Atom Single Ionization In A Strong Elliptically Polarized Laser Field

The ionization of atom in a strong laser field is an important aspect of the interaction between light and matter.The study of it not only has a high scientific value but also has a wide application prospects. On the one hand, the phenomena of strong field physics such as tunneling ionization and multiple ionization help us to realize the properties of atom itself and promotes the development of other subjects, on the other hand, many new techniques, for example ultra fast mea-surement, may come from those new phenomena. The attoclock technique is just resulted from the ionization of atom in an elliptically polarized field. Thus, the behavior of ionized electron performed in the light field of elliptical polar-ization receives increased attention in the recent years. The focus of this paper is to study the survival mechanism of ionized atom, the influence of Coulom-b potential on the angular distribution of atom and the constraints of ellipticity on the energy spectrum in an elliptically polarized light field. The results have a great significance for understanding the complex dynamics of electron in ex-treme condition and for further developing advanced optical techniques such as attoclock. The paper has six chapters and the main content is as follows:The first chapter briefly reviews the development history of ultrashort super-strong laser, introduces the interesting phenomena and research signifi-cance of strong field physics. The subject we have chosen and the main work we have done are also mentioned in this part.The second chapter introduces three theoretical systems in turn for study-ing strong field physics. They are classical theory, quantum theory and semi-classical theory, respectively. By comparing their advantages and disadvantages, we conclude that the semiclassical theory is most simple and effective and ex-tend it to the elliptically polarized field.The third chapter investigates the survival mechanism of ionized atom in an elliptically polarized field. In this part, we focus on the survival results of rare gas atoms and alkali metal atoms under different ellipticities. It is found that a fraction of atoms remain unionized after the laser pulse when the tunneled electrons are released in a certain window of initial field phase and transverse ve-locity. We name this window as the survival window.The survival window shifts with laser polarization ellipticity and its width varies with respect to laser in-tensity and atomic ionization potential. Neutral atom yield can be calculated by summing up tunneling probabilities in the window. Our theory can quantitative-ly reproduce the distribution of the survival yields vs laser ellipticity observed for helium in experiment. For other atom species with smaller ionization poten-tial such as magnesium, our theory predicts a wider distribution than the strong-field approximation model while closer to the three-dimensional semiclassical electron ensemble simulations, indicating the important role of the Coulomb ef-fects.The deviations between theoretical results and numerical simulations are due to the multiple scattering orbits of some electrons which cannot be approx-imated by survival window theory. In addition, we also study the sensitivity of survival rate which is the function of ellipticity to laser intensity. This is signifi-cant for measuring the peak intensity of a super-strong laser pulse.The forth chapter analyzes the effect of Coulomb interaction on the angular distribution of electron in an elliptically polarized laser field. It is found that the Coulomb effect of direct ionization plays the dominant role in producing the asymmetrical structure of angular distribution while the scattering can modulate slightly the emitting rate of all directions. Moreover, we find the scattering effect is determined by the displacement and velocity of electron short after the rec-ollision with nucleus. We provide two methods to correct the direct ionization, among it the " correction of effective Coulomb point" works well for atoms with large ionization potential, but it is not applicable for atoms with small ioniza-tion potential. And the "correction of effective Coulomb segment" just makes up this weakness. However, it is approximated greatly and has to be further im-proved. Ionization potential and laser ellipticity determines the position of peaks of angular distribution together and the laser intensity can be used to control the degree of sharpness.The fifth chapter studies the energy spectrum of atom ionization in an ellip- tically polarized field. It is found that the spectrum consists of two parts which are the smooth decreasing region and the oscillating plateau region, respectively. It is easily affected by the ellipticity. With the increasing of ellipticity, the peak of spectrum is moving to the direction of higher energy. At the same time, the smooth region keeps becoming wider while the oscillating region keeps becom-ing narrower. As the laser is circularly polarized, the energy totally becomes a smooth curve. The energy spectrum also has a relationship with the ioniza-tion potential and laser intensity. With the decreasing of ionization potential, the ionization rate is growing multiply and the dividing line of smooth region and plateau region keeps moving right. When the laser intensity is increased to a certain degree that is close to the over barrier value, there also appears a plateau in the smooth region and the whole spectrum presents a double plateau structure.The last chapter summarizes the major achievements of this paper and has a prospection for the further development and work of strong field physics.