We report the observation of the nucleation of pairs of screw dislocations of opposite topological charges in
second-harmonic generation processes in quadratic nonlinear media. The observation provides evidence about
the conservation and evolution of the topological charge of light waves parametrically interacting in the
nonlinear medium in the dynamical regime explored. @S1063-651X~98!12111-6#
Petrov, D., Torner, L. Observation of topological charge pair nucleation in parametric wave mixing. "Physical review E, statistical physics, plasmas, fluids, and related interdisciplinary topics", Desembre 1998, vol. 58, núm. 6, p. 7903-7907.