J. Valentine, S. Zhang, T. Zentgraf, E. Ulin-Avila, D. A. Genov, G. Bartal and X. Zhang. Three-dimensional optical metamaterial with a negative refractive index, Nature 455, 376-379 (2008)
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Metamaterials are artificial materials engineered to have properties that may not be found in nature. The greatest potential of metamaterials is the possibility to create a structure with a negative refractive index, since this property is not found in any non-synthetic material.