The Spatial And Temporal Characteristics Of Broadband Supercontinuum By A Two-color Field

High order harmonic generation (HHG), as one of nonlinear phenomena of intense lasers-atom/molecular interactions, is a hot topic in recent years. The HHG is not only a promising way to produce extreme ultraviolet coherent light source and attosecond pulses, but also an efficient tool to probe microscopic ultrafast processes. An accurate description of HHG should involve the single-atom/molecule and collective responses to the laser field, and the influence of macroscopic effects on the time-frequency characteristics of HHG can’t be neglected. The recent research focus on extending the width of supercontinuum and improving the intensity of the attosecond pulse, however, the spatial characteristics of the isolated attosecond pulse have not attracted much attention. This thesis will investigate time-frequency characteristics of HHG by a two-color field at the microscopic level, and then the influences of the propagation effects on the supercontinnm have been also studied. Furthermore, we concentrate on the spatial characteristics of the attosecond pulse. These main contents and conclusions of our work are:(1) The laser parameter influence on time-frequency of HHG in a two-colour laser pulse have been investigated. The laser’s parameters, namely, the relative intensity of the controlling field and its relative phase with respect to the fundamental field, determine the quantum path selection by affecting their ionization probabilities, In both cases of the ω+3ω and ω+2ω laser fields, it is shown that the quantum path selection in the multi-cycle pulse is more dependent on the parameters than that in the few-cycle pulse, and the selection of the quantum path in the multi-cycle ω+3ω pulse shows stability to the phase and intensity variation.(2) The time-frequency of the supercontinuum from stretched molecules in a linearly polarized two-color field has been studied. By adopting an infrared pulse combined with an ultraviolet (UV) attosecond pulse, the ionization process can be controlled effectively. In this excitation scheme, the harmonic spectrum is significantly enhanced by two orders, then smooth broadband supercontinuum with the bandwidth of about120eV is obtained, which leads to an isolated sub-60-as attosecond pulse with a high signal-noise ratio.(3) The spatial characteristics of xuv supercontinuum generation in the two-color laser field have been studied. By optimizing the synthesized two-color field, the spatial profile of the xuv supercontinuum varies from annular-like to Gaussian-like and then the spatial quality is improved effectively, which is beneficial for its potential applications. What’s more, our results show that the phase-matching properties of the short and long trajectories can be modulated effectively by adding a weak second harmonic field. This scheme can not only achieve the quantum path selection, but more importantly it can achieve on-axis phase matching for both of the two trajectories simultaneously. A bright interference pattern induced by the simultaneously phase-matched two trajectories can be observed by placing a near-field on-axis filter, which has great potential for experimentally characterizing the full single-atom dipole moment.