NONLINEAR SELF-DEFOCUSING IN DOPED SILICA SONO-GELS

Experiments with nonlinear self-refraction of Gaussian laser beams in silica sono-gels doped with copper tetrasulfonated phthalocyanine are reported. The propagation of laser beams inside nonlinear sol-gel samp les with different Cu-phthalocyanine concentrations has been monitored by measuring the spatial beam profile in the near field and in the fa r field behind the sample. The experimental results are analyzed by a new simple theoretical approach, in which we assume that the incident Gaussian beam induces a phase shift that varies as a Gaussian function of the beam radius. The beam propagation behind the sample is determi ned by the Huygens-Fresnel integral formalism. By solving the Huygens- Fresnel integral, analytical expressions for the spatial beam profile in both the near field and the far field after the nonlinear sample ar e obtained, Experiments are carried out with a diode pumped frequency doubled Nd-YAG laser at 532 nm. We obtain very large third-order nonli nearities in these doped sol-gel samples at temperatures just above ro om temperature. When we compare the predictions of the theory with the experimental data, we find experimental values of the nonlinear third -order susceptibility up to -2.3 X 10(-4) esu. (C) 1997 American Insti tute of Physics.