Highly anisotropic effective dielectric functions of silver nanoparticle arrays.

Variable-angle and Mueller matrix spectroscopic ellipsometry are used to determine the effective dielectric tensors of random and aligned silver nanoparticles and nanorods thin films. Randomly arranged particles are uniaxially anisotropic while 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. A structural tilt in the particle arrays results in monoclinic dielectric properties.