Localization of optical excitations and selective photomodification are studied experimentally in fractal aggregates of silver colloidal nanoparticles. The localized optical excitations of the fractal colloids cover a broad spectral range, from the visible to the infrared. We show that the absorbed laser energy is localized in an increasingly smaller number of particles with increasing the laser wavelength from 355 to 1900 nm. The size of the photomodified regions can be as small as 20 nm. The observed modification is explained by optically induced sintering (coalescence) of colloidal nanoparticles. [S0031-9007(97)05209-5]

Abstract. Linear and nonlinear optical properties of metal fractal clusters are studied. Experimental evidence is presented proving that clusters possess unique optical nonlinearities, characterized by rapid response, broadbandness and enormously high values of nonlinear susceptibility. A 106 times enhancement of the DFWM signal has been observed experimentally in silver particles aggregated into clusters. For the first time frequencyand polarization-selective photomodification of fractal silver clusters has been realized.

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