Theory Research Of The Influence Of Gas Target Location On Macroscopic High Harmonic

The interaction between intense laser and material brings a series of nonlinear optics phenomenon,such as the high harmonic generation(HHG),above threshold ionization(ATI),etc.High harmonic radiation can be used to obtain coherent short pulse,such as ultraviolet radiation(XUV)and X-ray.It is also the first choice to get coherent attosecond pulse.In order to improve the harmonic order,long wavelength laser was used.In recent years,it is more usual to get high harmonic by using few cycle infrared laser,but it will lead to harmonic intensity cut down.Some experimental phenomenon that is different from those of short wavelength driven laser was found.The position of the gas target has a great influence on yield of harmonic,and the high energy harmonic can be obtained when the gas target places before the laser focus.A lot of experimental parameters influence the results of the experiments.It is difficulty to study the influence of the different experimental parameters on the results just by experiments.We can study the harmonic radiation by the numerical results of simulating experiments.This paper mainly studies the effect of gas target location on harmonic.In this paper,we use the macroscopic propagation model to simulate that the intense laser drive macroscopic Xe gas and high harmonic generate.The simulation theory repeated to the experimental results very well,the location of gas target has strong influence on the yield of harmonic.We can get the following conclusions through the simulation study.Firstly,gas target before the laser focus compared with it usually after the focus,the harmonic is greatly improved.We analyze the reasons for the phenomenon that is not only about the harmonic emission mechanism but also about the spread of the laser field and harmonic in medium and free space.Secondly,the high energy harmonic intensity is improved if gas target in front of the focus when infrared laser is used.Thirdly,the improvement of the high energy harmonic has much to do with the laser intensity.The high energy part of the harmonic will not ascend when low laser intensity is used.Finally,we research harmonic generated from different locations of the gas target when changing the laser wavelength and otherparameters.We can draw a conclusion by analysis that this is related with the ionization rate of gas in the gas target.Only the ionization rate reaches a certain value be the harmonic improved when the gas target before the focus.