Ultra-sensitive refractive index sensor based on quasi-BIC formed in rectangular-split solid-ring metasurface with thin film lithium niobate

In this paper, a novel high-Q metasurface structure, where the rectangular-split solid-ring metasurface is composed of thin film lithium niobate, is proposed for application in refractive index sensor. This metasurface is based on the quasi-bound state in the continuum (quasi-BIC) arising from the symmetry breaking within the plane. The designed metasurface consists of a solid ring and a rectangular block. Simulation results demonstrate that both the designed nanoscale structures and the solid ring support a high-Q quasi-BIC, exerting a strong confinement effect on the near field. By adding the rectangular block, this metasurface compresses the electric field, leading to a more concentrated field distribution and achieving a fourfold enhancement in the squared field amplitude compared to the case without the rectangular block. We demonstrate that the rectangular block not only modifies the magnitude but also alters the direction of the electric field, effectively improving the Q-factor, approaching 105. Furthermore, we perform resonance fitting and analyze the resonance mechanism using the multipole theory. The results reveal that the proposed structure supports a typical magnetic dipole (MD) resonance with Fano features. Lastly, we analyze the sensitivity (S) and figure of merit (FoM) of the structure. The sensitivity reaches 241 nm/RIU, and the FoM reaches 3 × 105, achieving the highest FoM in the near-infrared regime. This work holds potential applications in high-sensitivity sensors and nonlinear frequency conversion technologies.