Supporting Material Optical properties of complex plasmonic materials studied with extended effective medium theories combined with RCWA

1. We first prepare a polymer solution with a concentration of 100 mg/ml by heating Polyvinyl alcohol (PVA, Mw 13000-23000, 89% hydrolyzed, Aldrich) at reflux in deionized water. Appropriate amounts of gold chloride (HAuCl4, 3H20, 99% Pure, Aldrich) are dissolve in the polymer solution under vigorous stirring so as to obtain metal/polymer blends with ratio of 3%, 5% and 10% in weight (w%). 2. The films are then deposited by spin coating (1500 rpm for 60 s) on cleaned glass substrates. The final height, measured with a profilometer is around 800 nm. We use a Mach-Zehnder interferometer to irradiate the film with an interference pattern (λ=473 nm, grating step = 2 μm, light flux = 100 mW/cm) thus performing a localized photo-reduction of the gold salt. 3. The films are finally heated at 130°C during 3 minutes on hot plate, which induces the nanoparticles growth and the surface relief grating formation.