Real-space nanoimaging of THz polaritons in the topological insulator Bi2Se3

Plasmon polaritons in topological insulators attract attention from a fundamental perspective and for potential THz photonic applications. Although polaritons have been observed by THz far-field spectroscopy on topological insulator microstructures, real-space imaging of propagating THz polaritons has been elusive so far. Here, we show spectroscopic THz near-field images of thin Bi2Se3 layers (prototypical topological insulators) revealing polaritons with up to 12 times increased momenta as compared to photons of the same energy and decay times of about 0.48 ps, yet short propagation lengths. From the images we determine and analyze the polariton dispersion, showing that the polaritons can be explained by the coupling of THz radiation to various combinations of Dirac and massive carriers at the Bi2Se3 surfaces, massive bulk carriers and optical phonons. Our work provides critical insights into the nature of THz polaritons in topological insulators and establishes instrumentation and methodology for imaging of THz polaritons. The real-space imaging of propagating THz polaritons, coupled light-matter excitations, in topological insulators has been elusive so far. Here, the authors report spectroscopic THz near-field images of the topological insulator Bi2Se3 revealing polaritons formed by coupling of THz radiation to optical phonons and various charge carriers.

Automated summary

This study successfully captures and analyzes the properties and dispersion of plasmon polaritons in thin Bi2Se3 layers using THz near-field imaging technique. The results show that the momentum and decay time of polaritons significantly increase compared to photons, but the propagation length is short. The study further reveals the coupling process between THz radiation and Dirac and Massive carriers on the surface of topological insulators and carriers and photons in the bulk.