Observation of Edge-Confined Acoustic Hyperbolic Polaritons in van der Waals Materials.

Hyperbolic polaritons (HPs) supported by van der Waals (vdW) materials enable exceptionally strong light-matter interactions through deep subwavelength confinement. This confinement can be further enhanced when a polaritonic mode couples to its mirror image in a metallic substrate, giving rise to acoustic hyperbolic polaritons (AHPs). While most previous studies have focused on volume-confined AHPs (v-AHPs), edge-confined AHPs (e-AHPs) remain experimentally elusive. Here, we provide the first near-field observation of e-AHPs by launching and imaging them in the prototypical sample of hexagonal boron nitride (hBN) on a gold substrate. Unlike v-AHPs propagating inside hBN, these e-AHPs are guided along the hBN edges and exhibit shorter polariton wavelengths, as revealed by both near-field imaging and extracted dispersion relations. Enhanced vibrational strong coupling in e-AHPs, experimentally verified by monitoring molecular-induced near-field responses, demonstrates their suitability for high-sensitivity molecule detection. These distinctive properties establish e-AHPs as a promising platform for optical sensing and on-chip nanophotonic devices.