Stephanie Law1
University of Delaware1
Topological insulators (TIs) are materials that have an inverted band structure. At the interface between a TI and a topologically-trivial material, linearly-dispersing surface states form. Electrons occupying these surface states are massless, two-dimensional, and spin-polarized. When light shines on a TI, Dirac plasmon polaritons (DPPs) can be excited from the surface state electrons. These DPPs are predicted to exist in the terahertz frequency range, show unusual dispersion relationships, and should be spin-polarized. In addition, by exciting DPPs, we can probe the properties of the TI film.<br/>In this talk, I will review our work on exciting DPPs in TI thin films and multilayers. We first explored DPPs in single TI layers as a function of film thickness and stripe width. By mapping the dispersion relationship, we were able to show conclusively that we were exciting DPPs and that these excitations had record large mode indices and long lifetimes. We attribute the long lifetime to the reduction in scattering pathways due to the spin-momentum locking in the material. Next, we explored in-plane coupling between the TI stripes. In this experiment, we kept the stripe width and film thickness constant and changed the gap size between the stripes. The DPP frequency decreased as the gap size decreased, consistent with dipole-dipole coupling between the stripes. Finally, we explored coupling within TI heterostructures. In this case, we used structures comprising two TI layers separated by a trivially-insulating spacer layer. We show clear evidence for coupling across the spacer layer and explore how the coupling changes as a function of structure parameters. We now have an understanding of the TI plasmonic “bandstructure” and can start using these materials for terahertz applications.