Dual-band, double-negative, polarization-independent metamaterial for   the visible spectrum
  We present the first dual-band negative index metamaterial that operates in the visible spectrum. The optimized four-functional-layer metamaterial structure exhibits the first double-negative (i.e., simultaneously negative permittivity and permeability) band in the red region of the visible spectrum with a figure of merit of 1.7 and the second double-negative band in the green region of the visible spectrum with a figure of merit of 3.2. The optical behavior of the proposed structure is independent of the polarization of the incident field. This low-loss metamaterial structure can be treated as a modified version of a fishnet metamaterial structure with an additional metal layer of different thickness in a single functional layer. The additional metal layer extends the diluted plasma frequency deep into the visible spectrum above the second order magnetic resonance of the structure, hence provides a dual band operation with simultaneously negative effective permittivity and permeability. Broadband metamaterials with multiple negative index bands may be possible with the same technique by employing higher order magnetic resonances. The structure can be fabricated with standard microfabrication techniques that have been used to fabricate fishnet metamaterial structures. 
Muhammad I. Aslam and Durdu \"O. G\"uney
physics.optics