Plasmon-phonon interactions in topological insulator rings

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 Bi2Se3 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. Plasmons ‐the collective oscillations of charge carriers in conducting materials‐ hold great potential for combining electronics and photonics at the nanoscale. These excitations can propagate along extended surfaces or they can be localized at so-called hotspots. The engineering of plasmon frequencies and spatial profiles has been mastered in metallic micro- and nanostructures such as nanospheres, nanorods [1], dimers [2, 3], particle arrays [4, 5], and a plethora of more exotic morphologies [6‐9]. Plasmon hybridization [10, 11], Fano resonances [12, 13], and electromagnetically induced transparency [14] are among the feats that have been realized and broadly used to understand and design plasmonic devices. The range of applications of plasmon excitations is vast and includes optical sensing [15‐18], quantum electrodynamics