Theoretical Analysis Of Above Threshold Ionization And High Harmonic Generation Based On Single-cycle Pulse

With the rapid development of the ultrafast laser technology, the single-cycle or even sub-cycle laser pulse has been realized in the laboratory at various spectrum bands. The great progress in experiments, have arouse lots of interests in the theoretical research in laser matter interaction in the single-cycle region. The analytical expression of a single-cycle pulsed beam (SCPB) is introduced which is an exact solution of Maxwell's equations deduced with the complex-source point model. With this SCPB model, we investigated the interaction between intense single- or sub-cycle pulses and hydrogen atom. The SCPBs are characterized by the intrinsic-chip and self-induced blueshift when compared with relatively longer pulses. These features will have direct influence on the above threshold ionization and high harmonic generation processes, and affects the total ionization yield and left-right asymmetries of electrons and so on.The interaction of the SCPBs with hydrogen atom has been simulated using numerical integration techniques to solve time-dependent Schrodinger equation. And the high harmonic generation, the total ionization yield and left-right asymmetries of electrons are studied thereafter. During the integration, a cos-1 -like absorbing boundary is applied to absorb the probability wave reflecting from the boundary. The initial wave function and wavefunction for other eigenstates are obtained by imaginary time propagation. The methods---eigenstate method and probability stream method, used to evaluate the ionization rate are compared in detail.It has been found that the envelop type of sub-cycle pulse have a greater influence on high harmonic generation than for few-cycle pulse. And in the sub-cycle region, the cutoff energy of high harmonic spectra decreases when the pulse width decreases. The relationship between cutoff energy and pulse width has something to do with the self-induced blueshift of the central frequency of the sub-cycle pulse.In addition, we analyzed the total ionization yields versus initial phase for sub-cycle pulses with different pulse width. There is surprising difference between the result of numerical simulation results and the popular used ADK theory which is based on the strong filed approximation. It implies that the ADK theory is no longer suitable in the sub-cycle domain. We find that more eigenstates have been involved in the progress of the ionization. This non-adiabatic process offers more channels for the ionization and consequently much more ionization yields are generated than predicted by the ADK theory.Furthermore, we have also do some research on left-right asymmetries of ionized electrons versus initial phases for different pulse width. It is found that symmetric point in the asymmetries-phase curve shifts with the changing of pulse width, and the speed of the shift become smaller with the pulse width grows. It is worth of thinking because the symmetric point changes little for few-cycle pulse. The reason of this phenomenon needs to have a further investment.