Negative index of refraction in artificial chiral materials

A new class of negative refractive-index metamaterials made of metallic spheres arranged in a three-dimensional lattice of helicoidal symmetry is reported. The studied metamaterial possesses several frequency bands which give rise to negative refraction. The proposed structures constitute a viable solution to realizing optical metamaterials since they can exhibit negative refraction in the frequency region of the surface-plasmon excitations of noble metals.