Launching Focused and Spatially Confined Phonon-Polaritons in Hexagonal Boron Nitride

Launching and focusing phonon-polaritons present novel opportunities for low-loss guiding of subdiffractionally confined light at the nanoscale. Despite significant efforts to improve control in polaritonic media, focused and spatially confined phonon-polariton waves have only been achieved in the in-plane anisotropic crystals (such as MoO$_3$) and remain elusive in the in-plane isotropic crystals (such as hexagonal boron nitride). In this study, we present a previously unexplored approach to launching phonon-polaritons by employing hBN subwavelength resonators coupled to gold nanoantennas. The integration of gold nanoantennas enables efficient coupling to the far-field component of mid-infrared light, while the geometry of the resonators defines the wavefront curvature, spatial confinement, and focusing of the launched polaritons. Using standard lithographic protocols, we achieve strong field enhancement and resonant mode localization, generating phonon-polaritons with precise spatial definition. Scattering-type scanning near-field optical microscopy reveals the real-space optical contrast of these modes. This innovative and practical approach introduces a new paradigm for fabricating nanoresonators that actively launch and focus phonon-polaritons, opening avenues for advanced nanophotonic devices.