中文摘要：

基于动量空间和坐标空间含时伪谱方法,通过求解氢原子在强激光作用下的含时薛定谔方程,系统地研究了原子在强激光作用下电离阈值附近的高次谐波发射随着激光电场强度的变化关系.研究发现,随着入射激光强度的增加,九次谐波发射效率呈现周期性振荡结构.结合原子的激发几率计算,发现电离阈值附近的谐波增强与高激发态的布居存在相互竞争关系.

英文摘要：

When an atom or a molecule interacts with an intense laser field,a coherent high-order harmonic emission is observed at a frequency that is an integer multiple magnitude of the initial frequency of the incident laser field.The harmonic emission has the characteristic of high emission efficiency at relatively high orders,and it also has a wide expansion in the frequency domain.Thus,the high-order harmonic generation can be utilized to generate coherent EUV or soft X-ray light sources as well as ultrashort at to second laser pulses.It is promising that the attosecond laser pulse will be an important tool for detecting and controlling the electron dynamics in atom and molecule systems.The mechanisms of high-order harmonics especially the high energy part of the harmonic spectrum can be explained by the well-known three-step model.The three-step model assumes that the electron in the bound state firstly are ionized by the potential barrier formed by the laser electric field and the atomic potential,then the ionized electrons oscillate in the laser field,and finally the electron with high kinetic energy gained in the laser field has the possibility to return back to the parent ion and recombines with the ground state of the system with a high energy photon emitted.As for harmonics with low orders,especially those with single photon energy near the ionization threshold,the Coulomb potential of the atom has significant influences on them.However,the effect of the Coulomb potential of the atom are not included in the three-step model,so the mechanism of near-threshold harmonics（NTH） cannot be clearly interpreted with the three-step model alone.In this circumstance,the study of the mechanism of near-threshold harmonic emission attracted people＇s attention in general.One important application of NTH is that it can be utilized to generate optical comb with EUV frequencies.Theoretically,Xiong et al.studied the mechanism of below-threshold harmonic（BTH） emission and found that the mechanism of this part of harmonics