Study On The Generation Of High-order Harmonic And Isolated Attosecond Pulses Of Atom In The Laser Fields

The attosecond pulse can probe ultrafast electron dynamics in atoms and reveal public material structure and dynamics characteristic . Therefore, it is very important to research the realization of attosecond pulse. It is well-known that the interaction of intense laser field with atoms and molecules can lead to high-order harmonic generation (HHG). Coherent sources of radiation in XUV and x-ray regions can be obtained by HHG. The end of the plateau of the harmonic generation will appear a supercontinuous characteristics, and that HHG is one of the important means by these supercontinuum to synthesize attosecond pulses.Theoretically, HHG can be well studied for atoms by the strong field approximation and solving the time-dependent Schr?dinger equation in intense laser field. Firstly, within the strong field approximation,we study the high-order harmonic behavior of atoms exposed to an intense laser field using Lewenstein model. The Lewenstein model for atoms is introduced, then we study high harmonic generation of hydrogen atom exposed to the long wavelength infrared laser field(800nm～2000nm), and analyze the wavelength dependence of conversion efficiency of high-order harmonic generation near the cut-off position. It is different from the wavelength dependence of high-order harmonic yield studied by most theorists in the plateau of the harmonic generation, and it is found that high-order harmonic yield is much lower near cut-off than in the beginning of the plateau of the harmonic generation, but the width of the attosecond pulse is shorter with increasing wavelength.Moreover, we study the generation of isolated attosecond pulses from a one dimensional(1D) model He~+ ion in an intense few-cycle chirped laser and its high frequence pulses by solving the time-dependentSchr equation with split-operator method, and analyze the characteristics of the attosecond pulse produced by HHG. Due to the presence of the chirped pulse and the high frequence pulse in the case of the combined fields, it is shown that the plateau of HHG is extended largely and the conversion efficiency of HHG is enhanced effectively. By superimposing some orders harmonics on different region for the second plateau, all of the pulses obtained are single attosecond ones, minimum pulse achieves 21as. To better understand the physical origin of HHG enhancement and attosecond pulse emission, we perform classical analysis and the time-frequency analysis.