Focusing versus defocusing nonlinearities due to parametric wave mixing - art. no. 183902

We show experimentally and theoretically that cascading due to two-wave par ametric frequency conversion in quadratic materials acts as an effective fo cusing or defocusing nonlinearity, depending not only on, mismatch, but als o on, the selected wave, and the dominant type of process (second-harmonic generation or down-conversion). A dramatic asymmetry of beam spreading and threshold lowering for soliton formation against mismatch is, the clear exp erimental signature of this behavior.