Vibrational Coupling to Quasi‐Bound States in the Continuum under Tailored Coupling Conditions

Abstract Photonic resonance modes can be spectrally coupled to the vibrational modes of molecules in the mid‐infrared regime through interactions between localized electric fields and nearby molecules. According to recent studies, radiative loss engineering of coupled systems is a promising approach for tailoring coupling conditions and enhancing the molecular signals. However, this strategy has only been realized using the localized surface plasmon resonances of metal nanostructures, which suffer from increased ohmic loss in the mid‐infrared region and face serious limitations in achieving high quality (Q) factors. In this study, silicon‐based metasurfaces formed on silicon‐on‐insulator wafers are adopted to achieve high Q factors and tune the coupling conditions between the quasi‐bound states in the continuum (qBICs) and molecular vibrations. The coupling between the resonance mode and polymethyl methacrylate molecules is tailored from weak to strong coupling regimes by simply changing the structural asymmetry parameter and utilizing the intrinsically high Q factors of the qBIC modes. In addition, the optimal asymmetry parameter that maximizes the enhanced molecular signal is identified, opening a route toward realizing highly sensitive surface‐enhanced infrared spectroscopy using complementary metal–oxide–semiconductor compatible all‐dielectric materials.