Plasmon-phonon interactions in topological insulator rings

TL;DR

This paper studies the plasmonic behavior of topological insulator rings, revealing tunable plasmon modes and strong plasmon-phonon hybridization, advancing the understanding and potential applications of TI-based THz devices.

Contribution

It provides the first detailed experimental and theoretical analysis of plasmon-phonon interactions in topological insulator ring structures.

Findings

Observation of bonding and antibonding plasmon modes

Tunable plasmon frequencies by varying ring diameter

Strong plasmon-phonon hybridization and Fano interference

Abstract

The great potential of Dirac electrons for plasmonics and photonics has been readily recognized after their discovery in graphene, followed by applications to smart optical devices. Dirac carriers are also found in topological insulators (TI) --quantum systems having an insulating gap in the bulk and intrinsic Dirac metallic states at the surface--. Here, we investigate the plasmonic response of ring structures patterned in Bi$_2$Se$_3$ TI films, which we investigate through terahertz (THz) spectroscopy. The rings are observed to exhibit a bonding and an antibonding plasmon modes, which we tune in frequency by varying their diameter. We develop an analytical theory based on the THz conductivity of unpatterned films, which accurately describes the strong plasmon-phonon hybridization and Fano interference experimentally observed as the bonding plasmon is swiped across the promineng 2\,THz phonon exhibited by this material. This work opens the road for the investigation of plasmons in topological insulators and for their application in tunable THz devices.