Bulk and surface sensing with quasi-bound states in the continuum and radiative modes in terahertz metasurfaces

We propose a metasurface-based THz molecular sensor comprising asymmetric metallic rods for simultaneous refractive index and thickness sensing. The asymmetric metasurface supports two distinct modes: a high-quality factor (high-Q) quasi-bound state in the continuum (quasi-BIC) mode and a radiative mode, enabling dual-channel sensing for enhanced detection capability. As a proof-of-concept, we employed terahertz time-domain spectroscopy (THz-TDS) to experimentally characterize the resonances’ shifts of the metasurface integrated into a liquid cell for detecting water concentration in water-ethanol mixtures. Both modes exhibit redshifts with increasing water content. Besides, the experimental characterization of the metasurface, simulations were performed to study the changes in resonant frequencies of the two modes for surface sensing, by adding a thin layer onto the metasurface with variable thickness. Both experimental and simulation results show a higher sensitivity for the quasi-BIC mode ( S = −0.15 THz/RIU for bulk refractive index sensing) compared to the radiation mode ( S = −0.09 THz/RIU). The simulated field distributions reveal that the quasi-BIC’s strongly localized field enhances the sensitivity. These findings demonstrate that simple metasurfaces can serve as effective THz sensors for absorptive analytes, offering multiple resonance markers for improved accuracy.