Study On Dynamics Of Frequency Combs Via High-order Harmonic Generation In Strong Laser Fields

The optical frequency comb realizes the direct link between microwave and optic frequency,which plays an extremely important role in the development of optical frequency precision measurement and ultra-fast science.At present,the high-order harmonic generation(HHG)from the matter driven by high-repetition and intense femtosecond laser provides an alternative method for the optical frequency comb generation.Therefore,the theoretical study of the frequency-comb light sources via HHG has important significance.In this thesis,the time-dependent generalized pseudospectral method is used to accurately and efficiently solve the time-dependent Schr(?)dinger equation(TDSE)of the femtosecond laser pulse trains and atomic interaction system.We have extended the secondorder split-operator technique in spherical coordinates for the time propagation of the TDSE.Once the time-dependent wavefunction is determined,one can compute the expectation value of the induced dipole moment.The frequency-comb HHG can be obtained by the Fourier transform of the induced dipole moment.The same spectral characteristics of frequency-comb HHG and HHG in general,that is a rapid drop at low orders followed by a broad plateau where all the harmonics have the same strength and a sharp cut-off region beyond which no further harmonic emission is seen.The difference is that the coherent and nested comb structure appears at each order harmonic in the frequency-comb HHG.Therefore,to study the nested comb structure and dynamics mechanism of the frequencycomb HHG,we first investigate the frequency-comb HHG of the femtosecond laser pulse trains interacting on a single-electron hydrogen atom system.By controlling the Keldysh ionization regime,the evolution of the structure and coherence of the frequency-comb HHG from the Keldysh multiphoton ionization regime to the tunneling ionization regime is explored.The result shows that the frequency-comb structure is dependent on the Keldysh regime determined by the laser intensity,and the time-dependent ionization probability and the electron population of the excited states are calculated to analyze the ionization process in detail.In the end,the coherent dynamics process of the frequency-comb HHG is demonstrated clearly.In addition,to probe the effect of the atomic system on frequency-comb structure,we study the frequency-comb HHG of helium atom driven by femtosecond laser pulse trains.Considering the high ionization energy of the helium atom,we put forward the model potential with angular momentum to solve TDSE accurately.In-depth,it is found that the frequency-comb HHG of helium atom in mid-infrared laser pulse trains has a greatly enhanced and suppressed structure compared with the regular structure,which absent in the hydrogen atomic system.We analyze the coherence process of these structures through the phase coherence of harmonics,and we also explore the dynamics process of frequency-comb HHG using the dynamic emission of harmonics.The results in this paper provide a deeper understanding of the underlying mechanism of coherent frequency-comb structures and shed light on current interest and significance to the experimental realization of controllable and frequency-comb vacuum-ultraviolet light sources.