Terahertz QBIC Ultrasensitive Biosensor

The quasi-bound state in the continuum (QBIC) observable in the far-field of metasurfaces provides a feasible platform to develop metasurface sensors with ultrahigh quality (Q) factor, as well as to achieve controllable amplification of the in-plane strong light-matter interaction. But so far, most of the QBICs excited in terahertz (THz) band are limited to theoretical research, and its biosensing applications are rarely reported. Here, we manipulate the interference coupling between electric quadrupole (ED) and magnetic dipole (MD) by introducing an asymmetry a 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 as well as effective sensing area achieved enormous increases of about 400% and 3300%, 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, and its direct limit of detection (LoD) for trace homocysteine molecules is 12.5 pmol/mu L. Its performance is about 40 times higher than the classical Dipole resonance. This work provides a new avenue to achieve rapid, precise, and nondestructive sensing of trace molecules.