Metamaterials are materials with artificial electromagnetic properties defined by their sub-wavelength structure rather than their chemical composition. Negative-index materials (NIMs) are a special class of metamaterials characterized by an effective negative index that gives rise to such unusual wave behaviour as backwards phase propagation and negative refraction. These extraordinary properties lead to many interesting functions such as sub-diffraction imaging(1,2) and invisibility cloaking(3-6). So far, NIMs have been realized through layering of resonant structures, such as split-ring resonators, and have been demonstrated at microwave(7,8) to infrared(9-13) frequencies over a narrow range of angles-of-incidence and polarization. However, resonant-element NIM designs suffer from the limitations of not being scalable to operate at visible frequencies because of intrinsic fabrication limitations(14), require multiple functional layers to achieve strong scattering(13,14) and have refractive indices that are highly dependent on angle of incidence and polarization. Here we report a metamaterial composed of a single layer of coupled plasmonic coaxial waveguides that exhibits an effective refractive index of -2 in the blue spectral region with a figure-of-merit larger than 8. The resulting NIM refractive index is insensitive to both polarization and angle-of-incidence over a +/- 50 degrees angular range, yielding a wide-angle NIM at visible frequencies.
Nature Mater.
Photonic Materials

Burgos, S. P., de Waele, R., Polman, A., & Atwater, H. (2010). A single-layer wide-angle negative-index metamaterial at visible frequencies. Nature Mater., 9(5), 407–412. doi:10.1038/nmat2747