  The specific optical nonlinearities inherent in aggregates of metal
nanoparticles under pico- and nanosecond pulsed laser irradiation are studied
in nanoparticle aggregates formed in silver hydrosols. The results of
experimental studies of the correlation between the degree of aggregation of
silver hydrosols and their nonlinear refraction index at the wavelengths 0.532
and 1.064 microns are discussed. The experiments revealed that nonlinear
refraction index changes its sign at 1.064 microns as the degree of the
hydrosol aggregation grows. The role of various processes occurring in resonant
domains of aggregates and the kinetics of these processes under laser
irradiation resulting in dynamic variation of the polarizability of aggregates
are analyzed. The areas under study included the kinetics of particles
displacement considering dissipative forces, heating of the particles and of
the surrounding medium depending on the wavelength, intensity and duration of
laser pulses. A theory of interaction of laser radiation with an elementary
type domain - two bound silver nanoparticles - was developed to describe the
kinetics of resonant domains photomodification in aggregates. This theory takes
into account thermal, elastic, electrostatic and light induced effects. The
experimental results on laser photomodification of silver particle aggregates
are discussed in the context of our model. These results include photochromic
and nonlinear optical effects, in particular, the nonlinear refraction and
nonlinear absorption.
