Tunneling Dynamics In Ultrafast And Intense Laser Fields

In the late 1980s, by applying the chirped pulse amplification technology, the ultrashort and intense laser pulse was produced and developed rapidly. Nowadays, people can easily achieve high-intensity femtosecond laser pulses with a peak intensity larger than 1013W/cm2 and a pulse width less than 100fs. With this kind of ultrashort intense laser pulse, it is possible to reveal the ultrafast dynamics of electrons in atoms and molecules and explore the behavior of matters in extreme strong field conditions. Tunneling ionization and multiphoton ionization are fundermental phenomena in the interaction of atoms, molecules with ultrashort intense laser pulse. The spending time of tunneling process and the mechanism of transition from multiphoton ionization to tunneling ionization have attracted considerable interests but are still unsolved. In this paper we mainly focus on the above two issues:firstly we study on the tunneling time problem and give a new definition of tunneling time; secondly we study the transition process from the multiphoton ionization to tunneling ionization by investigating the current density and make a comparison with Keldysh theory.There are three parts in this thesis as follows:Since the question of tunneling time was introduced by MacColl in 1932, there are many different definitions about tunneling time proposed, such as Larmor time, dwell time, phase delay time, traversal time and so on. Firstly, we give the detailed derivations and calculations about Larmor time and dwell time in the one-demensional square barrier tunneling model. Then, by applying the stability current density in square barrier tunneling model we give a new definition of tunneling time.Secondly, we study the current density in one-demensional hydrogen model by solving the one-demensional time dependent Schrodinger equation numerically. We investigate the dependence of the electron’s current density on the field strength of the trapezoid pulse. Based on the characteristics of the current density, the interaction between field and hydrogen atom can be divided into three different zones:multiphoton region, transition region and tunneling region. We also compare our methods with Keldysh theory and find a good agreement, which indicates that our method, analyzing the direction of current density, can be well used to distinguish the the mechanism of ionization.Finally, we extend our theory from one dimension to three dimensions by solving three-demensional time dependent Schrodinger equation numerically. We calculate the electron’s probability density distribution and current density distribution in different parameter regions. And again our theory works well in distinguishing different ionization regimes in three-demensional case.