One-dimensional surface phonon polaritons in boron nitride nanotubes.

Surface polaritons, which are electromagnetic waves coupled to material charge oscillations, have enabled applications in concentrating, guiding and harvesting optical energy below the diffraction limit. Surface plasmon polaritons involve oscillations of electrons and are accessible in noble metals at visible and near-infrared wavelengths, whereas surface phonon polaritons (SPhPs) rely on phonon resonances in polar materials, and are active in the mid-infrared. Noble metal surface plasmon polaritons have limited applications in the mid-infrared. SPhPs at flat interfaces normally possess long polariton wavelengths and provide modest field confinement/enhancement. Here we demonstrate propagating SPhPs in a one-dimensional material consisting of a boron nitride nanotube at mid-infrared wavelengths. The observed SPhP exhibits high field confinement and enhancement, and a very high effective index (neff~70). We show that the modal and propagation length characteristics of the SPhPs may be controlled through the nanotube size and the supporting substrates, enabling mid-infrared applications.