Absorption and extinction spectra of fractal and nonfractal small-particle composites are studied. General solutions of the coupled-dipole equations with the exact operator for the dipole interaction ͑including the near-, intermediate-, and far-zone terms͒ are found and compared with those in the quasistatic approximation. Broadscale numerical simulations of optical spectra for clusters containing a large number of particles ͑up to 10 000͒ are performed. A significant fraction of dipolar eigenmodes in fractal aggregates is shown to be strongly localized. The eigenmodes cover a wide spectral region providing resonant enhancement in the visible and infrared parts of the spectrum. In contrast to previous predictions, the absorption spectrum is shown to be significantly different from the spectral distribution of the density of dipole eigenmodes. It clearly indicates the importance of symmetry properties of the modes and corresponding selection rules for the absorption by different modes in random fractal composites. Our experimental data obtained for extinction spectra of silver colloid fractal aggregates are in good agreement with the results of numerical simulations.

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]

Direct observation of localized dipolar excitations on rough nanostructured surfacesUsing a photon scanning tunneling microscope ͑operating alternatively at the wavelengths of 594 and 633 nm͒ with shear-force feedback we image the topography of silver colloid fractals simultaneously with a near-field intensity distribution. We observe that near-field optical images exhibit spatially localized ͑within 150-250 nm͒ intensity enhancement by one to two orders of magnitude. These bright light spots are found to be sensitive to the light wavelength, polarization, and angle of incidence. We relate the observed phenomenon to the localization of resonant dipolar excitations in random nanostructured aggregates.

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A direct manifestation of electron energy quantization in metal nanoparticles is observed in two-photon excited luminescence. Experiments reveal the discrete spectra in broadband anti-Stokes photoluminescence from aggregates of silver colloid particles. A theory based on a spherical quantum-well model for metal nanoparticles is in good agreement with experimental observations.

Nonlinear optical properties of metal-dielectric composites, such as fractal colloid aggregates and clusters created by ion implantation, are studied. Strong fluctuations of local fields result in huge enhancements of optical nonlinearities in fractal colloid aggregates. The real and imaginary parts of the cubic susceptibility of silver colloid aggregates are measured. It is found that the coefficient of nonlinear absorption strongly depends on the laser wavelength and intensity. Optical limiting effect in fractal silver colloids is observed. Nondegenerate forward four-wave mixing technique is used to investigate the third-order nonlinear susceptibility for nanocomposite material with Au nanocrystals formed inside a SiO 2 glass matrix. The Au nanocrystals are formed by the ion implantation and annealing method that produces very high volume fraction of nanoparticles. The large value |χ(3)|=1.3×10-7 esu is measured. Two characteristic relaxation times, 5.3 ps and 0.66 ps, are estimated from the detuning curve of |χ(3)|, as the probe beam wavelength changes. A novel class of optical materials, microcavities doped with nanostructured fractal aggregates, is also studied. In our experiments, lasing at extremely low pump intensities, below 1 mW, and dramatically enhanced Raman scattering was observed in microcavity/fractal composites.

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Optical prossesses in nanostructured fractal composites are shown to be strongly enhanced. The enhancement occurs because of a localization of dipolar eigenmodes in subwavelength areas.