Negative-Index Metamaterials: Going Optical
  The race to engineering metamaterials comprising of a negative refractive index in the optical range has been fueled by the realization of negative index materials for GHz frequencies six years ago. Sheer miniaturization of the GHz resonant structures is one approach. Alternative designs make use of localized plasmon resonant metal nanoparticles or nanoholes in metal films. Following this approach, a negative refractive index has been realized in the optical range very recently. We review these recent results and summarize how to unambiguously retrieve the effective refractive index of thin layers from data accessible to measurements. Numerical simulations show that a composite material comprising of silver strips and a gain providing material can have a negative refractive index of -1.3 and 100% transmission, simultaneously. 
Thomas A. Klar, Alexander V. Kildishev, Vladimir P. Drachev, and   Vladimir M. Shalaev (School of Electrical and Computer En...