Koch Peter, Bartschke Juergen, L'huillier Johannes A
Opt Express. 2015 May 18;23(10):13648-58. doi: 10.1364/OE.23.013648.
We report on the influence of self-focusing and self-defocusing in the phase-mismatched frequency doubling crystal on the third harmonic generation (THG) efficiency in a two crystal frequency tripling scheme. By detuning the temperature of the doubling crystal, the impact of a phase-mismatch in second harmonic generation (SHG) on the subsequent sum frequency mixing process was investigated. It was found that adjusting the temperature not only affected the power ratio of the second harmonic to the fundamental but also the beam diameter of the fundamental beam in the THG crystal, which was caused by self-focusing and self-defocusing of the fundamental beam, respectively. This self-action was induced by a cascaded χ(2) : χ(2) process in the phase-mismatched SHG crystal. Self-defocusing was observable for positive detuning and self-focusing for negative detuning of the phase-matching temperature. Hence, the THG efficiency was not symmetric with respect to the point of optimum phase-matching. Optimum THG was obtained for positive detuning and the resulting self-defocusing in combination with the focusing lens in front of the THG stage was also beneficial for the beam quality of the third harmonic.
我们报道了在双晶三倍频方案中，相位失配倍频晶体中的自聚焦和自散焦对三次谐波产生（THG）效率的影响。通过调节倍频晶体的温度，研究了二次谐波产生（SHG）中的相位失配对后续和频混合过程的影响。结果发现，调节温度不仅影响二次谐波与基频的功率比，还影响THG晶体中基频光束的束径，这分别是由基频光束的自聚焦和自散焦引起的。这种自作用是由相位失配的SHG晶体中的级联χ(2):χ(2)过程引起的。对于相位匹配温度的正失谐可观察到自散焦，对于负失谐可观察到自聚焦。因此，THG效率相对于最佳相位匹配点不对称。对于正失谐获得了最佳的THG，并且由此产生的自散焦与THG级前的聚焦透镜相结合，也有利于三次谐波的光束质量。
