  Layered topological insulators, for example, Bi$_2$Se$_3$ are optically
hyperbolic materials in a range of THz frequencies. Such materials possess
deeply subdiffractional, highly directional collective modes: hyperbolic
phonon-polaritons. In thin crystals the dispersion of such modes is split into
discrete subbands and is strongly influenced by electron surface states. If the
surface states are doped, then hybrid collective modes result from coupling of
the phonon-polaritons with surface plasmons. The strength of the hybridization
can be controlled by an external gate that varies the chemical potential of the
surface states. Momentum-dependence of the plasmon-phonon coupling leads to a
polaritonic analog of the Goos-H\"anchen effect. Directionality of the
polaritonic rays and their tunable Goos-H\"anchen shift are observable via THz
nanoimaging.
