Anisotropic quasi-bound states in the continuum in an α-MoO3 metasurface for high-Q mid-IR sensing.

Bound states in the continuum (BICs) have attracted extensive attention in nanophotonics and sensing because they support exceptional optical confinement and ultrahigh quality factors. Dielectric BIC platforms based on high-refractive-index materials are particularly appealing because of their low radiative loss, strong field localization, and rich multipolar responses. Here, we propose a symmetry-protected quasi-BIC metasurface based on the anisotropic dielectric material α-MoO3 for mid-infrared (mid-IR) sensing. By introducing an in-plane symmetry-breaking perturbation through the displacement of the central cylinder within the unit cell, an anisotropic quasi-BIC resonance is excited in the mid-IR regime. Near-field distributions and multipole decomposition reveal that the resonance originates from the hybrid contribution of the electric quadrupole (EQ) and magnetic dipole (MD) modes. The optimized device exhibits a high sensitivity of up to 6000 nm/RIU and a Q factor of 553. These results deepen our understanding of anisotropic BICs and provide theoretical guidance for the design of compact photonic devices with enhanced sensing functionality.