Emergent optical plasmons at the surface of a doped three-dimensional topological insulator

Abstract

The conduction electrons in the topologically nontrivial surface states of three-dimensional topological insulators have been actively exploited to exhibit emergent exotic properties and functionalities. The existence of a trivial two-dimensional electron gas near the surface has also been widely established, stemming from surface carrier doping of the topological insulators. Here we investigate theoretically the coupling between the electrons in the topological surface states and two-dimensional electron gas, and reveal that the helical nature of the former can be effectively shared with the latter through proximity effect. Next we examine the collective modes of the combined system and predict an optical plasmon mode rooted in the enhanced interband transitions of the nontrivial bands with the same helicity. The emergence of the interband transitions also introduce pronounced variations in the Dirac plasmon mode of the topological surface states, characterized by its gapped feature. These findings may find important applications in plasmonic and spintronic devices.