Second-harmonic generation with focused beams under conditions of large group-velocity mismatch

We present a theoretical model that describes the focusing conditions for second-harmonic generation (SHG) of focused femtosecond pulses as a function of group-velocity mismatch (GVM), with direct application to efficient SHG using a “thick” nonlinear crystal. We observe a direct dependence of the optimal focusing ratio, L/b, on the strength of group-velocity mismatch. 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 SHG with focused femtosecond pulses in a “thick” crystal of KNbO3.

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