Surface and Volume Phonon Polaritons in Boron Nitride Nanotubes

Abstract

Phonon polaritons (PhPs) are quasiparticles created by coupling of photons to polar lattice vibrations. Previously, PhPs have been observed as both surface and volume confined waves. The dispersion of the polariton depends strongly on the nature of the material. Volume polaritons show asymptotic behavior near the longitudinal optical phonon frequency of the material, whereas surface polaritons instead approach the surface phonon frequency. Boron nitride nanotubes (BNNTs) were found to exhibit the dispersion of volume modes below the surface phonon frequency. However, around and above the surface phonon frequency, the behavior becomes that of a surface wave with an amplified near-field response. These findings improve our understanding of photonics within BNNTs and suggest potential applications that take advantage of the high fields and density of states in that spectral region.

Polaritonic Enhancement of Near-field Scattering of Small Molecules Encapsulated in Boron Nitride Nanotubes

“…We treat nanotubes as rolled-up hBN planes, therefore in cylindrical coordinates ǫ r = ǫ ⊥ and ǫ θ = ǫ z = ǫ . 9 We note that Phillips et al 20 treated similar systems as hollow cylinders with air using effective medium theory. In our case, since the diameters of our tubes are much smaller, the effective medium approximation gives only a negligible correction.…”

Near-Field Spectroscopy of Cylindrical Phonon-Polariton Antennas

“…The near-field response of an optical system can be found by investigating it with scattering scanning near field optical microscopy (sSNOM). [49][50][51] Figure 1: (a) SEM image of a portion of the SiC pillar (h ≈ d ≈ 1 µm) array with spacing P =2.5 µm. (b) Experimental FTIR and (c) simulated spectra of the investigated arrays with different P .…”

“…The near-field response of an optical system can be found by investigating it with scattering scanning near-field optical microscopy (sSNOM). − sSNOM has found large application in the field of nanophotonics, being able to directly study polariton propagation in a variety of materials, ranging from graphene plasmons , to anisotropic SPhPs . Near-field patterns in hyperbolic materials have been extensively studied with sSNOM ,− together with their spectral response. , Active and reversible tuning of the operation wavelength of SPhPs with phase change materials was also demonstrated with sSNOM. , While SPhPs and LSPhPs in silicon carbide (SiC) have been imaged in different geometrical configurations in combination with different materials, ,− a detailed study of the near-field response of SiC nanostructures is still lacking.…”

High-Temperature Polaritons in Ceramic Nanotube Antennas

“…This is due to the unbounded nature of a hyperboloid, which is in contrast to isotropic materials that possess bounded isofrequency contours of spherical or ellipsoidal topology. , One can calculate the LDOS in the near-field above a surface of hBN. − Figure b shows the LDOS at several near-field distances above an hBN–vacuum interface. Both Reststrahlen bands show large LDOS in the extreme near-field but decay as we move away from the interface . This implies that for hBN, these are dark modes that are not present in the far-field and would not produce strong thermal radiation.…”

“…Both Reststrahlen bands show large LDOS in the extreme near-field but decay as we move away from the interface. 39 This implies that for hBN, these are dark modes that are not present in the far-field and would not produce strong thermal radiation. However, by creating subwavelength particles (BNNTs) out of phonon polaritonic materials (hBN) we can introduce a mechanism to couple the dark modes to radiation in the far-field.…”