Imaging of anomalous internal reflections of hyperbolic phonon-polaritons in hexagonal boron nitride

TL;DR

This study uses near-field microscopy to visualize hyperbolic phonon polaritons in hexagonal boron nitride nanocones, revealing internal reflections, shape-dependent resonances, and potential for field enhancement applications.

Contribution

It provides direct imaging of internal hyperbolic phonon polariton reflections and demonstrates shape-dependent resonances in nanocones, advancing understanding of polariton behavior in hyperbolic materials.

Findings

Observation of hot rings on nanocone sidewalls due to internal polariton reflections

Resonance frequencies depend on nanocone shape and incident laser frequency

Enhanced near-field signals at specific resonances indicating low losses

Abstract

We use scanning near-field optical microscopy to study the response of hexagonal boron nitride nanocones at infrared frequencies, where this material behaves as a hyperbolic medium. The obtained images are dominated by a series of hot rings that occur on the sloped sidewalls of the nanocones. The ring positions depend on the incident laser frequency and the nanocone shape. Both dependences are consistent with directional propagation of hyperbolic phonon polariton rays that are launched at the edges and zigzag through the interior of the nanocones, sustaining multiple internal reflections off the sidewalls. Additionally, we observe a strong overall enhancement of the near-field signal at discrete resonance frequencies. These resonances attest to low dielectric losses that permit coherent standing waves of the sub-diffractional polaritons to form. We comment on potential applications of such shape-dependent resonances and the field concentration at the hot rings.