Optical properties of gold clusters in the size range 2-4 nm

We report experiments on gold clusters in the size range 2-4 nm, embedded in an alumina matrix. The metallic particles are produced with a laser vaporization source and codeposited with a dielectric vapor as a thin film on a substrate. Our technique allows varying the cluster size at a given metal concentration. These composite materials are studied through optical absorption and ellipsometric measurements, allowing determination of their complex index of refraction. Various complementary techniques provide information about their morphology, their chemical composition, the thickness of the films, and the size distribution of the clusters. The surface plasmon resonance in the absorption spectra is shown to be damped and blueshifted with decreasing cluster size. Theoretical calculations in the framework of the time-dependent local-density approximation allow a clear understanding of these experimental results.