  Hyperbolic materials exhibit unique properties that enable a variety of
intriguing applications in nanophotonics. The topological insulator Bi2Se3
represents a natural hyperbolic optical medium, both in the THz and visible
range. Here, using cathodoluminescence spectroscopy and electron energy-loss
spectroscopy, we demonstrate that Bi2Se3, in addition to being a hyperbolic
material, supports room-temperature exciton polaritons. Moreover, we explore
the behavior of hyperbolic edge exciton polaritons in Bi2Se3. Edge polaritons
are hybrid modes that result from the coupling of the polaritons bound to the
upper and lower edges of Bi2Se3 nanoplatelets. In particular, we use electron
energy-loss spectroscopy to compare Fabry-P\'erot-like resonances emerging in
edge polariton propagation along pristine and artificially structured edges of
the nanoplatelets. The experimentally observed scattering of edge polaritons by
defect structures was found to be in good agreement with finite-difference
time-domain simulations. Moreover, we experimentally proved coupling of
localized polaritons in identical open and closed circular nanocavities to the
propagating edge polaritons. Our findings are testimony to the extraordinary
capability of the hyperbolic polariton propagation to cope with the presence of
defects. This provides an excellent basis for applications such as nanooptical
circuitry, cloaking at the nanometer scale, as well as nanoscopic quantum
technology on the nanoscale.
