Observation of propagating Dirac plasmons in topological insulators

Topological insulators (TI) have been widely studied due to their unique band structure. The Dirac cone band dispersion as well as spin-momentum locking at the surface enables the existence of Dirac plasmons with resonance frequencies in the THz range [1]. People have observed localized Dirac plasmons by etching TIs into stripe arrays [2]. There is limited experimental evidence for propagating surface plasmon polaritons in TIs. Due to the nature of TI surface states, the propagating surface plasmon will be both charge density wave and spin density wave. Here we present our work on exciting propagating Dirac plasmons in TIs by lifting off gold gratings on top of the TI surface. We demonstrate the tunability of the plasmon resonance frequency by varying the grating coupler period. Future work may also include other degrees of tunablilty, and the simulation of plasmon dispersion relation. The research on propagating Dirac plasmons give rise to new dimensions of plasmonic and spintronic applications with TI.
[1] P. Di Pietro, et al., Nat. Nanotechnol., 8, 556-60, 2013
[2] T. P. Ginley and S. Law, Adv. Opt. Mater., 6, 180113, 2018