Three-dimensional optical metamaterial with a negative refractive index

Metamaterials are artificially engineered structures that can have properties, such as negative refractive index, not attainable with naturally occurring materials. The first demonstrations of negative refractive index metamaterials (NIMs) were at microwave frequencies, but a wide range of applications, for example in imaging and directional light sources, would open up if optical NIMs were available. So far, only thin, effectively two-dimensional optical metamaterials have been demonstrated, limiting device applications. Valentine et al. now report a three-dimensionally optical metamaterial, a multilayered, cascaded 'fishnet' structure, which unambiguously exhibits negative refractive index over a broad spectral range. A prism constructed with this material demonstrates negative refraction of visible light, and is easily probed from free space, paving the way for practical optical device applications. One day perhaps even the 'invisibility cloak' much touted in the media when this paper was published AOP.