Non-sequential Double Ionization Of Ar Atom In Few-cycle Two-color Circularly Polarized Laser Fields

A series of interesting and novel high-order non-linear phenomena have occurred in the interaction of atoms and molecules with the ultra-fast and ultra-strong laser pulse,such as highorder harmonic generation and non-sequential double ionization.The two electrons involved in strong-field non-sequential double ionization are strongly correlated,which provides a simple and effective pathway to explore the common electron correlation behavior in nature.Since L'Huillier discovered the non-sequential double ionization in 1982,non-sequential double ionization has been one of hot topics in the strong field physics community.A large number of studies have indicated that the electron correlation behavior and microscopic dynamics in nonsequential double ionization depend on laser parameters,such as the intensity,wavelength,pulse width,and polarization properties of the laser pulse.Ionized electrons are mainly driven by the laser electric field,so the motion trajectory of ionized electrons largely depends on the specific wave shape of the laser electric field.Therefore,a combined laser field composed of two intense laser pulses with different frequencies is proposed to drive non-sequential double ionization of atoms and molecules.By adjusting the relative parameters of the two laser pulses,one can change the wave shape of the combined laser electric field and further control the nonsequential double ionization.One class of such combined electric fields is two-color circularly polarized laser fields.By adjusting the relative intensity of two circularly polarized laser pulses and other parameters,a two-dimensional laser electric field with flexible and adjustable wave shape can be realized.This is extremely beneficial for adjusting the microscopic dynamics and controlling the correlation behavior of electrons in strong-field non-sequential double ionization.For a few-cycle laser pulse,the carrier envelope phase of the laser pulse also has significant influence on the wave shape of the laser electric field.Therefore,this paper proposes to drive the non-sequential double ionization of Ar atoms by few-cycle counter-rotating twocolor circularly polarized laser pulse,and theoretically investigate the dependence of the correlated electron microscopic dynamics on the relative phase of two pulses in few-cycle twocolor circularly polarized fields.For current computing resources,the numerical solution of the three-dimensional timedependent Schr?dinger equation of two-electron system of atom or molecules in the visible or near-infrared non-linearly polarized field is still a huge challenge.In recent years,classic ensemble models have been developed to deal with the ionization of two electrons in strong laser field.This method has the advantages of simple calculation process,high calculation efficiency,and well in agreement with experimental results.It has been widely used in the theoretical research of non-sequential double ionization.In this paper,the three-dimensional classical ensemble model is used to investigate the non-sequential double ionization of Ar atom by few-cycle counter-rotating two-color circularly polarized laser fields.Numerical results indicate that double ionization probability sensitively depends on the relative phase of the two components and achieves its maximum at the relative phase 0.7?.We select four specific relative phases for deep analysis,the momentum distributions of two electrons in the field plane for four different phases is mainly distribute near the three sides of the negative vector potential curve of the combined laser electric field.As the relative phase increases,both the negative vector potential and correlation electrons momentum distributions rotate clockwise,indicates that the relative phase affects the emission direction of ionized electrons.Based on the order of electron ionization after recollision,we theoretically separate the first and the second electrons.The first and second electrons distribute in two different areas in the momentum distribution.It means that the first and second electrons can be distinguished from the measured electron momentum distribution.We trace the classical non-sequential double ionization trajectories and perform statistical analysis of the recollision time and the electron ionization time.By comparing the negative vector potential of the laser electric field with the electron momentum distribution,we can also obtain the first and second electrons release time.We carefully examine the return direction of the free electron and find that the return angle is closely related to the relative phase.The relative phase can effectively control the return direction of free electrons at a wide range of angles.Based on the time delay between the double ionization and the recollision,the ionization mechanism is divided into recollision-induced direct ionization and recollision-induced excitation with subsequent field ionization.The relative phase strongly influences their contribution to non-sequential double ionization of Ar atom.