Phonon polariton confinement in isotopically pure MOVPE-grown BN triangles

Phonon polaritons, quasiparticles formed by the resonant hybridization of light and lattice vibrations, exhibit unique properties like the possibility of hyperbolic dispersions. In this context, exfoliated hexagonal boron nitride (hBN) has emerged as a promising material for phonon polariton-based research. However, to advance toward practical applications, it is essential to demonstrate efficient phonon-polariton propagation and confinement in large-area epitaxial BN. To address this topic, we use metalorganic vapor phase epitaxy (MOVPE)-grown BN and investigate the phonon polariton properties using scattering-type scanning near-field optical microscopy (s-SNOM) and nanoscale Fourier transform infrared spectroscopy (nano-FTIR). We report remarkably long phonon polariton propagation lengths, indicating the high crystalline quality of the BN layer. By using epitaxially grown isotopically pure triangular islands, we further demonstrate efficient phonon-polariton confinement with mode patterns tunable by the incident light wavelength. Our results pave the way for implementing all-epitaxial, microscale, high-quality polariton resonators for nanophotonics and quantum optics.