The Generation Of High-order Harmonic And Isolated Attosecond Pulse From The Interaction Of He~+ Ion With Intense Laser Fields

With the rapid development of ultra-intense laser technology, the theoretical research on the interaction between intense lasers and matters, the explanation for the new experimental phenomena, the prediction for the behavior and the law of matters in the intense laser field, which is still out of labs'reach, have become active forefront research topic. From intensive studies in theory and experiments in the past two decades, it is apparent that the strong-field atomic, molecular, and optical (AMO) physics research not only touches on the most basic aspects of the physical world, but also plays an important role in science and technology. Just as femtosecond pulses have presented the natural time scale of electronic motion in the mean field potential of solids and molecules, attosecond pulses are powerful tools for studying and controlling the motion of electrons inside atoms and molecules. Concrete examples are controlling molecular motion, exciting inner shell electrons, tracing the motion of bound electrons, and manipulating above-threshold ionization.High-order harmonic generation (HHG), which is a phenomenon occurs in the multi-photon physics when atoms or molecules are driven by a short intense laser pulse, has experimentally been the furthest advanced method that can produce single attosecond pulses. Moreover, for its potential applications such as getting a coherent light in the extreme ultraviolet (XUV), a coherent soft x-rays source, it has been rapidly developed in the past decade. Both the experimental and the theoretical studies have shown that the HHG spectrum has the general character: a fast decreases for the first few harmonics, then exhibits a broad plateau, and ends up with a sharp cutoff. The HHG process can not only be well understood by the means of the well-known semiclassical three-step model, but also can be understood by Leweisten used fully quantum theory. Firstly, an electron tunnels through the barrier which is formed by the Coulomb potential and the laser field. Secondly, the electron oscillates and gains kinetic energy under the influence of the laser field. Finally, when the laser field inverses the electron begins to slow down, if the laser fields satisfy certain conditions the velocity of the electron would decrease to zero and gain inverse velocity. If so, the electron will be pulled back to recombine with the parent ion and emits a harmonic photon.Concerning the practical application, people's essential pursuit on the HHG study is extending the width of harmonic plateau, and enhancing the emission efficiency. Three-step model predicts the cutoff frequency law of the harmonic plateau in the monochromatic laser field is: E cutoff = Ip+3.17Up, where I p is ionization potential; U p= E 2 4ω2 is the pondermotive energy gained by electron in the laser pulse, E andωare the peak intensity and the frequency of the incident laser field, respectively. From the cutoff rule we can see there are two methods can be used to extend the cutoff of the harmonic plateau. One is changing the ionization potential of the atomI P , the other is changing theU P , which is related to the laser field parameters. However, for a special kind of atom, the ionization potential is fixed, in order to increase the IP we can only choose the atom with higher ionization potential. Because the He+ ion has higher ionization potential, in this thesis I choose it as the study target. Furthermore, because UP is related to the peak value and the frequency of laser field, we can adjust the two parameters to change the cutoff position of the harmonic plateau. The essential purpose of this thesis is extending the harmonic plateau as broaden as possible and enhancing the harmonic emission efficiency, superposing a properly selected range of the harmonic spectrum and getting an intense isolated attosecond pulse.First, this thesis investigates the HHG emission of He+ ion by using the coherent superposition of the ground state and the first excited state with equal weight as the initial state. We choose the combination laser field of a linear chirped laser pulse and a static electric field, with the form [ ]E (t ) = E0 f (t )cosωt +δ( t )+αE0 , as the driving laser pulse. Through calculation, we find that the choice of the coherent superposition state can effectively enhance the harmonic efficiency by 6-7 magnitudes of order; by choosing the combination of the chirped laser pulse and static electric field, the cutoff position can be extended to IP + 26.53UP compare with the I p +15.88U P in the chirped laser field and I p +4.72U P in the combination of fundamental laser pulse and the electric field. Moreover, a supercontinuum is obtained; by superposing properly selected range of harmonic spectrum, in the supercontinuun, an isolated 42as attosecond pulse is obtained. Following, we use wavelet transform and semi-classic method to explain the enhancement of the harmonic efficiency, the extending of the cutoff position and the generation of an isolated attosecond pulse. Furthermore, based on practical and present experimental technique, we use CO2 laser replace the static electric field, and find that both the cases have the similarly functions.From the classic three-step model we can see that the HHG can be controlled by different steps. Recently studies show that through the choice of the two-color field the electron motion, the second step of the'simpleman', as well as the HHG can be controlled. However, due to the inherent characteristic of the two-color scheme, the efficiency of the continuous harmonics is low, which straightforward affects the intensity of attosecond pulse and its application. Therefore, a great deal of effort has been devoted to enhance the harmonic efficiency. Moreover, control of quantum trajectories is an efficient method to generate an isolated attosecond pulse. The attosecond pulses can be obtained by superposing several consecutive harmonics in phase, and the harmonics are in phase, are very important for the generation of isolated attosecond pulses. It is shown that there are two paths, so-called long path and short path, which contribute to every harmonic. Because the two paths have different emission times, the harmonics are not phase-locked. As a result, superposing properly range of harmonic spectrum leads to an irregular attosecond pulse with two separate bursts. To resolve this problem, it is essential to control the quantum path so as to pick out one single path, and finally, filter several consecutive harmonics in the cutoff region. Furthermore, the laser parameters directly affect the electron motion and the HHG process. With the introduction of the chirp, the symmetry of the fundamental laser field is destroyed by changing its frequency, period, wavelength, the laser peak, and so on. Correspondingly, the electron motion can also be changed. Because high harmonics emitted by many-cycle pulses may not be synchronized on an attosecond time scale, thus setting a lower limit to the achievable x-ray pulse duration. However, the dynamically induced harmonic chirp can be compensated by introducing an appropriately designed frequency chirp on the laser beam.Just as we mentioned above, concerning the effects of the two-color laser field, chirped laser field and the superposition states has been respectively studied, we provide a new method: investigate the HHG emission and the generation of attosecond pulses of He+ ion through the chirped two-color laser field (combination of chirped fundamental laser and a subharmonic laser field) with the coherent superposition states as the initial state. Through our calculation, with the introduction of the chirped parameters the motion of the electron can be changed, and the cutoff position can also be remarkably extended. By adjusting the chirped parameter a 1670eV supercontinuum can be obtained, and superposing a properly selected range of harmonic spectrum an isolated 38as attoseocnd pulse can be obtained. To explore the underlying mechanism responsible for the enhancement of the harmonic efficiency and the cutoff extension, and the generation of attosecond pulses, we perform the time-frequency analysis of the emitted pulses by means of the wavelet transform of the induced dipole acceleration and a classical simulation by virtue of the three-step model and ADK theory.