Abstract
Metals, semiconductors, metamaterials, and various two-dimensional materials with plasmonic dispersion exhibit numerous exotic physical effects in the presence of an external bias, for example an external static magnetic field or electric current. These physical phenomena range from Faraday rotation of light propagating in the bulk to strong confinement and directionality of guided modes on the surface and are a consequence of the breaking of Lorentz reciprocity in these systems. The recent introduction of relevant concepts of topological physics, translated from condensed-matter systems to photonics, has not only given a new perspective on some of these topics by relating certain bulk properties of plasmonic media to the surface phenomena, but has also led to the discovery of new regimes of truly unidirectional, backscattering-immune, surface-wave propagation. In this article, we briefly review the concepts of nonreciprocity and topology and describe their manifestation in plasmonic materials. Furthermore, we use these concepts to classify and discuss the different classes of guided surface modes existing on the interfaces of various plasmonic systems.
Highlights
Various metals, gas plasmas, highly doped semiconductors, and metamaterials that can be modelled as weakly interacting gases of charged particles do not support light propagation within their bulk at frequencies lower than their plasma frequency, a direct consequence of their negative permittivity in this regime
We review the rich physics of unidirectional surface modes on complex plasmonic
We review the rich physics of unidirectional surface modes on complex plasmonic materials, relating them to the properties of the bulk modes, and distinmaterials, to the properties of thewaves bulk modes, and distinguishing between guishing relating betweenthem various classes of surface based on the concepts of reciprocity various classes of surface waves based on the concepts of reciprocity and topology
Summary
Gas plasmas, highly doped semiconductors, and metamaterials that can be modelled as weakly interacting gases of charged particles do not support light propagation within their bulk at frequencies lower than their plasma frequency, a direct consequence of their negative permittivity in this regime. As discussed in the following, the interface between a magnetized plasma and a trivial opaque material is an example of such a topological system, with a difference in gap Chern number equal to unity [18,19,20], which results in a single unidirectional surface mode spanning the bandgap (Figure 1d), with no backward-propagating mode in this frequency interval (these properties make this system the analogue of “quantum Hall insulators” in condensed-matter physics). This results in a surface mode that is backscattering-immune, for a limited range of propagation angles, in the presence of defects with certain symmetries We denote this surface mode as “topologically protected” it must be noted that this is a relatively weak form of protection, especially compared with the case of strictly unidirectional surface modes in nonreciprocal topological platforms.
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