Topological insulators are tunable waveguides for hyperbolic polaritons

TL;DR

This paper explores how layered topological insulators like Bi$_2$Se$_3$ can serve as tunable waveguides for hyperbolic polaritons, with their properties controllable via surface state doping and external gating, enabling observable effects like the Goos-H"anchen shift.

Contribution

It demonstrates the tunability of hyperbolic phonon-polaritons in topological insulators through surface state doping and external gating, revealing new control mechanisms for polaritonic waveguides.

Findings

Hybrid modes from phonon-polariton and surface plasmon coupling can be tuned by gating.

Directional polaritonic rays exhibit a controllable Goos-H"anchen shift.

THz nanoimaging can observe the tunable directionality and shifts.

Abstract

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.