  Plasmon polaritons in topological insulators attract attention from a
fundamental perspective and for potential THz photonic applications. Although
polaritons have been observed by THz far-field spectroscopy on topological
insulator microstructures, real-space imaging of propagating THz polaritons has
been elusive so far. Here, we show spectroscopic THz near-field images of thin
Bi2Se3 layers (prototypical topological insulators) revealing polaritons with
up to 12 times increased momenta as compared to photons of the same energy and
decay times of about 0.48 ps, yet short propagation lengths. From the images we
determine and analyze the polariton dispersion, showing that the polaritons can
be explained by the coupling of THz radiation to various combinations of Dirac
and massive carriers at the Bi2Se3 surfaces, massive bulk carriers and optical
phonons. Our work provides critical insights into the nature of THz polaritons
in topological insulators and establishes instrumentation and methodology for
imaging of THz polaritons.
