Near-field study of THz collective excitations in topological insulators

Collective excitations at terahertz (THz) frequencies of thin films of Bi2Se3 and Bi2Te2.2Se0.8 topological insulators are investigated by a combination of hyperspectral nano-imaging and detectorless scattering-near-field optical microscopy (s-SNOM). By mapping the THz near-field response, we provide first experimental evidence for the activation of bulk plasmons polaritons and hyperbolic phonon-polaritons coupled with the Dirac-plasmons associated with the topological surface-states. The unveiled low-loss, highly tunable and strongly confined terahertz electromagnetic modes prospect promising application in quantum nano-photonics.