The Study Of Multichannel Highorder Harmonic Generation And Its Propagation Effect From Superposition States

The high-order harmonic generation(HHG) is a hot topic in the research field of strong laser pulses interacting with atoms and molecules. There will be radiate the high energy new pulse light after the interaction between atoms or molecules and intense laser field. The radiation frequency(above the five harmonic) is the integer multiple of the incident laser frequency. The nonlinear optical phenomenon which is known as the effect of high order harmonic and the radiation wave by the process is called the high-order harmonic. HHG possesses many advantages. It can generate attosencond laser pulses and provide a coherent XUV and X-ray source. In addition, it also is used in studying the interior structure of atoms and molecules. Therefore, the research of HHG is the most fundamental and critical task. The superposition of initial state, which in physics is an important model, has wide appliance in describing the material and electromagnetic interaction. We will present the application of the superposition in the HHG and reveal the underlying physical mechanism. The strong field approximation is advantageous to save computational time and present clear physical picture. In this paper, we adopt the SFA method respectively to calculate the dipole moment of the single state and superposition by solving the time-dependent Schr?dinger equation(TDSE). What is more, we analyze the influence of propagation effect to the HHG by solving the Maxwell wave equation(MWE). That may explain the disappearances of hyper-Raman(HR) lines in experimental measurements. The main content of this dissertation is as follows:The numerical simulations of the single atom response for different superposition are presented in the first part. We analyze and compare the high-order harmonic spectra and the contribution of each channel under the two circumstances. The state population is also shown in the paper. The results show that a strong resonance peak appears in the higher order harmonic spectrum. This is mainly due to direct transition between excited state and ground state for 1s2 p superposition state, while for 1s2 s derives from the transition from 2s state to continuum state and then return 1s state. It is more notable that multichannels in HHG can be resolved in single atom response due to the strong coupling of the bound states.In the second part, we investigate the HHG when taking into account the propagation effect. We find that the noisy which appear in the single atom response vanish and the harmonic become more distinguishable and resolvable. More importantly, some channels resolved in the single atom response may be suppressed. The reasons come from the following:(1) the phase matching. The phase shifting removes some channels,(2) the change of relative strength of each channel. The spectra which have a weak intensity are overwhelmed by strong spectra,(3) the depletion speed of the excited state is quick, while the ground state is stable and almost no ionization. The results can be applied to demonstrate why the hyper-Raman lines have not been identified in the corresponding experimental measurements. The results present a strong proof to the experiment for the disappearance of HR lines.