Study Of Coulomb Effects In The Non-dipole Interaction Of Strong Laser Fields With Atoms

Good beam alignment,perfect coherence and robust disturbance resistance,all these characteristics make Laser the perfect experimental light source.Re-search into the interaction of Laser with atoms,molecules and solids leads us to a better understanding of physical processes.With the increase of Laser peak intensity due to new technologies like chirped pulse amplification,hopefully we will reach the nonlinear quantum field theory area.Meanwhile,the duration of Laser pulse is getting shorter and shorter in the lab.Interaction of strong short Laser pulse with matter is a hot research area in today's physics.Effects of strong field on electrons compete with the Coulomb potential,leading to a series of nonlinear and non-perturbative phenomena,such as multiphoton ionization,higher-order above threshold ionization,high order harmonic generation,non-sequential double ionization,et al.Study of these strong field phenomena at least gives us two advantages.On one hand,ultra-strong ultra-fast Laser can be used to probe the dynamics of particles on a quantum level,one of the examples is to probe chemical reaction when it happens.On the other hand,after knowing the mechanism of strong field process,we can use it as a means to manipulate the movement of microscopic particles.Therefore,thorough understanding is very helpful to later research and applications.This dissertation focuses on a small area of strong field physics:the photo-electron transversal momentum distribution in strong field ionization.There are five chapters all together,including:Chapter one:concise introduction to the progress of Laser and accounting for the review of chirped pulse amplification method.Some of the very sig-nificant strong field phenomena are listed,and in the process emphasizing the importance of rescattering effects.Meanwhile we in this chapter explains the dy-namics of electron in a plane electromagnetic wave in detail,in order for easier understanding of formula and concepts in later chapters.Chapter two:this chapter gives a summary of some useful theoretical meth-ods used in strong field research,including time-dependent Schrodinger equa-tion method,strong field approximation method and semi-classical monte carlo simulation method.we focus on the discretization of wave function and nu-merical evaluation of the time propagation.We give a concise introduction to B-Spline method,and explains four kinds of time propagation methods:split operator method,Crank-Nicholson method,Chebyshev polynomial approxima-tion method and Arnoldi-Lanczos recurrence method.Discussion of strong field approximation is mainly about the saddle point approximation.Classical monte carlo simulation is one of the brilliant methods in strong field simulation,and we combine quantum tunneling with CTMC to get into a better description of the three-step process.Chapter three:this chapter discusses the non-dipole effects in strong field ionization,along with the study of the involvement of Coulomb potential in the transversal momentum distribution in the rescattering process.In the first part of this chapter,we discuss three experimental research performed at an earlier time:the Coulomb focusing experiment reminds us of the effects of Coulomb potential on photoelectron transversal momentum.Photon momentum partition research leaves us awake about non-dipole effects in strong field ionization.And the experiment performed using a mid-infrared Laser reminds us of combined effects of non-dipole interaction and Coulomb potential.By studying the nu-merical simulation results and the sub-cycle electron dynamics,we unmask the Coulomb compensation mechanism in the rescattering process.Relating this mechanism to the saddle points in the scattering function we quantitatively ex-plain the peak shift of the transversal momentum distribution.Later on,by com-parison with classical scattering scheme,we connect the previously-mentioned Coulomb compensation mechanism with the Glory effect.Aware that the for-ward scattering amplitude of Glory effect is Bessel square type,we quantitative-ly interpret the interference pattern in the momentum spectrum.Chapter four:Considering the essential aspects of the three-step model in strong-field physics and the characteristics of the problem studied here,we de-vise a new notion of strong-field trajectory and develop a n analytical model describing photoelectron motion,with Coulomb potential included.Further-more,we analyze the evolution of photoelectron transversal momentum under the combined influence of Coulomb potential and laser field.It is found that the transversal momentum distribution is substantially narrowed by the impact of Coulomb potential,compared with its initial Gaussian-like distribution.Detailed investigation of the analytical model finds that,the electron transversal momenta accumulate around a specific kind of trajectories due to Coulomb force,induc-ing saddle points structure in the scattering function of the tunneled electron.This kind of structure corresponds to the peculiar cusp-like structure in the final transversal photoelectron momentum distribution.Physically speaking,when the photoelectron returns to the vicinity of its parent core,the attraction effects of the ion on the tunneled electron compensates the transversal momenta deviation of those electron trajectories adjacent to that corresponding to the saddle points.Thus this compensation effects result in the narrowing of the final transversal momentum distribution.And in a longer wavelength or mid-infrared laser field ionization experiment,this effect will lead to a more interesting phenomenon.Chapter five:this chapter gives a summary of this dissertation and discusses some future progresses in strong field physics.