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

ABSTRACT 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.