Permittivity-asymmetric dielectric metasurface with tunable dual quasi-BICs for biosensing

Quasi-bound states in the continuum (quasi-BICs) in all-dielectric metasurfaces offer a promising approach to achieving high quality ( Q ) factor resonances with enhanced light–matter interactions. In this paper, we propose a permittivity-asymmetric dielectric metasurface comprising elliptical cylinder dimers, enabling the excitation of dual quasi-BICs. The Q -factors of the dual quasi-BICs exhibit inverse quadratic dependence on the asymmetry parameter, reaching up to 12,941 and 29,040, respectively. Through detailed investigations of far-field multipole decomposition and near-field electromagnetic distributions, we demonstrate that the dual quasi-BICs are governed by magnetic quadrupole (MQ) and magnetic dipole (MD), respectively. Furthermore, their resonance positions can be independently tuned via geometric adjustments with minimal linewidth variation. The proposed metasurface shows excellent label-free biosensing capabilities, achieving refractive index sensitivities S of 101 and 534 nm/RIU and figure of merits (FOMs) of 990.2 and 9888.9, respectively. Our work offers strong potential for high-performance multi-band biosensing applications.