Impact of phonon nonlocality on nanogap and nanolayer polar resonators

Polar dielectric nanoresonators can support hybrid photon-phonon modes termed surface phonon polaritons with length scales below the diffraction limit. In the deep subwavelength regime the optical response of these systems was shown to diverge from that predicted through a standard dielectric description. Recently, we developed an analytical, dielectric approach and applied it to spheres and planar heterostructures, reproducing anomalous features observed in experiment and microscopic calculations. In this Rapid Communication we develop tools to describe the nonlocal response of polar nanoresonators of arbitrary symmetry, and use them to investigate systems with nanogaps and nanolayers of practical technological relevance. We demonstrate that the available field enhancement is strongly reduced, as the electromagnetic energy leaks away from the hot spots, while phononic resonances are shifted by resonator effects.