Investigation Of High-order Harmonic Generation Of Atom And Molecule Exposed To Intense Laser Fields

It is well-known that the interaction of intense laser field with atoms and molecules can lead to high-order harmonic generation (HHG). Atomic HHG spectra show a common Characteristic which has been proved by a lot of experiments: the first few harmonics decline sharply, followed by a broad plateau with almost constant intensity for many harmonics, and then the intensity of high harmonics decrease abruptly at cut-off. The highest harmonic photon energy is I p + 3.17Up, where U p is the ponderomotive energy and I p is the ionization potential. The appearance of plateau structures from HHG power spectra make HHG possible as sources of radiation in x-ray and XUV areas, furthermore, HHG is a preferred source for the generation of attosecond pulse. HHG has been a subject of great interest owing to these potential applications in recent years. We study high-order harmonic generation and ionization behavior of atom and molecule exposed to an intense laser field in this thesis. The major investigationsare listed as follows:Firstly: We study stabilization of one-dimensional atom exposed to intense laser fields by solving time-dependent Shr?dinger equation in KH frame with pseudospectral method. Our results show that atomic ionization is suppressed in the high frequency and superintense laser field but not in the moderate intense laser field.Secondly: Solving time-dependent Schr?dinger equation numerically, we investigate high-order harmonic generation and ionization probability of one dimensional model hydrogen atom, two dimensional model hydrogen atom and three dimensional real hydrogen atom subjected to intense laser fields. Our results demonstrate that high-order harmonic generations and the ionization probabilities of model atoms are very close to those of real hydrogen atom both in the multi-photon ionization regime and in the over-barrier ionization regime; in the tunneling ionization regime, the plateau feature and cut-off position of high-order harmonic generations of model atoms and real hydrogen atom are the same, and the trend of changing of the ionization probabilities with time is similar, but the values of ionization probabilities of model atoms are different from those of real hydrogen atom. We think that the difference of ionization probabilities among one-dimensional model hydrogen atom, two dimensional model hydrogen atom and real hydrogen atom are made for their different momentum distributions. Thirdly: we solve time-dependent Schr?dinger equation of real hydrogen atom exposed to intense laser field and static electric field with split-operator method, and investigate the abnormal behaviors of high-order harmonic generation (HHG) in intense laser field, which is caused by a static electric field. Our study indicates that intensity of odd-order harmonic generation decreases, even-order harmonic generation appears and the HHG spectrum exhibits a double-plateau structure in the presence of a static electric field, and that the first plateau of HHG power spectra is narrowed and the second plateau is widened with increasing of the intensity of a static electric field. By using wavelet analysis, we find that the radiation of high harmonic near cut-off is made up of attosecond pulses which is separated by one optical cycle. This provide a promising way to produce attosecond pulse in the experiment.Fourthly: An efficient method to generate a short attosecond pulse is presented by using intense few-cycle chirped laser in combination with an ultraviolet (UV) attosecond pulse. We show that high-order harmonic generation (HHG) plateau near cut-off is enhanced by one order of magnitude compared with the chirped laser case and the HHG supercontinuum spectrum is generated by adding a UV attosecond pulse to the IR few-cycle chirped laser at a proper time. By enhancing the long quantum path and suppressing the short one corresponds to one major return, then an isolated 57-as pulse with a bandwidth of 62eV is obtained directly. Secondly, quantum paths contributing to high-order harmonic generation can be controlled efficiently by adding a static electric field to an intense laser field. Using semiclassical simulation and wavelet analysis, an isolated 150-as pulse can be produced by superposing several harmonics near cut-off of the second plateau.Fifthly: High harmonic generation and attosecond pulse train of molecule N 2 exposed to two-color laser field are investigated with Lewenstein model. According to our study, we find that the spectrally unresolved structure in the plateau of the harmonic generation owing to the interference of two comparable short quantum path and long quantum path in a one-color laser case. In the two-color case, the interference of these quantum paths can be weakened and high harmonics in the plateau can be distinguished by adjusting the relative phase of the third harmonic field with respect to the fundamental field. Especially, whileφ=π, only the short quantum paths contribute the high harmonics in the plateau, the interference of these quantum paths can be ignored and a regular attosecond pulse train can be obtained in the plateau. Secondly, we study the ionization of homonuclear diatomic molecule N 2and O 2 by using molecular strong-field approximation theory. Our results show that the ionization rate of molecule N 2will decrease monotonically with increasingθ( from 0 to 90 ) , which is the angle of the polarization of the laser with respect to the molecular axis. The ionization rate of molecule N 2 is maximum near 0 . For molecule O 2, the ionization rate will enhance with increasingθ( from 0 to 90 ) and reach a maximum near 45 , and then drop down toward 90 .