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

TL;DR

This study uses near-field THz imaging to visualize and analyze polaritons in Bi2Se3 topological insulators, revealing their dispersion, decay, and coupling mechanisms, advancing understanding of THz polaritons in these materials.

Contribution

It provides the first real-space images of propagating THz polaritons in Bi2Se3, analyzing their properties and coupling mechanisms, and establishes methods for THz polariton imaging.

Findings

Polaritons exhibit up to 12 times increased momentum compared to photons.

Decay time of polaritons is approximately 0.48 ps.

Polaritons are explained by coupling to Dirac, massive carriers, and phonons.

Abstract

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.