Optimization Of Phase Matching Of High Harmonic Generationin Ultra Intense Laser

With the development of laser technology, the power of laser has been greatlyimproved, even more than the internal atomic Coulomb field. In this physicalcondition, many phenomena produced through interaction with laser and matter can’tbe explained by perturbation theory, such as tunneling ionization, high-orderharmonic generation, non-sequential ionization and so on. Thus the research of atomsand molecules under ultra-strong femtosecond laser field enters a new stage. In recentyears, research on high-order harmonic theory and experiments with femtosecondlaser field has received a lot of attention, because the high-order harmonic generationis one of the ways to obtain coherent extreme ultraviolet light source or even softX-rays. However, due to the phase mismatch between the high harmonics and thefoundamental frequency wave in high harmonic generation process, the higherharmonic conversion efficiency is very low. If we can achieve phase matching,high-order harmonic generation conversion efficiency will be greatly improved.In this paper, we introduce the progress of the high order harmonic research intheory and experiments, and the development of high order harmonic phase matchingtechnology, and also describes the theory of high order harmonic generation in detail.We analyze the influence of various parameters on the phase mismatching and theintensity of high order harmonic in several cases. The result indicates that, harmonicintensity begins to increase with the increase of gas pressure, after reaching themaximum, harmonic intensity reduces with the increase of pressure. Optimumpressure corresponding with different orders harmonics is different, which is theresult of the increase of the harmonic intensity due to the pressure increase and thedecrease of the harmonic intensity due to the phase mismatching caused by pressureincrease. By controlling focusing condition, such as the use of large confocal parameter, we can improve the intensity of high order harmonic. By discussing theimpact of the length of the gas cell for harmonic generation under several conditions,we can conclude that the harmonic intensity is stronger, when the gas cell is fourtimes the length of the absorption length.