Ultrasensitive Tunable Terahertz Sensor via Critical Coupling-Enhanced Fano Resonance

High-performance terahertz (THz) sensing holds significant promise for label-free medical diagnostics and environmental monitoring, owing to unique fingerprint spectra of biomolecules. However, THz sensing technology still faces limitations, including low sensitivity, low quality factor (Q-factor), restricted figure of merit (FOM), and nontunability. Here, we propose a cavity-enhanced scheme for an ultrasensitive tunable THz sensor, integrating a graphene-strip metasurface, a microcavity with dielectric analytes, and a gold reflective layer. We reveal a new physical mechanism allowing tunable critical coupling to maximize light-matter interaction, which involves strong coherent Fano effect arising from dipole localized surface plasmon resonance of graphene-strip metasurface, coupled with surface plasmon polaritons excited in gold reflective film, and a highly confined cavity mode with narrow absorption linewidth. Hybrid coupling mechanism leads to highly localized field within the microcavity, significantly enhancing light-matter interaction and facilitating THz ultrasensitive sensing. Results show the critical coupling resonance scheme substantially improves the sensitivity and FOM. The sensor demonstrates a high sensitivity of 2.7 THz per refractive index unit (RIU) with a normalized sensitivity of 1.5 RIU^-1 and a record-high FOM exceeding 2.54 × 10⁶. The sensing performance is tuned by electrically controlling graphene gate voltage. The proposed scheme is promising for on-chip integrated THz biochemical sensors.