Observation of Dirac plasmons in a topological insulator

TL;DR

This study provides experimental evidence of Dirac plasmons in a topological insulator, demonstrating their tunable properties and confirming theoretical predictions through infrared spectroscopy of engineered micro-ribbon arrays.

Contribution

First observation of Dirac plasmon excitations in a topological insulator using infrared spectroscopy and micro-ribbon arrays, confirming their dispersion relation without fitting parameters.

Findings

Plasmonic excitations are robust across 6-300 K.

Dispersion matches theoretical predictions for Dirac plasmons.

Frequency tunability demonstrated by changing ribbon width.

Abstract

Plasmons are the quantized collective oscillations of electrons in metals and doped semiconductors. The plasmons of ordinary, massive electrons are since a long time basic ingredients of research in plasmonics and in optical metamaterials. Plasmons of massless Dirac electrons were instead recently observed in a purely two-dimensional electron system (2DEG)like graphene, and their properties are promising for new tunable plasmonic metamaterials in the terahertz and the mid-infrared frequency range. Dirac quasi-particles are known to exist also in the two-dimensional electron gas which forms at the surface of topological insulators due to a strong spin-orbit interaction. Therefore,one may look for their collective excitations by using infrared spectroscopy. Here we first report evidence of plasmonic excitations in a topological insulator (Bi2Se3), that was engineered in thin micro-ribbon arrays of different width W and period 2W to select suitable values of the plasmon wavevector k. Their lineshape was found to be extremely robust vs. temperature between 6 and 300 K, as one may expect for the excitations of topological carriers. Moreover, by changing W and measuring in the terahertz range the plasmonic frequency vP vs. k we could show, without using any fitting parameter, that the dispersion curve is in quantitative agreement with that predicted for Dirac plasmons.