  We identify a route towards achieving a negative index of refraction at
optical frequencies based on coupling between plasmonic waveguides that support
backwards waves. We show how modal symmetry can be exploited in
metal-dielectric waveguide pairs to achieve negative refraction of both phase
and energy. By properly controlling coupling between adjacent waveguides, a
metamaterial consisting of a one-dimensional multilayer stack exhibiting an
isotropic index of -1 can be achieved at a free-space wavelength of 400 nm. The
general concepts developed here may inspire new low-loss metamaterial designs
operating close to the metal plasma frequency.
