Terahertz label-free biomolecular sensing in quasi-bound states in a continuum metasurface

We present a terahertz metasurface that supports a quasi-bound state in the continuum (quasi-BIC) under symmetry-breaking effect, featuring sharp resonances ideal for biosensing applications. The design consists of an asymmetric bar pair structure supporting an antisymmetric dark mode that remains non-radiative (dark) under perfect mirror symmetry. Introducing in-plane mirror symmetry breaking with respect to the incident polarization transforms the dark mode into a quasi-BIC mode. The metasurface exhibits a sharp resonance on a flat transmission background, ensuring high spectral selectivity and contrast. The metasurface response was validated through simulations, coupled-mode theory, and terahertz time-domain spectroscopy. Biological analytes (human serum albumin, N-acetyl-L-tryptophanamide, and poly-L-lysine) were uniformly coated on the metasurface and experimentally analyzed for variation in the spectral shift. The results indicate a considerable resonance shift consistent with simulations. Unlike bare substrate measurements, which show negligible spectral change, the quasi-BIC resonance enables strong light–matter interaction and detection of minute refractive index variations. These results highlight the potential of quasi-BIC metasurfaces for highly sensitive, label-free biosensing in the terahertz regime.