Mapping propagation of collective modes in Bi<sub>2</sub>Se<sub>3</sub> and Bi<sub>2</sub>Te<sub>2.2</sub>Se<sub>0.8</sub> topological insulators by near-field terahertz nanoscopy.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 34795216.
- Also identified by DOI 10.1038/s41467-021-26831-6 and PMC identifier 8602307.
- Licence recorded as CC BY.
- The licence permits redistribution, so the abstract is shown in full and the full text is available from the publisher.
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<sub>2</sub>Se<sub>3</sub> and Bi<sub>2</sub>Te<sub>2.2</sub>Se<sub>0.8</sub>, hyperbolic materials with topologically protected surface states. By mapping the near-field scattering signal from a set of thin flakes of Bi<sub>2</sub>Se<sub>3</sub> and Bi<sub>2</sub>Te<sub>2.2</sub>Se<sub>0.8</sub> 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.