Angular dispersion suppression in deeply subwavelength phonon polariton bound states in the continuum metasurfaces.

Quasi-bound states in the continuum (qBICs) achieved through symmetry breaking in photonic metasurfaces are a powerful approach for engineering resonances with high quality factors and tunability. However, miniaturization of these devices is limited as the in-plane unit-cell size typically scales linearly with the resonant wavelength. By contrast, polariton resonators can be deeply subwavelength, offering a promising solution for achieving compact devices. Here we demonstrate that low-loss mid-infrared surface phonon polaritons enable metasurfaces supporting qBICs with unit-cell volumes up to 10 times smaller than the free-space volume . Using 100-nm-thick free-standing silicon carbide membranes, we achieve highly confined qBIC states with exceptional robustness against incident-angle variations, a feature unique among qBIC systems. This absence of angular dispersion enables mid-infrared vibrational sensing of thin, weakly absorbing molecular layers using a reflective objective, a method that typically degrades resonance quality in standard qBIC metasurfaces. We introduce surface-phonon-polariton-based qBICs as a platform for ultraconfined nanophotonic systems, advancing the miniaturization of mid-infrared sensors and devices for thermal radiation engineering.