The Control And Application Of Quantum Path In Atoms Driven By High Intensity Femtosecond Laser Field

Along with the rapid development of laser technique, the interaction between femtosecond laser and matter has become a hot topic in many fields. When exposed under the strong laser field, the atoms and molecules will be excited and ionized resulting in a dynamic wave-packet. Both the generation and evolution of the wave-packet can be defined as the so called quantum path. The quantum path indicates the most essential information inside the physical and chemical reactions. This concept has helped us obtaining an intuitive and clear picture about the complex and abstract phenomena in quantum mechanics. The control of quantum path has applications in a wide range, which include high order harmonic generation (HHG) and attosecond pulse production, the ultrafast imaging in biology, tomographic imaging of molecular orbitals and molecule alignment. In this thesis we combined the quasi-classical and quantum model to investigate the dynamics of atoms driven by intense femtosecond laser field theoretically, then proposed several schemes of quantum path control as well as their important applications in ultrafast science. The contents include: 1) Select the long and short quantum paths of HHG by adjusting the relative phase of a bichromatic field consisting of the fundamental and its third harmonic fields. Apply the quantum path selection to generate regular attosecond pulse train. 2) Proposed a method to generate single attosecond pulse in a multi-cycle regime, in which an over-saturated intense laser is applied to deplete the ground state population so that the recombination step in HHG is controlled. 3) Proposed a scheme to generate efficient isolated attosecond pulse by using a two-color multicycle laser field. The basic principle is controlling the ionization and propagation of electron wave packet by adding a weak second harmonic field. 4) Investigated the launch time of attosecond pulses driven by a two-color field. The free electron trajectory can be modulated by varying the relative intensity of the two components of the driving field. As a result, the control of the launch time of attosecond pulses from HHG can be realized in a very high accuracy. 5) Investigated the interference of electron quantum path in laser assisted X-ray photoionization, found out that for specific delay between X-ray and laser pulse, the interference structure in photoelectron spectra can reveal the duration of the X-ray attosecond pulse. Based on this effect, a simple method was used to characterize single attoscond pulse.