  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.
