Focusing versus defocusing nonlinearities in self-trapping due to parametric frequency conversion

Summary form only given. Solitons due to parametric mixing in quadratic (/spl chi//sup (2)/) media constitute an active area of research. In few years, spatial solitons, filamentation, temporal trapping, and spatio-temporal effects have been observed. The cascading limit, i.e., an effective self-focusing Kerr-like action occuring for large positive mismatches /spl Delta/k = 2k(/spl omega/) - k(2/spl omega/) is often invoked to understand trapping in second-harmonic generation (SHG). The self-focusing nature of /spl chi//sup (2)/ mixing for large /spl Delta/k > 0 is then taken for granted, irrespective of frequency (/spl omega/ or 2/spl omega/) and launching conditions (SHG versus downconversion or optical parametric amplification, OPA). Here, we present the first experimental evidence that this is not so. We show that the SH beam tends to defocus (focus) for /spl Delta/k > 0 (/spl Delta/k < 0). This entails that the threshold energy for soliton formation in OPA is expected to be lower for /spl Delta/k < 0, where the dominant injected 2/spl omega/ beam experiences self-focusing, as opposed to SHG.