  Squeezing light into subwavelength dimensions is vital for on-chip
integration of photonic technologies. One approach to overcome the diffraction
limit is coupling light to material excitations, leading to polariton states.
Here, we showcase how low-loss mid-infrared surface phonon polaritons enable
metasurfaces supporting quasi-bound states in the continuum (qBICs) with deeply
subwavelength unit cells. Utilizing 100 nm thick free-standing silicon carbide
membranes, we achieve highly confined qBIC states with a unit cell volume ~
10^4 times smaller than the diffraction limit. This results in remarkable
robustness of the platform against the incident angle that is unique among qBIC
systems. We also demonstrate vibrational strong coupling with a thin layer of
spin-coated molecules, leveraging the small mode volume. This work introduces
phononic qBICs as a novel ultra-confined nanophotonic platform, paving a way
for the miniaturization of mid-infrared devices for molecular sensing and
thermal radiation engineering.
