Terahertz Plasmon Polaritons in Large Area Bi2Se3 Topological Insulators

Assessing the nature of topological quantum materials, and in particular probing the existence of topological surface states, is a very challenging task. Terahertz (THz) frequency scattering near-field optical microscopy has emerged as an effective technique to investigate the presence of massless surface carriers by locally probing collective surface excitations, i.e., plasmon polaritons, whose dispersion critically depends on the density and nature of surface carriers. Here, thin (14-19 nm) films of Bi2Se3 are experimentally investigated through a combination of x-ray diffraction, Hall-bar magneto-transport, and near-field detectorless optical holography at THz frequencies, from 2 to 4.3 THz. The dispersion of surface plasmon polaritons are determined for different Bi2Se3 film thicknesses, proving the presence of massless surface carriers. The results open intriguing opportunities in THz nano-plasmonics and topological nano-photonics including the development of superlenses and metasurfaces, making use of plasmon polaritons.