Ultrasensitive Terahertz Metasurface Biosensor Based on Quasi‐Bound States in the Continuum

ABSTRACT The terahertz (THz) spectral regime offers unique opportunities for next‐generation biochemical sensing due to its non‐destructive, label‐free probing capability and strong sensitivity to molecular vibrations. However, conventional THz biosensors remain hampered by intrinsically low‐quality factors and limited sensitivity, severely restricting their utility for trace‐level biochemical and chemical detection. Here, we report an ultrasensitive THz metasurface biosensor that harnesses quasi‐bound states in the continuum (QBICs) with sharp resonances and enhanced light–matter interactions to overcome these limitations. As a proof of concept, the device achieves label‐free detection of a sulfur‐containing amino acid cysteine, with an ultrahigh sensitivity of 492 GHz RIU −1 and an ultralow detection limit down to 0.00025 mg mL −1 . The synergy between QBIC‐induced field confinement and meticulous structural optimization of the metasurface underpins this performance, marking a significant advance over conventional THz metasurface biosensing schemes. These results establish QBIC‐based metasurfaces as a promising platform for ultrasensitive and high‐precision biochemical and chemical sensing, with broad implications for medical diagnostics, food safety, and environmental monitoring.