Nanometer-Scale Cavities for Mid-Infrared Radiation via Image Phonon Polariton Resonators.

Surface polaritons play a pivotal role in strong light-matter interactions at the nanoscale due to their ability to confine light to deep-subwavelength dimensions. A promising class of materials exhibiting such a polaritonic response is polar dielectrics, which support surface phonon polaritons (SPhPs). While SPhPs offer significantly lower losses compared to other polaritons, their potential has been underutilized due to their limited ability to reach large confinement factors. Here, we demonstrate a system composed of silver nanocubes deposited on a SiC polar dielectric, which experimentally realizes the antisymmetric-image-phonon-polariton mode, a hybridized SPhP mode that can confine mid-infrared radiation to extremely small mode volumes, almost a billion times smaller than their free-space volume, with quality factors an order-of-magnitude greater than those of surface plasmons or graphene plasmons, surpassing values of 180. Our method is general, scalable, and applicable to any polar dielectric, opening the path for controlling and manipulating strong light-matter interactions at the nanoscale in the long-wavelength range.