Probing Phonon Polaritons in Boron Nitride Nanotubes with Thermal Emission Spectroscopy

In polaritonic dielectric materials, optically active phonons cause the coupling between electromagnetic waves and the vibrational motions of the polar lattice, leading to phonon polaritons, coupled oscillations of light and matter. Thermally exciting such materials leads to strong fluctuating optical dipole moments and unique thermal emission characteristics in well-defined spectral regions. Hexagonal boron nitride (hBN) is an emerging 2D material that possesses high-frequency and low-loss optical phonons in two spectrally distinct mid-infrared frequency bands. The hyperbolic nature of these frequency bands leads to a large local density of states (LDOS). However, in 2D form, the polaritonic states are dark modes, bound to the material. In this study, by exploiting the cylindrical form of hBN, i.e. boron nitride nanotubes (BNNTs), we create subwavelength particles capable of coupling these dark modes to radiative ones. Through direct measurement of thermal emission of a disordered system of BNNTs, we confirm their radiative polaritonic modes and show that the antenna behavior can be observed even in a disordered system.