  Strongly anisotropic media where the principal components of the dielectric
tensor have opposite signs are called hyperbolic. Such materials length exhibit
unique nanophotonic properties enabled by the highly directional propagation of
slow-light modes localized at deeply sub-diffractional scales. While artificial
hyperbolic metamaterials have been demonstrated, they suffer from high
plasmonic losses and require complex nanofabrication, which in turn induces the
size-dependent limitations on optical confinement. The low-loss, mid-infrared,
natural hyperbolic material, hexagonal boron nitride is an attractive
alternative. We observe four series of multiple (up to seven) 'hyperbolic
polariton' modes in boron nitride nanocones in two spectral bands. The resonant
modes obey the predicted aspect ratio dependence and exhibit record-high
quality factors (Q up to 283) in the strong confinement regime (lambda/86 in
the smallest structures). These observations assert hexagonal boron nitride as
a promising platform for studying novel regimes of light-matter interactions
and nanophotonic device engineering.
