High-performance mid-infrared refractive index sensing all-dielectric metasurface with dual resonance excited by bound states in the continuum and toroidal dipole mode

Converting bound states in the continuum (BICs) domain to quasi-BICs regime effectively achieves high Q-factor excitation, enabling refractive index sensing. In this paper, we propose an all-dielectric metasurface consisting of four silicon nanopore cylinders. While maintaining a four-fold symmetry, an asymmetric distribution of nanopores is introduced, creating two resonances in the mid-infrared range. Through near-field analysis and multipole decomposition, it is demonstrated that magnetic dipoles (MD) and toroidal dipoles (TD) dominate the resonance modes, and the sensing performance of the structure is investigated. The quality factor (Q) and sensitivity reach 6688 and 1140nm/RIU, respectively. The sensing characteristics in different refractive index (RI) media are analyzed, and the results show that the refractive index sensing capability of the dielectric metasurface is influenced by its structural parameters and exhibits a significant linear response relationship. Furthermore, by varying the structural parameters of the dielectric metasurface, we also discovered the differences in sensing performance within different wavelength ranges. We study all polarization angle robust quasi-BICs in the mid-infrared spectral region, which can provide new references for polarization -insensitive metasurfaces with dual resonances. Polarization-insensitive technology can avoid the need for compensating the polarization state of the light signal in the sensor and improve the performance and reliability of the sensor.