Boosting vibrational strong coupling via quasibound states in the continuum in hybrid metal-dielectric metasurfaces

Bound states in the continuum (BIC) in all-dielectric metasurfaces have recently been extensively applied to enhance vibrational strong coupling in the midinfrared band due to strong field confinement and low optical loss. However, the fields of such qBIC modes are primarily confined within the structural units, hindering the effective access of molecules to electromagnetic hotspots, which significantly reduces coupling efficiency. Here, we achieved collective vibrational strong coupling between plasmonic accidental qBIC modes and molecular vibrations by embedding polymethyl methacrylate molecular layers within a metal-insulator-metal nanodisk array on a dielectric spacer. We proved the strong dependence of coupling strength on the number of molecules participating in coupling and achieved a record-breaking Rabi splitting up to 18 meV $(141\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1})$, which is three times higher than that of reported hybrid systems with qBIC-based modes. The dramatic enhancement of Rabi splitting stems from the up to 60 times field enhancement of the plasmonic qBIC mode, as well as the perfect spatial overlap between the field hotspot and the molecular vibrations. We have also constructed a full-quantum model to quantitatively describe the coherent and incoherent coupling dynamics of the hybrid system, revealing the crucial role of the Rabi phase in the coupling spectral response and the effect of incoherent coupling strength on the vibrational polariton dynamics. The proposed hybrid system with ultrafast energy transfer and long coherence lifetime provides a platform for developing ultrafast and high-compact polariton devices in the midinfrared frequency.