Terahertz ultrasensitive biosensor based on wide-area and intense light-matter interaction supported by QBIC

A quasi-bound state in the continuum (QBIC) has unique attraction in optical switch, nonlinearity, communi-cation, and sensing due to its ultrahigh radiation quality (Q) factor. The QBIC observed in metasurfaces also provides a feasible platform to achieve in-plane strong light-matter interaction, as well as to develop ultrasen-sitive biosensor. However, the existing metasurface designs are difficult to realize highly efficient excitation and high-performance sensing of QBIC in terahertz (THz) band. Here, we manipulate the interference coupling be-tween electric quadrupole and magnetic dipole by introducing an asymmetry alpha into the metallic metasurface structure, which excites ultrahigh quality QBIC resonance with Q factor of up to 503. Correspondingly, light field energy constrained by the metasurface and effective sensing area achieved enormous increases of about 400% and 1300%, respectively, which greatly expands the spatial extent and intensity of light-matter interaction. Simulations and experiments show that the proposed QBIC metasurface deliver a high refractive index sensitivity reaching 420 GHz/RIU, where RIU is the refractive index unit, and its direct limit of detection (LoD) for trace homocysteine (Hcy) molecules is 12.5 pmol/mu L. Its performance is about 40-times better than that of the classical Dipole mode. This work provides a new avenue to achieve rapid, precise, and nondestructive sensing of trace molecules, and has potential applications in the fields of biochemical reaction monitoring, photocatalysis and photobiomodulation.