Negative index metamaterials at optical frequencies

We have also studied the possibility of forming the isotropic 3D negative index material from an assembly of Ag nanoparticles. It appears that various open- and close-packed structures exhibit "soft" electromagnetic response and extraordinary transmission at certain wavelengths that much exceed the period of the structures. It appears that various nanoparticles assemblies are close to having negative index but remain "positive". They are discussed from a perspective to be used as "transparent metals".