Coupled Dirac Plasmons in Topological Insulators

Topological insulators are predicted to house spin-polarized 2D Dirac plasmons. In topological insulator thin films, Dirac plasmons on the top and bottom surfaces are coupled, giving rise to an unusual dispersion relationship. These plasmons are of interest both for fundamental science as well as applications in terahertz sensing and waveguiding. In this article, conclusive evidence is presented for coupled Dirac plasmons excited in the surface states of topological insulator thin film stripe arrays by investigating how the plasmon frequency depends on both film thickness and stripe width. Bi₂Se₃ films of thickness 50–200 nm are patterned into periodic stripes of width 1–4 µm to excite plasmons. The plasmon frequency dependence on both film thickness and stripe width is found to be in good agreement with theoretical models for coupling 2D Dirac plasmons. Alternative explanations such as 2D massive plasmons are shown to be inconsistent with data. Finally, effective mode indexes of up to 211 are reported, demonstrating incredible confinement of light in the terahertz frequency range.