Babinet complementary metasurfaces for terahertz quasi-bound states in the continuum and sensing

Quasi-bound states in the continuum (quasi-BICs) supported by metasurfaces play a pivotal role in high-performance optical sensing, owing to their ultrahigh quality factors that significantly boost light–matter interactions with target analytes. Distinct from conventional symmetry-breaking approaches for BIC to quasi-BIC transitions relying on single structures, we investigate dimer metasurfaces based on Babinet complementary metasurfaces (BCMs). Via multipole decomposition and field distribution analysis, we reveal that the multipole decomposition components of the quasi-BICs resonant modes in BCMs are essentially identical, with magnetic dipoles dominating the resonant response, which provides critical theoretical guidance for quasi-BICs design. Furthermore, we demonstrate the sensing performance of the developed metasurface by investigating transmittance spectra under varying arginine concentrations. Notably, the transmittance at the quasi-BICs resonance exhibits a significant variation, whereas the corresponding frequency shift is negligible. This work lays a theoretical basis for quasi-BICs evolution in BCMs and promotes high-performance metasurfaces for sensing.