Effect of roughness on surface plasmon scattering in gold films

Attenuated total reflection and rear-side light emission were measured on 50 nm thick gold films evaporated on glass substrates previously coated with LiF layers of 0-500 nm thickness. The complex dielectric constant of metal films was determined as a function of the fluoride thickness. Roughness parameters (sigma, delta) were calculated from the angular distribution of the emission intensity and also from atomic force microscope images. Roughness amplitudes (delta) were found to be proportional to the fluoride thickness up to 350 nm, but over this value began to decrease. Both the average grain diameter and the correlation length (sigma) increased with the layer thickness in the whole observed range, indicating the flattening out of LiF at large thicknesses. Dielectric functions and surface plasmon wave-vectors of gold layers change drastically under the influence of increasing roughness amplitudes, which effect cannot be quantitatively described by the unmodified Fresnel equations.