Curving h-BN thin films can create extra phonon polariton modes.

Hexagonal boron nitride (h-BN) thin films support volume-confined phonon polariton modes within the bulk material as well as surface-confined modes at the edges of thin films. In this Letter, we theoretically investigate the phonon polaritons in curved h-BN thin films. One-dimensional guided phonon polariton modes are found, which are caused by the curved geometry and do not exist in extended flat films. These modes are guided along a specific direction with relatively low propagation losses. So far, one-dimensional guided phonon polariton modes have only been proposed in nanowire and nanoribbon structures. Our study offers another way with the advantage of keeping the h-BN film intact, which can avoid huge scattering losses due to the structural defects. These investigations may offer an easy and robust approach toward phonon-polariton-based nanophotonic circuitry.