  A type of 3-dimensional optical negative-refractive-index metamaterials
composed of all dielectric nanospheres is proposed and demonstrated
theoretically. The metamaterials are constructeded by pairing together two
kinds of dielectric nanospheres as concentric shells embedded in a host medium.
Mie-based extended effective theory shows that the dielectric core and the
dielectric shell provide the negative permeability and the negative
permittivity, respectively, both due to the strong Mie resonances. Within the
coupled resonant frequency region, the negative index of refraction can be
achieved.
