Monte-Carlo Simulation of Polarization-Selective Spectral Hole Burning in Fractal Clusters

Fractals, objects of noninteger dimensionality embedded into usual three dimensional space, possess many nontrivial mathematical properties. There exists a series of physical systems whose geometry can be adequately described as fractal. We shall restrict ourselves to the study of one of such physical realizations of fractals, namely fractal clusters, simply called below as fractals. More specifically, we shall consider theoretically the selective photomodification of fractals induced by powerful laser radiation. Experimentally, this phenomenon has recently been observed in silver clusters in colloidal solutions and gels. The effect of selective photomodification consists in burning out the spectral hole in the absorption contour of a fractal at the frequency close to that of the exciting laser radiation. The spectral hole exists at times on the order of months or longer and, thus, can be considered as persistent. The primary hole is highly dichroic it is observed only in the polarization of the probe light coinciding with that of the exciting radiation and is practically absent for the normal polarizations. In solutions this dichroism relaxes with the rotational diffusion times of the clusters, and in gels it does not significantly change for months. The experimental data indicate that the photomodification has a threshold in the intensity of the laser radiation. Below we shall exploit the last feature to develop the theory of the photomodification which should explain its selectivity and give further predictions to stimulate experimental study.