Mapping propagation of collective modes in Bi2Se3 and Bi2Te2.2Se0.8 topological insulators by near-field terahertz nanoscopy
Eva A. A. Pogna, Leonardo Viti, Antonio Politano, Massimo Brambilla, Gaetano Scamarcio, Miriam S. Vitiello
Abstract
Abstract Near-field microscopy discloses a peculiar potential to explore novel quantum state of matter at the nanoscale, providing an intriguing playground to investigate, locally, carrier dynamics or propagation of photoexcited modes as plasmons, phonons, plasmon-polaritons or phonon-polaritons. Here, we exploit a combination of hyperspectral time domain spectroscopy nano-imaging and detectorless scattering near-field optical microscopy, at multiple terahertz frequencies, to explore the rich physics of layered topological insulators as Bi 2 Se 3 and Bi 2 Te 2.2 Se 0.8 , hyperbolic materials with topologically protected surface states. By mapping the near-field scattering signal from a set of thin flakes of Bi 2 Se 3 and Bi 2 Te 2.2 Se 0.8 of various thicknesses, we shed light on the nature of the collective modes dominating their optical response in the 2-3 THz range. We capture snapshots of the activation of transverse and longitudinal optical phonons and reveal the propagation of sub-diffractional hyperbolic phonon-polariton modes influenced by the Dirac plasmons arising from the topological surface states and of bulk plasmons, prospecting new research directions in plasmonics, tailored nanophotonics, spintronics and quantum technologies.