Processes in Resonant Domains of Metal Nanoparticle Aggregates and   Optical Nonlinearity of Aggregates in Pulsed Laser Fields
  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. 
Anatoliy P. Gavrilyuk, Sergei V. Karpov