Hyperbolic whispering-gallery phonon polaritons in boron nitride nanotubes

Light confinement in nanostructures produces an enhanced light–matter interaction that enables a vast range of applications including single-photon sources, nanolasers and nanosensors. In particular, nanocavity-confined polaritons display a strongly enhanced light–matter interaction in the infrared regime. This interaction could be further boosted if polaritonic modes were moulded to form whispering-gallery modes; but scattering losses within nanocavities have so far prevented their observation. Here, we show that hexagonal BN nanotubes act as an atomically smooth nanocavity that can sustain phonon-polariton whispering-gallery modes, owing to their intrinsic hyperbolic dispersion and low scattering losses. Hyperbolic whispering-gallery phonon polaritons on BN nanotubes of ～4 nm radius (sidewall of six atomic layers) are characterized by an ultrasmall nanocavity mode volume (Vm ≈ 10–10λ03 at an optical wavelength λ0 ≈ 6.4 μm) and a Purcell factor (Q/Vm) as high as 1012. We posit that BN nanotubes could become an important material platform for the realization of one-dimensional, ultrastrong light–matter interactions, with exciting implications for compact photonic devices.