Launching focused phonon-polaritons via hexagonal boron nitride subwavelength resonators

The generation and control of phonon-polaritons present new possibilities for guiding sub-diffraction-limited light with minimal losses at the nanoscale. Despite extensive efforts to enhance control in polaritonic systems, focused phonon-polariton waves have only been demonstrated in anisotropic crystals, like MoO₃, and remain challenging to achieve in isotropic crystals, such as hexagonal boron nitride (hBN). In this work, we introduce hBN sub-wavelength resonators that couple to far-field mid-infrared light and generate focused (λ/55) phonon-polaritons, fabricated using standard lithographic techniques. These resonators allow for the localization and significant field enhancement of a resonant mode within the cavity, enabling the launch of phonon-polaritons with spatial definitions determined by the cavity geometry. Using scattering-type scanning near-field optical microscopy, we directly observe the real-space optical contrast of these phonon-polariton modes. Our findings offer a new, practical method for fabricating nano-resonators capable of launching and focusing spatially defined phonon-polaritons, opening up possibilities for phonon-polariton-based nanophotonic devices and potentially advancing phonon-polaritonic holography.