Wedge Dyakonov Waves and Dyakonov Plasmons in Topological Insulator Bi₂Se₃ Probed by Electron Beams

Bi₂Se₃ has recently attracted a lot of attention because it has been reported to be a platform for the realization of three-dimensional topological insulators. Due to this exotic characteristic, it supports excitations of a two-dimensional electron gas at the surface and, hence, formation of Dirac-plasmons. In addition, at higher energies above its bandgap, Bi₂Se₃ is characterized by a naturally hyperbolic electromagnetic response, with an interesting interplay between type-I and type-II hyperbolic behaviors. However, still not all the optical modes of Bi₂Se₃ have been explored. Here, using mainly electron energy–loss spectroscopy and corresponding theoretical modeling we investigate the full photonic density of states that Bi₂Se₃ sustains, in the energy range of 0.8 eV–5 eV. We show that at energies below 1 eV, this material can also support wedge Dyakonov waves. Furthermore, at higher energies a huge photonic density of states is excited in structures such as waveguides and resonators made of Bi₂Se₃ due to the hyperbolic dispersion