Plasmon-Phonon Interactions in Topological Insulator Microrings

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 (TIs)-quantum systems having an insulating gap in the bulk and intrinsic Dirac metallic states at the surface. Here, the plasmonic response of ring structures patterned in Bi<inf>2</inf>Se<inf>3</inf> TI films is investigated 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. An analytical theory based on the THz conductance of unpatterned films is developed, which accurately describes the strong plasmon-phonon hybridization and Fano interference experimentally observed as the bonding plasmon is swiped across the prominent 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. Topological insulator Bi<inf>2</inf>Se<inf>3</inf> microring arrays are investigated by means of terahertz spectroscopy. Both bonding and antibonding plasmon modes are observed in the spectra, together with a strong plasmon-phonon hybridization around 2 THz. An analytical theory is developed, which accurately describes the observed features. This work opens the road for the investigation and design of topological insulators-based plasmonic devices.