Surface-confined edge phonon polaritons in hexagonal boron nitride thin films and nanoribbons

In this paper, we point out that the phonon polaritons in hexagonal boron nitride may not have to be volume-confined. Surface-confined phonon polaritons can occur at the edges of ultra-thin boron nitride films. Based on numerical calculations, we have investigated the effective indexes of such phonon polariton modes, and found that these modes normally have much lower losses than the volume-confined ones. Surface-confined phonon polaritons in boron nitride nanoribbons are also investigated that can be seen as the coupling of the phonon polariton modes at two edges of the nanoribbon, and the corresponding losses can be further reduced. Then, we give a deep discussion on the volume-confined and surface-confined phonon polaritons. In the end, the effects of defects and substrates on such modes are presented. The proposed low-loss surface-confined phonon polaritons may be useful in phonon polariton based photonic devices.