Ultrasensitive trace-analyte detection empowered by a quasi-BIC terahertz metasensor.

Ultrasensitive detection of trace biomarkers is crucial for early disease diagnosis. Terahertz metasensors based on quasi-bound states in the continuum (quasi-BIC) have emerged as a promising platform for label-free biosensing due to their high quality (Q) factors and strong field confinement. Here, we propose and experimentally demonstrate a plasmonic metasensor for trace-level biomarker detection. The sensor consists of an asymmetric circular split-ring resonator (CSR) enclosed by a square ring (SR), fabricated on a cyclo-olefin copolymer substrate. By breaking the structural symmetry via offsetting the split of CSR, a high-Q quasi-BIC resonance is excited, exhibiting a refractive index sensitivity of up to 200 GHz RIU-1. We successfully apply this metasensor for the ultrasensitive detection of L-tyrosine with trace-level concentration, which exhibits a detection limit down to 0.33 μg mL-1. This work presents a viable route for developing high-performance THz biosensors, holding great potential for point-of-care diagnostic applications.