Nonlinear refraction of silver hydrosols during their aggregation

The relation between the degree of aggregation of silver hydrosols and their nonlinear refractive index n₂ is studied experimentally. It is found that the sign of n₂ at a wavelength of 1.064 μm changes with increasing the aggregation degree, which corresponds to the replacing of self-focusing by self-defocusing. The observed effects are explained based on the analysis of a change in nonlinear dispersion of the medium, taking into account the interaction between phases and the photochromic effects, which are typical for colloidal structures with fractal geometry. It is shown that the change in the sign of the nonlinear refractive index of hydrosols upon irradiation by laser pulses of duration of less than 10^-7 s is caused by the perturbation of resonances of silver and water and by the competition between Kerr nonlinear polarisations involving these resonances.