The Numerical Study Of High-order Harmonic Generation From Atom In The Intense Laser Field

Recently, with the rapid development of the ultra-short intense laser technology, the ultra-short intense laser pulse with the maximal energy about 10²² W/cm² can be obtained in experiments. These surprising developments prompt the research on the interactions between materials and the intense laser fields and make the research on atom, molecule and cluster in intense laser field becoming very interesting and excited topics in the basic theory research.When intensity of the laser field reaches up to 1013 W/cm2, the activity of materials is no longer determined by the intrinsic property of materials, but determined by the interaction of the laser-materials. Some phenomena can not be explained by the perturbation theory, such as multiphoton ionization, above-threshold ionization, tunneling ionization, over the barrier ionization and high harmonic generation (HHG). A lot of experiments have demonstrated that HHG spectra have a generic shape: there is a sharp decline in conversion efficiency followed by a plateau in which the harmonic intensity varies slowly with harmonic order, and eventually ended by an abrupt cut-off. The appearance of the plateau can not be explained by the perturbation theory, as the intensity of the laser field is comparable to the that of the coulomb field within the Hydrogen atom. Then the effect of the laser field can not be treated as a perturbation. Various nonperturbative methods have been developed and adopted to deal with the interaction of laser-materials. The HHG not only provides a new source of XUV and the soft X-ray radiations, but also shows a new possible way for extremely short pulses. The research on the HHG within the interaction of laser-materials is an activity domain.The basic of laser-material interaction is the interaction of the laser with atom. The working of this paper is base on laser-atom interaction. The aim is to find the method of enhancing the efficiency of HHG and to gave the analysis in qualitative and quantitative for the enhancement of conversion efficiency of HHG, consequently, to provide the theory direction for experiment.Dominating workings in this paper are listed as following:1) Theory recipe. The precondition of researching the laser-atom interaction is solving time-dependent Schr(o|¨)dinger equation (TDSE) of laser-atom interaction. In this part, the asymptotic boundary condition (ABC) of 1-D atom in laser field presented recently is reviewed, the simple deduce of TDSE is given. Simultaneity, the asymptotic boundary condition (ABC) for the problem of 1-D has been generalized to the problem of 2-D and symplectic algorithm is reviewed. All working of this paper is based on the ABC and symplectic algorithm.2) The results of laser-1-D model atom interaction. The methods of enhancing the efficiency of HHG have been given: taking the equal weight coherent superposition of the ground state and the first excited state as the initial condition and choosing the moderate two-color laser field as the radiating field. The two schemes can greatly enhance the conversion efficiency of HHG. Finally, the analyses in qualitative and quantitative of the enhancement of conversion efficiency of HHG have been given by the average distance and the transition probability.3)The results of laser-2-D model atom interaction. Basing on the 2-D ABC, the TDSE of laser-2-D model atom interaction has been asymptotic discreted. We consider the ellipse polarized laser field, and take different ellipticity. Then the harmonic spectra have been shown in the circle polarized, ellipse polarized and linear polarized laser field, respectively. For the characteristic of HHG spectra in three laser fields, we give explaination through the average distance, ground state probability and the probability density of wave function. We give the reasonable availability of 1-D model when dealing with the interaction of linear polarized laser field with atom by the average distance of 2-D model atom in the linear polarized laser field. We have investigated the harmonic spectra generated by 2-D multi-well model in the linear polarized laser field and circle polarized field and study the variation law and give the theoretical analysis.