Negative-index metamaterials: Going optical

The race toward engineering metamaterials comprising of negative refractive indexes in the optical range started with the realization of negative-index materials for gigahertz frequencies six years ago. Sheer miniaturization of the gigabertz 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.