The Theoretical Research On The Optimization Of Atomic High-order Harmonic Generation

With the ultrafast motion of electrons (about several hundredattoseconds) in atoms or molecules, it is necessary to adopthigher-time resoluted tool to research the motion of electrons,morever, attosecond time-resolved laser pulse is considered as themost effective tool to research the ultrafast process. Therefore, thegeneration of isolated attosecond pulse is very popular both in theoryand experiment. So far, the high-order harmonic generation has beenthe most effective way to obtain isolated attosecond pulse. The widerthe high-order harmonic spectrum, the narrower the isolatedattosecond pulse, so broaden the high-order harmonic spectrum isone of the feasible schemes to obtain the narrower isolatedattosecond pulse.The paper presents two different schemes to optimize the plateauof high-order harmonic spectrum for helium atom, by numericallysolving the time-dependent schr dinger equation to optimizate thehigh-order harmonic generation, and to invesigate the numericalsimulation of the high-order harmonic generation. The two schemesto broaden the high-order harmonic spectrum in this paper are asfollows: Firstly, we obtain a two-color laser field which is synthesizedby a driving pulse and a controlling pulse. Then, adding a low-frequency field to two-color laser field to research the effect ofthe synthetized laser field on high-order harmonic generation;Secondly, adding a static electric field to two-color laser field toresearch the effect of the synthetized laser field on high-orderharmonic generation. The high-order harmonic generation withdifferent laser parameters have been studied by this two schemes.The results show that, by adding a low-frequency field or a staticelectric field to a synthesized two-color laser field, not only thebandwidth of the supercontinuum is broaden, but also the shortquantum path is selected to effectively obtain the isolated attosecondpulse. In addition, by ajusting the intensity of the low-frequency fieldto research intensive change affected the high-order harmonicspectrum, it shows that, the supercontinuum can be furtherbroadened by adjusting the intensity of the low-frequency field.Morever, we also use classical simulations to research thehigh-order harmonic spectrum, and calculations by quantum methodare well accorded with it by the classical simulations. And areasonable explanation about extending the plateau of high-orderharmonic spectrum was made through the classical returningkinetic-energy map, the ADK ionization rate and the time-frequencydistribution.