Topology, spin, and light

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Science  26 Jun 2015:
Vol. 348, Issue 6242, pp. 1432-1433
DOI: 10.1126/science.aac4368

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Ocean waves form between air and water, and both winds and currents decay exponentially with distance from the water surface. Similar evanescent surface waves may occur whenever two substances with differing physical properties meet—but some are special cases because they must exist for topological reasons. Such mandated surface modes occur in the low-energy quasi-particle spectrum of p-wave superconductors and superfluids and constitute the defining feature of the electronic properties of topological insulators. On page 1448 of this issue, Bliokh et al. (1) suggest that we add evanescent solutions of Maxwell's equations to this list of special cases, and in particular, the coupled oscillations of the electromagnetic field and the near-surface electrons in a metal that are known as surface plasmon-polaritons.