In this review paper we discuss various possible physical realizations of artificial media with negative real parts of material parameters (wire media, split-ring resonators, omega and chiral particles, active composites). The physical phenomena behind the effect of negative parameters are explained, and analytical models for the material properties are given. Further, some simple electromagnetic systems which contain slabs of double-negative materials are studied: two-layer waveguides and one-dimensional electromagnetic crystals. Unusual properties of waves in these structures are discussed. The review describes results obtained mainly in the Radio Laboratory of the Helsinki University of Technology.
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