Mid-infrared surface phonon polaritons in boron-nitride nanotubes

Boron-nitride nanotubes (BNNTs) are structural analogs of carbon nanotubes formed from hexagonal boron-nitride (h-BN) sheets. h-BN exhibits strong phonon resonances which result in a negative permittivity band in the mid-infrared (mid-IR) part of the electromagnetic spectrum. Recently, we have experimentally demonstrated that BNNTs can support one-dimensional surface phonon polaritons (SPhPs) in the mid-IR. In this work, we thoroughly study the modal characteristics of the SPhP modes of BNNTs with respect to their structural attributes, such as diameter, hollowness, and ellipticity, as well as dispersion as a function of frequency. As is discussed in detail throughout the paper, the SPhP modes of BNNTs offer deep-subwavelength field confinement and large field enhancement and provide a viable means for nanophotonics, near-field optics, and thermo-optics at mid-IR wavelengths. (paper)