Hyperbolic and dirac plasmons in topological Insulators (Conference Presentation)

Plasmon polaritons have revolutionized our world of nanophotonics. They have created a platform for enhanced light-matter interactions, propagation of light beyond the diffraction limit, and nanofocusing of electromagnetic energy. However, for applications in data processing and telecommunication, dissipation of optical energy in metallic waveguides is much beyond what can be tolerated for nanocircuitry. Recently other groups of polaritonic waves, namely photon polaritons, exciton polaritons, and Dirac plasmons have been demonstrated as possible candidates for nanophotonics1. Interestingly, a topological insulator like Bi2Se32, as well as heterostructures like graphene/hBN 3, can support coexisting polaritonic waves of all the kinds stated above. At THz frequencies and near to the Fermi energy level, those materials support both hyperbolic phonon polaritons and Dirac plasmons, whereas at infrared and visible ranges4,5 exists another channel for exciton-polariton mode. Here, we mainly discuss the dispersion of the surface polaritons and their spatiotemporal behaviors, at all the energy ranges stated above. We however mainly focus on an aspect of topological insulators which is less discussed beforehand, i.e. topological magnetoelectric effect6. We study the criteria for existence of propagating optical modes which are transversely bound at the interface of two materials. In particular, quite general cases are considered, where the materials involved are assumed to be anisotropic, but also demonstrating magneto-electric effects7. We also discuss the situations where the coexistence of Dirac and hyperbolic polaritons result in level repulsion. We further study the effect of topological magnetoelectric effect on the appearance of hybrid optical modes with various polarization states. In addition to surface polaritons, existence of wedges support another channel for long range propagation of hyperbolic polaritons, due to the coupling of two edge polaritons. We study here the behavior of hyperbolic wedge polaritons at visible and ultraviolet energy ranges. We discuss the radiation damping and long range propagation of hyperbolic wedge and surface polaritons, both theoretically and experimentally using electron energy-loss spectroscopy and finite-difference time-domain method. References 1 Basov, D. N., Fogler, M. M. & de Abajo, F. J. G. Polaritons in van der Waals materials. Science 354, aag1992 (2016). 2 Wu, J.-S., Basov, D. N. & Fogler, M. M. Topological insulators are tunable waveguides for hyperbolic polaritons. Phys. Rev. B 92, 205430 (2015). 3 Woessner, A. et al. Highly confined low-loss plasmons in graphene-boron nitride heterostructures. Nat. Mater. 14, 421-425 (2015). 4 Esslinger, M. et al. Tetradymites as Natural Hyperbolic Materials for the Near-Infrared to Visible. Acs Photon. 1, 1285-1289 (2014). 5 Talebi, N. et al. Wedge Dyakonov Waves and Dyakonov Plasmons in Topological Insulator Bi2Se3 Probed by Electron Beams. Acs Nano 10, 6988-6994 (2016). 6 Dziom, V. et al. Observation of the universal magnetoelectric effect in a 3D topological insulator. Nat. Commun. 8 15197 (2017). 7 Talebi, N. Optical modes in slab waveguides with magnetoelectric effect. J. Opt.-Uk 18, 055607 (2016)