Optical absorption and localization of eigenmodes in disordered clusters

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Abstract
Results of a large-scale computational project for the calculation of the dispersion relations of eigenmodes (surface plasmons) and optical-absorption spectra of disordered clusters (fractal and uncorrelated) are reported. Fractals (cluster-cluster aggregates and the random-walk clusters, both original and diluted) and random-gas clusters consisting of 100–300 monomers are studied. High-accuracy results of Monte Carlo simulations are obtained. Transition of the eigenmodes from extremely localized to fully delocalized is found. Scaling of the dispersion relation of the eigenmodes, i.e., their localization radius or coherence length as a function of the spectral variable X, predicted earlier is quantitatively confirmed for diluted clusters. In contrast to the dispersion relations, the absorption spectra as functions of X do not show pronounced scaling in the intermediate region, but scale in the binary (spectral-wing) region. We suggest a new plot for the absorption profiles, namely absorption as a function of the coherence length of excitations. In such plots for most clusters, scaling is pronounced, but the indices differ dramatically from the predictions of the strong-localization theory. Possible reasons for the observed behavior are discussed.