Excitation And High-order Harmonic Generation Of Atom In Multi-color Laser Fields

The interaction of intense laser fields with atoms and molecules can produce high-order-harmonic generation(HHG). HHG could seed coherent light sources of radiation in extreme ultraviolet(EUV or XUV) and soft x-ray areas, and extract structural information of atoms or molecules. More importantly, HHG can generate attosecond pulse(AP) that is the shortest detective tool in time scale so far. The emergence of laser pulse with long-wavelength, few-cycle and stable carrier-envelope phase provides the possibility of controlling effectively the generation of high intensity and energy HHG,and the coherent transfer of atomic population. We will investigate how to enhance harmonic yield and generate ultrashort intense AP by the method of waveform synthesis, how to manipulate the coherent transfer of atomic population and prepare the excitation states and superposition states by femtosecond laser pulse in this thesis, the major contents are listed as follows:Firstly, we demonstrate theoretically the superiorities of waveform synthesizing in the generation of AP by optimizing the parameters of laser pulse with Genetic Algorithm(GA) and solving time-dependent Schr?dinger equation(TDSE) with pseudospectral method. Our results show that a sub-cycle field can be obtained by synthesizing two or three long pulses, which can effectively increase the ionization rate by concentrating most field energy close to the center of the whole pulse, and suppress electron wave packet spreading by reducing the electron travel time. The harmonic yield can be enhanced by about two orders under the optimized field compared to a single mid-IR laser pulse. In addition, from these intense harmonics we can obtain about 30 as single AP if harmonic phases are compensated.Secondly, we study the controlling of coherent transfer of atomic population and the preparing of the low-excitation states of helium atom by femtosecond laser pulse by way of solving TDSE with angular-momentum-dependent model potential. Our results show that laser pulse can quickly and efficiently manipulate the coherent transfer of atomic population(even can reach 100%) and further prepare low-excitation states and superposition states on the condition that all other energy levels are far away from target state energy level. However, when some levels are very close to target state level, the maximum of population that is transferred to the target state is lower than 100%, because that closing results in that the target state level and those levels suffer simultaneously from the coherent action of laser pulse.