Frequency-doubling of focused ultrashort pulses under conditions of significant temporal walk-off

摘要

A theoretical model that describes second harmonic generation of focused ultrashort pulses in the presence of large group velocity mismatch (GVM) is presented. We observe a direct dependence of the optimal focusing on the strength of GVM. Our model also describes the temporal duration of the second harmonic pulses under these conditions as well as the change in optimal phase mismatch. The theoretical results are compared with an experiment for second harmonic generation with focused femtosecond pulses in a "thick" crystal of KNbO_3 with a length much bigger than the time duration - GVM ratio.