Spectroellipsometric characterization of plasma-deposited AU/SiO2 nanocomposite films
The optical properties of bulk noble metals are due to interband (d band to the s-p conduction band) transitions at shorter wavelengths and intraband (free electron) absorbtion at longer wavelengths. In metal nanoclusters the intraband contribution is modified due to the confinement of the electrons within the particle. Instead of monotonically increasing with wavelength, the absorbtion spectrum is dominated by the resonant coupling of the incident field with the quanta of collective conduction electron plasma oscillations, so-called surface plasmons. The frequency and the shape of the surface plasmon resonance (SPR) band is dependent on the concentration, size, and shape of the metal clusters, as well as the dielectric properties of the surrounding medium. This dependence allows for the extraction of microstructural characteristics of the nanocomposite material from their optical properties. This can be done, for example, by fitting the measured dielectric function of the metal clusters with the calculated response using the Mie theory, or, for larger metal volume fractions where dipole-dipole interactions become important, with the response predicated by effective medium theories.