Investigation Of High-order Harmonic Generation From Superposition States

An unique plateau appears in the high-order harmonic generation (HHG)spectrum as the intense laser pulses interact with the atoms and molecules. The HHGcan be used to produce coherent radiation whose wavelength ranges from extremeultraviolet to X-ray and ultrashort laser pulse whose duration is on the attosecondscale. At present, the low intensity of HHG is the main factor that limits theapplication of it, therefore, how to improve its intensity becomes the research focus.In the macroscopic aspect, with the loose focusing scheme been adopted to involvemore atoms in the HHG process, and combined with the optimized phase matchingcondition, the intensity of HHG is greatly improved. In the microscopic aspect,exciting the atom by the prepulse firstly and then radiating the atom in thesuperposition state by the main pulse, the conversion efficiency of HHG can belargely promoted.To further promote the emission efficiency of HHG, the mechanism of the HHGfrom atom in the superposition state should be clarified, which will be investigated inthis thesis by the time-dependent pseudospectral method, and on this basis, theinfluence of the motion of bound electron on the HHG will be further analyzed. Themain contents of this thesis are as follows:In the first part, the HHG spectra from the system whose initial state is thesuperposition state consists of different states under the radiation of ultrashort laserpulse is calculated and the emission mechanism of the high-frequency andlow-frequency components of HHG is analyzed. It is found that, along with theincrease in the energy of harmonics, a double-plateau structure will appear on theHHG spectrum. There exists a difference between the emission mechanism of twocomponents of HHG: for the low-energy harmonic plateau, it is produced by the recombination of the electron which is ionized from the excited state with the excitedstate of its parent ion as it return to the nuclear zone in the laser field; in contrast, forthe high-energy harmonic plateau, it is produced by the recombination of the electronwhich is ionized from the excited state with the ground state of its parent ion as itreturn to the nuclear zone in the laser field. Utilizing the harmonics which lie indifferent harmonic plateaus, the population of different initial states of the system canbe diagnosed.In the second part, the HHG spectra from the system whose initial state is thesuperposition state consists of same states under the radiation of ultrashort laser pulsesis investigated. Splitting the time-dependent dipole moments of the system accordingto transition with different symmetries, the influence of the symmetry of the systemon the HHG can be investigated. It is found that, the symmetry of the bound state ofthe system at the time when the ionized electron return to the parent ion plays adeterministic role in HHG. As a result, we can utilize the HHG from the superpositionstate to detect the motion of the bound electrons.