si blocks, which significantly enhances the Q factor and robustness. Finally, the Si cap is added on top of the microstructure to form an F-P cavity, which can improve the local field enhancement area to achieve both high sensitivity and FOM. Our approach presents a promising pathway for the design of high Q nanostructures, with significant potential applications in sensing technology.