Using Elliptically Polarized Laser Field To Study The Characteristics Of Harmonic Radiation Short Orbits

The process of high harmonic generation（HHG）emitted by atoms,molecules and solids in the intense laser field is one of the frontiers in the field of intense field physics,which has an extremely important application in the study of attosecond ultrafast dynamics.The high-order harmonic radiation is a nonlinear process,which can be described as the electromagnetic wave radiated by the target materials such as atoms and molecules under the action of intense laser pulse,whose frequency is integral multiple of the incident laser frequency.Using high-order harmonics,we can explore the structure of matter and the dynamics of electrons in the space scale of atom size,and track the motion of electrons and nuclei in the sub femtosecond time scale.In this paper,by numerically solving the time-dependent Schrodinger equation（TDSE）,we calculate the high-order harmonic radiation process of atoms with different ionization energy,and further explore the contribution of short orbit components to the high-order harmonic spectrum through the filtering scheme.When the laser intensity is low and the wavelength is long,the harmonic radiation process contributed by the short orbit is significantly suppressed in the Ip range above the threshold.Further analysis shows that this phenomenon can be attributed to the influence of the tunneling exit of the electron wave packet on the short orbit,which provides a possible observation means for studying the spatial characteristics of the electron wave packet tunneling.In the first chapter,we briefly introduce the progress of laser technology and attosecond science,as well as some basic physical phenomena in the process of the interaction between intense laser field and matter.This paper focuses on the radiation mechanism and characteristics of high-order harmonics,which paves the way for the research of this paper.In the second chapter,the important theoretical models and related numerical methods in the field of strong field physics are introduced.First,the semiclassical three-step model is introduced,which is the most basic model to understand the strong field physical process,and lays the foundation for the subsequent theory.Then we introduce the strong field approximation（SFA）theory,which greatly reduces the number of quantum orbitals and simplifies the calculation.Then we consider the Coulomb effect on the basis of the basic strong field approximation model.That is to say,when the electron tunneling through the barrier,it does not escape immediately,but vibrates near the parent nucleus for a period of time（delay caused by Coulomb）under the joint action of Coulomb field and external laser field,until the electron instantaneous energy is greater than 0,then it is judged as ionization.The modified strong field approximation model considering Coulomb effect is called MSFA.Finally,we introduce the split operator method and wavelet analysis method for solving the time-dependent Schrodinger equation.These methods are important numerical simulation and analysis methods in this paper.In Chapter 3,based on the TDSE calculation scheme of short orbit contribution,we study the high-order harmonic radiation process of He atom.The results show that with the increase of laser wavelength and the decrease of laser intensity,the harmonic generation of HHG spectrum in Ip-2Ip energy region is obviously suppressed.The width of this region depends on the ionization energy of the target atom.The amplitude of harmonics is calculated by MSFA,which further reveals the dependence of suppression on laser parameters.Then the influence of Coulomb effect on this phenomenon is tested by changing the shielding potential parameters.The results show that the smaller the laser intensity and the longer the wavelength,the weaker the Coulomb effect and the more obvious the harmonic suppression in this region.In Chapter 4,the influence of ellipticity on harmonic generation is studied by using elliptically polarized laser field.It is observed that the low energy region is not sensitive to the ellipticity,and the change of the ellipticity does not change the harmonic production greatly.From another point of view,it shows that the below threshold HHG has no orientation dependence,and the contribution of short orbit is mainly due to multiphoton ionization.In the fifth chapter,the research work of this paper is summarized.