| High-order harmonic generation (HHG) results from a nonlinear response of atoms and molecules to intense laser fields. During the recent twenty years, HHG has been attracting the focus of many researchers since its potential application, such as the generation of extreme ultraviolet (XUV) light source and even soft X radiation source, the generation of attosecond pulses and the detection of ultrafast processes. A theoretical description of HHG in atomic or molecular gases involves two aspects:the single-atom or single-molecule process in which the harmonics are generated, and multiatom or multimolecule (propagation) effects, which occur during the propagation of laser pulses and harmonics through a nonlinear medium. In this thesis, the influences of the propagation effects to the HHG have been investigated, and the results show that the propagation effects can be used to improve the temporal characteristics of the harmonics and produce isolated broadband attosecond (as) pulses. The contents include:(1) The influences of the propagation effects on the supercontinuum generated by ionization gating have been investigated. It is shown that the intensity of the harmonics from the short quantum path can be enhanced selectively, while the intensity of the harmonics from the long quantum path is suppressed during harmonics propagating through the atomic gas due to the difference between the conditions of the short and the long quantum paths. Then the harmomics in the supercontinuum after propagation are mainly originated from the short quantum path and can be used to produce isolated broadband attosecond pulses directly. (2) The influences of the propagation effects on the supercontinuum generated by anω+ω/2 two-color laser pulse have been investigated. The variation of the relative phase between the driving laser pulse and the control laser pulse generated during the propagation in the atomic gas is negligible. Moreover, by adjusting the focus conduction of the laser beam, the phase matching of the short quantum path can be achieved during the propagation, and a sub-100 as isolated pulse can be generated from harmonics in the supercontinuum. (3) The influences of the propagation effects on the supercontinuum generated by attosecond ionization control have been investigated, and the results show that an isolated attosecond uv pulse is a robust tool for producing efficient broadband supercontinuum at macroscopic level. Moreover, the propagation effects can be used to select one of the long and short quantum paths. Then a smooth supercontinuum and efficient isolated attosecond pulses can be generated. (4) Using a 2ω/3 control laser pulse to control the ionization and acceleration in HHQ the asymmetry in the HHG for asymmetric molecules can be further enhanced, which leads to a broadband supercontinuum in the plateau of the high harmonic spectrum with a multicycle driving laser pulse. The selection of the short quantum path for the harmonics in the supercontinuum can achieved after propagation. Therefore, isolated attosecond pulses can be generated by directly filtering the harmonics in the supercontinuum. |
