Sub-wavelength electromagnetic phenomena in plasmonic and polaritonic nanostructures: from optical magnetism to super-resolution
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Effective medium theory of sub-wavelength metallic, semiconducting and dielectric nanostructures that encompasses their electric, magnetic and magnetoelectric response at optical frequencies is introduced. Theory development is motivated by the recent surge of interest in electromagnetic metamaterials: nanostructured composites with unusual or naturally unavailable electromagnetic properties. Unlike numerous other studies, this work focuses on strongly sub-wavelength (unit cell size a λ/n) structures inasmuch as non-subwavelength composites, in general, cannot be described with effective medium parameters. The theory starts from purely electrostatic description of non-magnetic composites and uses plasmon eigenfunctions as the basis. Magnetism and other retardation phenomena are taken into account as perturbations of electrostatic equations. Theoretic description is validated by experimental data on extraordinary optical transmission through subwavelength hole arrays in crystalline silicon carbide films. It is shown that one of the most amazing applications of optical metamaterials, known as the “superlens”, enables deeply sub-wavelength spatial resolution not limited by Abbe’s resolution of a microscope. Theoretical grounds and designs of proof-of-principle verification experiments for near-field sub-wavelength imaging are presented. Theoretical principles and formulas are applied to the problem of engineering an optical negativeindex metamaterial (NIM) that may be used to improve the near-field superlens. NIM engineering begins with simple two-dimensional examples (cylinder arrays, wire pairs) and advances to more complicated metamaterials (strip-film and strip-wire arrays, tetrahedral clusters). Finally, the concept of liquid negative-index metafluids (NIMF) based on plasmonic nanoclusters is introduced and exemplified using tetrahedral cluster colloids. Clusters of plasmonic nanospheres, known as Artificial Plasmonic Molecules (APM), can be easily fabricated in macroscopic amounts and, depending on their symmetry, may exhibit three-dimensionally isotropic electromagnetic response.