Optical Properties of Small Metal Particle Composite Systems: Gold - Silicon Dioxide

Small metal particle/insulator composite systems have acquired great importance for many scientific and technological applications. One of the most fundamental quantities to describe the nature of these systems is their dielectric function. We have made the first measurements of the dielectric function for a metal/insulator composite system over a wide energy range. The composite materials of this study were extensively characterized by a variety of techniques and were prepared by a unique method which allowed for independent control of particle size and volume fraction. The Au-SiO(,2) composites show the expected resonance due to the sphere dipole oscillation mode at the position predicted by the simple Maxwell-Garnett Theory (MGT). The resonance also shows the long lifetime predicted by MGT theory. In contrast to this, MGT theory predicts a strength for the resonance which is much greater than is actually observed. More complex theories for the composite dielectric function which include the treatment of the detailed interactions between the particles of the composite predict a large redshift of the resonance while retaining the strength of the resonance. This redshift actually increases the discrepancy between theory and experiment. The ability to vary the composite particle size independently of the volume fraction led to the discovery that, contrary to the predictions of existing theories, the composite dielectric function depends on particle size. We are able to show that several sum rules on the dielectric function can be applied to the MGT EDF; this allowed us to carry out, for the first time, experimental tests of the validity of the MGT sum rules in metal/insulator composite materials. In addition, we have measured the wavevector dependence of the dielectric function and find much less dispersion of the dielectric function than is predicted by theory. These unexpected results imply that as yet undetermined phenomena are involved in determining the electromagnetic response of small metal particle systems.