Mid-infrared strong coupling between quasibound states in the continuum and epsilon-near-zero modes in a thin polar dielectric film

Epsilon-near-zero (ENZ) materials offer promising candidates for the study of strong light-matter interactions due to their exotic optical properties. Quasibound states in the continuum (QBICs) supported by all-dielectric metasurfaces possess the characteristics of high quality factors with flexible tunability, allowing for significant field enhancement and more degrees of freedom for manipulating light-matter interaction at the nanoscale. Here, we propose a two-dimensional all-dielectric metasurface combined with a thin polar dielectric film, which supports the ENZ mode and the inherent transverse optical (TO) phonon mode. We demonstrate a strong QBIC-ENZ coupling with giant and highly tunable coupling strengths. The proposed metasurface can not only support QBIC modes, but can also excite the ENZ mode in polar dielectric film. With the help of the strong field localization and flexible modulation properties of QBIC, we further realize a selective QBIC-ENZ-TO three-mode strong coupling in the hybrid system, demonstrating a large Rabi splitting over 22.1 meV (178 ${\mathrm{cm}}^{\ensuremath{-}1}$) at room temperature, which is four times higher than that of the QBIC-ENZ two-mode coupling. We also demonstrate that the three-mode coupling strength can be easily tuned by varying the thickness of the ENZ thin film, which provides an effective approach for the manipulation of the strong light-matter interaction. The proposed all-dielectric platform exhibits strong coupling strength and flexible design tunability, which is expected to provide a different route for the design of ultracompact optical devices at mid-infrared frequencies.