Dirac plasmons and beyond: the past, present, and future of plasmonics in 3D topological insulators
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Prospective Article
Dirac plasmons and beyond: the past, present, and future of plasmonics in 3D topological insulators T. Ginley, Y. Wang, Z. Wang, and S. Law, Department of Materials Science and Engineering, University of Delaware, Newark DE 19716, USA Address all correspondence to S. Law at [email protected] (Received 30 May 2018; accepted 9 August 2018)
Abstract We review progress studying unique plasmonics in topological insulators (TIs). First, we describe exfoliation and deposition synthesis approaches. TI materials have substantially improved: it is now possible to grow samples with few trivial electrons and controllable doping. We then describe the theory behind the unique behavior of the coupled, 2D Dirac plasmons. While reviewing experimental efforts, we note that Dirac plasmons have been conclusively demonstrated in TIs and they show remarkable properties including long lifetimes, large mode indices, and huge modulation depths. Finally, we describe the opportunities that are present now that high-quality materials can be obtained, including spin and nanoparticle plasmons.
Introduction Topological insulators (TIs) such as Bi2Se3, Bi2Te3, Sb2Te3, and their alloys, have been known as good thermoelectrics for decades. However, it was only within the past 10 years that these materials were discovered to be three-dimensional (3D) TIs.[1,2] The unique behaviors of these materials stem from the inverted band structure at the Γ point.[3] As an example, take a material whose valence band comprises states with p-type symmetry and whose conduction band comprises states with s-type symmetry, as shown schematically in Fig. 1(a). If the material has a substantial spin-orbit coupling, a band inversion can occur at high-symmetry points in the Brillouin zone as shown in Fig. 1(b). This leads to a band structure renormalization, as shown in Fig. 1(c). At the Γ point, the conduction band will now have p-type symmetry while the valence band will have s-type symmetry. At an interface between this inverted material and a topologically trivial material, topological surface
states (TSS) will form in a Dirac cone, shown as dashed lines in Fig. 1(d). These surface states exhibit spin-momentum locking, meaning that the momentum of the electron determines the spin. This leads to topological protection of the electrons occupying the TSS since the electrons cannot backscatter into other TSS without also undergoing a spin flip. The linear dispersion of these TSS results in small electron masses and large Fermi velocities, similar to electrons found in graphene. Due to their unique band structure, TIs are proposed for applications in spintronics and other next-generation electronic devices.[4] In this paper, we will discuss the prospect to use TIs as novel plasmonic materials for optoelectronic applications in the far-infrared and THz. The rest of the paper is organized as follows: we first describe how to synthesize TI thin films, followed by a discussion on the theory behind TI plasmons. We then describe a variety of experimental investi
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