Is there an experimental verification of a negative index of refraction yet?

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Abstract
We show that recent claims of experimental verification of a negative index of refraction in metallic metamaterials, “left-handed” microwave frequencies, are questionable. At these frequencies the imaginary part of the dielectric permittivity is significant or even dominates its real part, and hence the effective medium behaves as a metal, i.e., with losses below the cutoff frequency of relevance. Then, the refractive index is complex, and there is not a distinction between right- and left-handed material because the electromagnetic wave is inhomogeneous. Just by using this idea we explain the transmittivities that were recently obtained in experiments by Shelby et al. [Science 292, 77 (2001) and Appl. Phys. Lett. 78, 489 (2001)].