Topological insulators (TIs) are layered materials that ideally exhibit an insulating bulk with conducting surfaces. The electrons in these surface states are two-dimensional, linearly dispersing, and exhibit spin-momentum locking. Light can couple to these surface electrons, exciting two-dimensional Dirac plasmons, similar to those excited in graphene. Unlike graphene, however, TI plasmons are expected to be spin-polarized. This causes them to be protected from non-magnetic backscattering, potentially leading to extremely long propagation distances. There are a variety of potential applications for TI plasmons, including THz sensors, optically-driven spintronics, and THz metamaterials. In this talk, I will discuss our recent results measuring the dispersion relationship of coupled Dirac plasmons in topological insulator films. Because the TI films are extremely thin, plasmons excited on the top and bottom surfaces of the film will couple, resulting in Dirac plasmon acoustic and optical modes. By patterning the films into stripes, localized plasmons can be excited. Since we are exciting the optical mode in our films, by changing both the stripe width and film thickness, the coupled plasmon dispersion relationship can be mapped out. Our results show that we are indeed exciting coupled 2D Dirac plasmons and not massive 2D plasmons from either the bulk or from a band-bending two-dimensional electron gas.
Finally, I will discuss recent results on TI films grown with molecular beam epitaxy on high-quality buffer layers. One of the challenges when studying TI films is that the Fermi energy is usually pinned near the bottom of the conduction band, leading to a large density of trivial carriers in the bulk states. These trivial carriers can open up additional scattering pathways for the topological carriers, reducing plasmon lifetimes. We have found that growth of TI films on high-quality buffer layers brings the Fermi energy closer to the Dirac point, reducing the density of trivial carriers. I will close by showing data for Dirac plasmons excited in these high-quality TI layers. Overall, TIs represent an exciting new material class for studies of Dirac plasmon physics as well as plasmonic applications in the THz.