A Broadband Negative Index Metamaterial at Optical Frequencies

A broadband metamaterial presenting negative indices across hundreds of nanometers in the visible and near-infrared spectral regimes is demonstrated theoretically, using transformation optics to design the metamaterial constituents. The approach begins with an infinite plasmonic waveguide that supports a broadband but dark (i.e, not easily optically accessed) negative index mode. Conformal mapping of this waveguide to a finite split-ring-resonator-type structure transforms this mode into a bright (i.e, efficiently excited) resonance composed of degenerate electric and magnetic dipoles. A periodic array of such resonators exhibits negative refractive indices at optical frequencies in multiple regions exceeding 200 nm in bandwidth. The metamaterial response is confirmed through simulations of plane-wave refraction through a metamaterial prism. These results illustrate the power of transformation optics for new metamaterial designs and provide a foundation for future broadband metamaterial devices.