Percolation and fractal composites: Optical studies

Local field distributions are studied in random metal-dielectric films near percolation (percolation films) and fractal aggregates of colloidal partic les. For both systems, it is shown that optical excitations are localized i n small nanometer-scale areas, "hot spots," where the local fields are much larger than the field of an incident electromagnetic wave. The large local fields result in giant enhancement of various optical phenomena. The surfa ce-enhanced white-light generation and second-harmonic generation have been obtained in percolation films. For fractal aggregates of silver particles, a giant effect of local optical activity has been observed. The effect is due to surface-plasmon excitations localized on chiral-active particle conf igurations in fractals.