Reflecting upon the losses in plasmonics and metamaterials
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Introduction Recent years have seen significant progress in the two interrelated fields of plasmonics (or metal optics)1–6 and optical metamaterials (MMs)7–9 artificially engineered materials with specially designed nanostructured building blocks (meta“atoms”) that yield material properties not found in nature. Both of these research directions rely upon the most remarkable feature of subwavelength metallic objects—the high degree of concentration of electromagnetic fields achievable in the vicinity of metal surfaces. This degree of concentration, which is well beyond that allowed by the diffraction limit, arises from coupling the energy and momentum of a photon to a free-electron gas. The subwavelength coupled oscillations, known as surface plasmons (SPs), both enable efficient light manipulation at the nanoscale in plasmonic structures and the resonant properties of MMs. The ability of plasmonic nanostructures to concentrate the electromagnetic energy near subwavelength unit cells or “artificial atoms” allows their arrangement in a regular manner, thus engendering new MMs with optical properties unattainable in natural materials.9 Fascinating MM designs and demonstrations as well as ideas originating from the related field of transformation optics (TO)10,11 range from a negative index of refraction, focusing and imaging with subwavelength resolution, invisibility cloaks, and optical black holes to nanoscale optics, data processing, and quantum information applications.12,13 While significant steps in developing functional nanoplasmonic and MM devices have been made, the ultimate goal of
their widespread practical implementation has been impeded by many factors, the largest of which remains the inherent loss associated with absorption in the metal. The loss limits the propagation distance of plasmons. Even metals with the highest dc conductivity such as silver and gold, which have long been used as plasmonic elements and MM unit cells, exhibit excessive losses at optical frequencies.14 It is well known that the rate of energy loss in a metal is on the order of 1014 s–1 for noble metals, and it becomes even larger at shorter wavelengths. Moreover, additional losses arise in metals when they are patterned at the nanoscale, since nanopatterning often results in smaller grains, rough surfaces, and semi-continuous films. As the importance of loss has become more evident to the plasmonics community, a multi-pronged effort to mitigate loss has been developed. One can loosely classify the variety of approaches to deal with the losses into three broad categories. • Engineering the shape and size of plasmonic structures with the goal of reducing the fraction of energy confined inside the metal and thus reducing the loss. • Introducing optical gain into the plasmonic/MMs structure with the goal of compensating the loss. • Considering materials other than noble metals such as highly doped semiconductors, intermetallics, and graphene. We note that new intermediate carrier density materials used in plasmonics and MMs offer additional prop
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