All-dielectric metasurface tunable refractive index sensor based on asymmetric dimers with high-quality factor

In this paper, an all-dielectric metasurface refractive index sensor based on Fano resonance is proposed. The unit structure consists of two silicon ellipsoidal columns with rectangular holes deposited on a silicon dioxide substrate. Based on the theory of bound states in the continuum (BIC), the transition from BIC to quasi-bound states in the continuum (QBIC) can be achieved by modifying the tilting angle of the elliptic column, thereby enabling the excitation of the sharp Fano resonance at 1183.7 nm and 1298.54 nm wavelengths, which in turn yields a maximum quality factor (Q-factor) of 15,575. The near-field distribution indicates that both QBICs are predominantly governed by magnetic dipole (MD) modes. Furthermore, the refractive index sensing performance of the metasurface structure is analyzed by varying the refractive index of the surrounding ambient medium, resulting in a maximum sensitivity (S) of 563 nm/RIU. The sensing characteristics of the structure for seawater temperature and salinity are subsequently investigated, and the maximum sensitivities are determined to be 52 pm/K and 0.265 nm/‰, which demonstrated excellent sensing performance. The proposed metasurface structure has a wide range of potential applications in fields, such as biosensing, optical switching, marine environment monitoring and sensing, and more.