The High-Order Harmonics And Attosecond Pulse From A Model Atom Irradiated By Bichromatic Laser Field

In recent years, with the rapid development of laser technologies, especially the progress in femto-second laser pulses, the peak intensity of laser pulses has reached and even exceeded that of the Coulomb field in a hydrogen atom. Under the action of such strong pulses, a series of phenomena which can no longer be interpreted by the conventional perturbation theory occurs. The High-order harmonic generation (HHG) is just one of them.As we know, when an atom, molecule or cluster interacts with strong laser fields, high-order harmonics of the incident laser pulse frequencies can be generated. A variety of experiments has demonstrated the following characteristics of the HHG spectra: a sharp decline of the intensity for the first few harmonics, followed by a plateau consisting of many harmonics with the roughly same intensity and then an abrupt cutoff at a certain frequency. The importance of the HHG researches lies in the fact that it is not only a promising way of generating coherent light in the extreme ultraviolet and x-ray region but also a means to produce attosecond x-ray pulses. Study of attosecond pulse of the order of magnitude of ultrashort pulse has been very active after laser interaction with the material to enter the field of strong field physics. Theoretically two methods for attosecond pulses generation have been raised: One is the Stimulated Raman Scattering (SRS), another is the HHG. Despite the energy conversion efficiency of the HHG is lower than SRS in the mechanism of attosecond pulse generation, its advantage is that it can be removed a single-attosecond pulse from the pulse sequence. Therefore, the preferred source of the study of attosecond pulse is the HHG. The main task of this paper is the study of high-order harmonics and attosecond pulse from a one-dimensional model atom irradiated by bichromatic laser.During the HHG process, the two most important orbits contributing to each harmonic are the so-called short and long trajectories, characterized by electron travel times in the continuum of about one-half and about an optical cycle, respectively. The phase of the two paths has quite different oscillation behaviours. For harmonics in the plateau, both trajectories are present. The interference between the two components results in an irregular radiation with at least two bursts within half an optical cycle. A selection of a single quantum path has more valuable applications because of its regular attosecond pulse train in time domain. For this purpose, in this paper we use one-dimensional model atom of Coulomb potential, the high-order harmonics from one-dimensional model Ne atom irradiated by bichromatic laser shows double-plateau structure, and the second plateau is supercontinuum spectrum. A selection of a single quantum path can be achieved in different laser intensity ratio by adjusting the difference of the phase between the fundamental and second frequency, an isolated 80as pulse was straightforwardly obtained.First, the SMT (simple man's theory) theory and the ionization mechanism of one-dimensional model atom irradiated by bichromatic laser were performed to analyse and explain the double- plateau structure. In our programme harmonic spectrum have a similar double- plateau structure, and there are two main ionization times whose ionization rate large, but the ionization yield are different in every ionization figure. According to the SMT theory we can obtain the main ionization time of the first and second plateau. As the HHG is essentially a stimulated recombination process, combine with the factors affecting the efficiency of harmonic we can obtain the efficiency of the first plateau is higher than the second one. The two plateaus have different main ionization time, and the efficiency of the first plateau is higher than the second one, so the double- plateau structure can be interpreted.Secondly, we got the quantum path of HHG from the result of time-frequency analysis of HHG. We found that in different laser intensity ratio a selection of a single quantum path can be achieved by adjusting the difference of the phase between the fundamental and second frequency. The result of time-frequency analysis of the second plateau 70 ~ 110ωshowed that single attosecond pulse can be obtained. But there are differences of width and strength among the attosecond pulse. In accordance with the HHG is a stimulated recombination process, the SMT theory and the ionization mechanism of one-dimensional model atom and the feature of bichromatic laser were performed to analyse and explain the mechanism controlling quantum path of HHG. The ionization rate of one-dimensional model atom irradiated by bichromatic laser and the result of time-frequency analysis of the HHG were employed to explain the differences among the attosecond pulse.