Probing Azimuthal Anatomy of Hyperbolic Whispering Gallery Modes in hBN

TL;DR

This paper introduces a novel method using an auxiliary cavity to decouple excitation and detection in s-SNOM, enabling detailed visualization of hyperbolic whispering gallery modes in hBN with high azimuthal momentum.

Contribution

The study presents a new strategy to visualize complex polaritonic modes by decoupling excitation and detection, revealing detailed azimuthal mode structures in hyperbolic cavities.

Findings

Direct observation of subwavelength polaritonic WGMs with high azimuthal momentum

Dynamic tuning of effective refractive index to maintain azimuthal momentum

Numerical simulations confirm experimental mode structures

Abstract

Scattering-type scanning near-field optical microscopy (s-SNOM) is a powerful tool for investigating polaritonic modes. However, an inherent limitation of this technique is that excitation and detection occur at the same location. This constraint makes it challenging to resolve excitations with complex spatial structures, which can exhibit delicate dependence on the in-coupling conditions. Here, we present a strategy to overcome this limitation by introducing an auxiliary cavity, which serves as a stationary near-field excitation source. This configuration allows the s-SNOM tip to act solely as a detector, and decouples excitation from detection. We apply this approach to whispering gallery modes (WGMs) of hyperbolic phonon-polaritons in hexagonal boron nitride resonators. Through spatially resolved near-field maps we directly observe subwavelength polaritonic WGMs with large and discrete azimuthal momentum ($k_\phi / k_0$ up to 15). This allows us to map the frequency and angular behavior of the modes. Notably, we observe dynamic tuning of the effective refractive index by the WGMs to preserve consistent azimuthal momentum \(k_\phi\) under varying excitation conditions. Numerical simulations support the experimental observations and confirm the observation of hyperbolic WGMs. This approach enables direct visualization of previously hidden mode structures in hyperbolic cavities and opens new pathways for momentum-controlled polaritonic device engineering.