THEORETICAL AND EXPERIMENTAL STUDY ON GEOMETRY RELATED PHASE-MATCHING CONDITION FOR THIRD HARMONIC GENERATION

We present a theoretical- analytical and numerical- study on the phase-matching condition for third harmonic (TH) generation by focusing gaussian laser pulses in dispersive non-linear (NL) media. We address the influence of the irradiation geometry on the efficiency of TH generation process. We find that for a fixed length of the NL medium, the intensity of TH signal at the output of the NL medium can be increased by two alternative approaches. First, by focusing the driving laser radiation at the frontier of the NL medium rather than in the center of the NL medium. Second, by increasing the driving beam waist radius while laser intensity is kept constant. Both methods are demonstrated to be successful for increasing the TH signal because they minimize destructive superposition of the TH emissions before and after the focus due to Gouy phase-shift. When the two methods are combined, i.e. loose focusing of the driving laser at the frontier of the NL medium, we can boost the TH signal by more than one order of magnitude as compared to the case of non-optimized irradiation geometry condition. The theoretical results on the dependence of the TH signal to the driving beam waist radius are supported by experimental data.