SMALL-PARTICLE COMPOSITES .1. LINEAR OPTICAL-PROPERTIES

Absorption and extinction spectra of fractal and nonfractal small-part icle composites are studied. General solutions of the coupled-dipole e quations with the exact operator for the dipole interaction (including the near-, intermediate-, and far-zone terms) are found and compared with those in the quasistatic approximation. Broad-scale numerical sim ulations of optical spectra for clusters containing a large number of particles (up to 10 000). are performed. A significant fraction of dip olar eigenmodes in fractal aggregates is shown to be strongly localize d. The eigenmodes cover a wide spectral region providing resonant enha ncement in the visible and infrared parts of the spectrum. In contrast to previous predictions, the absorption spectrum is shown to be signi ficantly different from the spectral distribution of the density of di pole eigenmodes. It clearly indicates the importance of symmetry prope rties of the modes and corresponding selection rules for the absorptio n by different modes in random fractal composites. Our experimental da ta obtained for extinction spectra of silver colloid fractal aggregate s are in good agreement with the results of numerical simulations.