Dual-Polarization Quasi-BIC Refractive Index Sensing via Dielectric Symmetry Breaking in TiO$_2$-BeS Metasurfaces

Abstract

A dual-polarization dielectric metasurface sensor based on TiO$_2$ nanobar pairs with a 20\,nm BeS gap insert is numerically investigated in the near-infrared. The BeS layer introduces dielectric symmetry breaking without requiring geometric asymmetry, enabling polarization-selective excitation of two distinct resonances. Under TE illumination, the structure supports a quasi-BIC resonance at 879.2\,nm with $Q=128$, whereas TM excitation produces a broader magnetic dipole resonance at 910.8\,nm with $Q=36$. The spectral separation between the two modes enables simultaneous tracking of both polarization channels within a single measurement window. For background refractive indices from 1.00 to 1.05, the TE and TM resonances exhibit sensitivities of 243.1 and 178.8\,nm/RIU, respectively. The corresponding figures of merit reach 35 and 7\,RIU$^{-1}$, with detection limits on the order of $10^{-5}$\,RIU. Field distributions show strong confinement of the TE mode inside the gap region, leading to enhanced overlap with the surrounding analyte. Because the two resonances respond differently to refractive-index variations, the metasurface produces a polarization-dependent spectral fingerprint that may provide additional selectivity beyond conventional single-channel dielectric sensors. The proposed platform further shows good tolerance against dimensional variation and consistent resonance behavior across independent FDTD solvers.