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

## TL;DR

Scientists observed a new type of light wave in hexagonal boron nitride on gold, which could lead to highly sensitive molecular detection and compact nanophotonic devices.

## Contribution

First experimental observation of edge-confined acoustic hyperbolic polaritons in van der Waals materials.

## Key findings

- Edge-confined AHPs are guided along hBN edges with shorter polariton wavelengths.
- Enhanced vibrational strong coupling in e-AHPs enables high-sensitivity molecule detection.
- Near-field imaging and dispersion relations confirm the unique properties of e-AHPs.

## 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.

This work demonstrates a new type of light wave—edge‐confined acoustic hyperbolic polaritons—in hexagonal boron nitride on gold. These waves travel along crystal edges with nanoscale confinement and strongly interact with molecular vibrations, enabling highly sensitive detection of tiny molecular layers. The findings open pathways toward compact mid‐infrared sensors and integrated nanophotonic devices.