  We investigated emission and propagation of polaritons in a two dimensional
van der Waals material hexagonal boron nitride (hBN). Our specific emphasis in
this work is on hyperbolic phonon polariton emission that we investigated by
means of scattering-type scanning near-field optical microscopy. Real-space
nano-images detail how the polaritons are launched in several common
arrangements including: light scattering by the edges of the crystal, metallic
nanostructures deposited on the surface of hBN crystals, as well as random
defects and impurities. Notably, the scanned tip of the near-field microscope
is itself an efficient polariton launcher. Our analysis reveals that the
scanning tips are superior to other types of emitters we have investigated.
Furthermore, the study of polariton emission and emission efficiency may
provide insights for development of polaritonic devices and for fundamental
studies of collective modes in other van der Waals materials.
