The High-Order Harmonics Generation From A One-Dimensional Model Atom And A United Two-Atom System By Two-Color Laser Field

The interaction of intense laser pulses with atoms, molecules, cluster, and solids can lead to high-order harmonics generation (HHG) of the incident laser frequency, as a consequence of highly nonlinear dynamics. Currently HHG is a potential way to generate radiations in x-ray and XUV regions. The unremitting pursuit in HHG studies is to simultaneously widen and heighten the HHG plateau on a large scale. The cutoff frequency is predicted byωcutoff = I p + EK(where I p is the ionization potential and is the kinetic energy in pulse). By virtue of the cutoff law, we notice that for a single atom model, taking the ion with larger ionization potential as target is one of the most direct way to extend the harmonic plateau. In fact, the maximal cutoff position was currently achieved from the interaction of intense laser pulse with He atom with the largest ionization potential among allatoms. Using highly charged ions, which have much higher ionization potentials, has Ek been proposed to further extend cutoff position. Unfortunately, the small ionization yield leads to a poor harmonic efficiency. Another direct way is to increase kinetic energy. To realize the aim, it is necessary to raise the intensity of the laser pulse when laser wavelength is kept constant. However, an atom will be depleted completely when the laser intensity rises up to a certain threshold amount, so that the corresponding harmonic emission process also terminates. To overcome current difficulty, new approaches must be suggested.Firstly, when choosing the appropriate laser intensity and ensuring the considerable population of ground state,we find that the harmonic efficiencies near the cutoff position is enhanced over five orders of magnitude by changing the relative phase of the driving and controlling field, in which the two-color laser pulse is employed to interacting with one-dimensional model atom, i.e, the He atom, with its initial state being considered as the ground state. In order to explain this phenomenon, we first discuss the reason for the appearance of the double plateau. It shows that there exist the great difference between the ratios of ionization probabilities of the first plateau and the second one. Since the second plateau can afford the great ratio of ionization probability, the electron hops into each high-energy continuum state with a great probability. This induces the sufficient population of the ionized electron after it is driven by laser pulse. There are three key factors that affect the efficiency of the high harmonic generation. First, the richer ground state population brings out, the higher recombination efficiency at the recombination instant. Second, when the population of electron at the ground state of is fixed, at the recombination instant the higher population of electron at the continuum causes the higher the HHG efficiency. Third, the properties of the atom itself, namely, the coupling strengths between the bound state and the continuum are uncontrollable for a given atom. Therefore, the HHG efficiency of the first plateau is high in comparison with that of the second plateau, which gives rise to the appearance of the second plateau. With the change of the phase difference, the driving and controlling fields become uniform and the magnitude of the component field becomes large. Accordingly, these results induce the great breadth of the coupled laser pulse in each half optical circle and the great difference of the ionization probability between the first and the second plateaus. Hence harmonic efficiencies are improved and the double plateau vanishes.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. On the theoretical side, there are two methods raised for the attosecond pulses generation: 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.Based on the analysis of single-atom harmonic spectra, we know that there are plenty of ionized electrons whose kinetic energies are too high in the process of the laser-atom interaction and that they can not be converted into high-energy photons due to their positions far away from parent-ions. We suppose if these electrons are provided with another recombination object, so that more energetic electrons can participate in the recombination process, then the plateau width will be extended on a large scale. So we design a united two-atom model to simulate the actual plasma environment, in which the atom-separation is optimized.To demonstrate the physical feasibility of our project, we systematically investigate the HHG power spectrum of the one-dimensional united two-atom model. As for the United Two-Atom System, it is very important to choose a proper inter-nuclear separation. In this thesis, we chose the inter-nuclear separation X R =πα0 / 2 (α0is the quiver radius). From the united-atom HHG power spectrum we found that not only the width of the plateau is extended on a large scale, but also the efficiency of the HHG power spectrum is enhanced with near two orders. In the following, we give detailed analysis of the generation mechanism of plateau, both qualitative and quantitatively with the help of the classical SMT theory and the Morlet wavelet-transformation technique. Due to the existence of two collision-centers in a untied two-atom system, the ionized electron driven by the laser electric field does not only recombine with the ion, but also recombine with the other ion. Because the extended harmonics arising from the ionized electron driven by the laser electric field only recombine with the other ion, the HHG power spectrum goes beyond the continuous spectrum. Finally, through wavelet transformation for several harmonics in the extended HHG plateau, we obtain super-short attosecond pulses with the duration of only eighty-six attoseconds and relatively high intensity.