Probing Azimuthal Anatomy of Hyperbolic Whispering Gallery Modes in hBN

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 introduces a tip-independent and spatially separated excitation source, allowing the s-SNOM tip to primarily probe the resulting modal field distribution with minimal perturbation of the resonator. 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 ($k_{\varphi }/k_0$ up to 15). Direct access to the modal field distributions enables identification of the azimuthal mode numbers and their frequency-dependent excitation spectra. We further show that, over finite spectral intervals, the modes continuously adjust their effective refractive index while preserving a fixed azimuthal mode number before transitioning to the next resonant state. 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 opportunities for studying and engineering confined polaritonic states.