Anisotropic effective medium approach for metamaterials made of arrays of metallic nanopillars

The optical response of a surface covered with an array of gold nanopillars is studied both experimentally, using spectroscopic ellipsometry, and numerically. When treating the gold pillars as an effective layer, we find that it is possible to extract anisotropic effective optical indices that fit the experimental data. However, a detailed numerical study of the behavior of the metamaterial under illumination shows the presence of localized, specific coupling effects with the underlying layer, suggesting that homogeneization, and effective indices should be considered with caution and have only limited applicability.