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 × 10 6 . The sensing performance is tuned by electrically controlling graphene gate voltage. The proposed scheme is promising for on‐chip integrated THz biochemical sensors.