Highly anisotropic effective dielectric functions of silver nanoparticle arrays

Variable-angle and Mueller matrix spectroscopic ellipsometry measurements of highly anisotropic plasmonic materials composed of random and aligned silver nanoparticles and nanorods are presented. We show that the effective dielectric tensors of randomly dispersed particles are uniaxially anisotropic and those of aligned particles are biaxially anisotropic, with the anisotropy predominantly at the plasmonic resonances. The strong resonances in nanorod arrays result in the real part of the effective in-plane permittivities being opposite in sign over a significant range in the visible, suggesting the potential to design materials that display tunable negative-refraction.