Refractive index sensing based on bound states in the continuum in all-dielectric metasurface

Bound states in the continuum (BICs) are waves that lie inside the radiation continuum but are completely bound without any leakage of energy. Theoretically, they have infinite lifetimes and radiation quality factors, which have non-radiative characteristics with zero linewidth. By introducing structural perturbation, BICs suffer limited leakage and transform into observable quasi-BICs with a finite quality factor. Metasurfaces based on BICs have become an emerging tool in the field of high-sensitivity sensing because they possess high-quality factors and can enhance the interaction between light and matter. Here, we design an all-dielectric metasurface based on symmetry-protected quasi-BICs, consisting of an asymmetric elliptic pillar. By breaking the C2 symmetry of the structure, the BIC state transforms into quasi-BIC, and a sharp Fano resonance is obtained in the near-infrared band. The quality factor and working frequency can be adjusted by fine-tuning the asymmetric parameters. The eigenmode analysis of the structure is performed. Besides, the refractive index sensing performance of the proposed metasurface is investigated. With the increase in the environmental refractive index, the resonant wavelengths of the transmission spectra have a redshift, which indicates that the designed metasurface is sensitive to the changes in the environmental refractive index. The sensing sensitivity is calculated as 277 nm/RIU, and the FOM (figure of merit) is 396. This design provides a new way for the design of high-sensitivity and microchannel integrated sensors, which has potential applications in the fields of biological and chemical sensing.