Plasmonics in topological insulators: Spin-charge separation and the influence of the inversion layer

We demonstrate via two examples that topological insulators (TI) offer a new platform for plasmonics. Firstly, we discuss that the collective excitations of a thin slap of a TI possess spin-charge separation. This gives rise to long-lived acoustic plasmons that cannot be scattered by charged impurities. Secondly, we argue that the depletion layer mixes Dirac and Schr\"odinger electrons which can lead to novel features such as high modulation depths and interband plasmons.