Investigation On High Harmonic Generation By Free Electrons In Strong Laser Field And Its Application

Since the invention of the chirped pulse amplification (CPA) technique in the1980s, it is now possible to generate a laser pulse with intensity as high as1022W/cm2and even higher intensity may be expected through the construction of ELI (Extreme Light Infrastructure). As a consequence, the research on high-power laser-matter interaction has drawn worldwide attention from both experimentalists and theorists. Various new research directions have emerged, such as multi-photon ionization, above-threshold and tunneling ionization, high harmonic generation (HHG), atom stabilization, laser-modified quantum electrodynamics (QED) processes and so on. The investigations in these new directions are important not only for examining some fundamental theories related to strong fields, but also for their potental applications in a large number of other research fields such as material sciences, biology, chemistry and even nuclear physics.Our works in this thesis will focus upon some electrodynamics processes related to electons radiations in strong laser fields, which can be summarized as follows.1. High harmonic generation (HHG) from free electrons in intense laser fields. In the framework of both classical and semi-classical theory, we have rederived the frequency spectra and angular distribution of the radiation power from the free electrons in strong laser field, and performed a detail numerical calculation to investigate the effects of the initial electron energy, the laser intensity and polarization on the differences between the classical and semi-classical results. It is shown that the discrepancy of the frequency spectra is more significant when using higher electron energy, lowering the laser intensity or increasing the order of the harmonics in the head-on collisions. This conclusion has been verified for both the plane wave fields and the laser pulses with finite durations. Moreover, the polarization sometimes also matters:there is no radiation in the direction of electron current J based on classical theory for linearly polarized laser fields while it does not happen for the results of semi-classical theory. Finally, we also compare the influences of classical radiation damping and the quantum recoil on the radiation spectrum. It is shown that the quantum recoil effect is more significant for weak field with high photon energy while the classical radiation damping becomes important in the high-intensity field domain since it depends more on the radiation power.2. The interference effect of laser-assisted bremsstrahlung emission in Coulumb fields of two nuclei. When Coulumb field is superinterposed upon an intense laser field, the basic QED process for a free electron will change from Nonlinear Compton Scattering (NLCS) into the laser-assisted bremsstrahlung (LAB). Now if the electron is scattered by a two-atom molecules, the interference between the two LAB’s from different nucleus centers should be expected. By numerically evaluating the cross sections for laser-assisted bremsstrahlung, we do observe the interference pattern in the radiation spectrum when the laser wavelength approaches the internuclear distance. In addition, we have demonstrated that both the internuclear distance and orientation of the molecule can also influence the interference which implicates that by expecting the LAB interference, we might be able to obtain the information about the molecular structure in the intense laser fields.3. The influence of the intensity gradients of a laser beam upon HHG spectra from free electrons. The ultra-high laser intensity in the lab is usually achieved via tight focusing, which will cause very large intensity gradients and can influence the emission spectra of free electron HHG. We describe the field-gradient effectively as a cycle-averaged ponderomotive potential and the whole interaction process is taken as a laser-assisted bremsstrahlung radiation. By numerically evaluating the cross section, it is shown that the main effect due to the ponderomotive potential is the broadening of the spectrum for the low harmonics. The line width is mainly dependent on the waist width of the laser beam.4. The elastic scattering of electrons by H+, H and H-in strong laser field. In order to detect the stable multiply charged negative ions induced in a high-frequency strong laser field, we numerically evaluated the classical trajectories and cross-sections for electrons scattered by the dressed state of H+and H, H-. The corresponding electronic charge distribution for H and H-is derived through High-Frequency Floquet Theory (HFFT) in Kramers-Henneberger (K-H) frame for a linearly polarized strong laser field. As a crude approximation, we have taken the Coulumb potential formed from the charge distribution to be stiff. Finally, it is shown the electrons passing H almost without being deflected. As to the case of H+and H", the cross-section of the former will always be larger than that of the latter and the differences between them will be more significant if the laser intensity is increased or the incident electron energy is lowered.