Quasi‐Bound States in the Continuum Calibrated Broadband Metasurface Enhanced Mid‐Infrared Absorption Spectroscopy

ABSTRACT Infrared metasurfaces featuring artificially designed structures provide a versatile platform for tailoring sensor properties, holding great promise for next‐generation broadband surface‐enhanced mid‐infrared absorption spectroscopy. In particular, the over‐coupled metasurfaces provide broader sensing bandwidths with a simpler fabrication compared to under‐coupled pixelated metasurfaces. However, over‐coupled metasurfaces has encountered several technical bottlenecks, particularly in the numerical simulation of electromagnetically induced absorption mechanisms and in the extraction of broadband signal. Herein, we propose a metasurface design based on extinction property analysis that modularly controls quasi‐bound states in the continuum and dual over‐coupled resonances to enable trace detection and spectral fingerprinting identification, respectively. The quasi‐bound states in the continuum with surface sensitivity of 0.79 nm/nm serves as an intrinsic calibration reference, delivering a sharp spectral marker for high‐fidelity signal retrieval. The calibrated framework allows accurate retrieval of broadband vibrational signatures, while the over‐coupled resonances collectively amplify molecular absorption from 1800 to 1000 cm −1 . Our results demonstrate that extinction‐based analysis offers superior frequency resolution for visualizing the coupling between resonators and molecules. It underscores the potential of dual over‐coupled metasurfaces for identifying complex analytes such as microplastics and biomarkers, paving the way for advanced mid‐infrared sensing platforms.