Study On Controlling Chirp And Ellipticity Of High-order Harmonic Generation From He Atom Irradiated By Two-color Laser Fields

High-order harmonic generation(HHG)can be observed when ultrashort intense laser pulses interact with atoms,molecules,and solids.The frequency of HHG can reach extreme ultraviolet and even X-ray band,it can be characterized nanoscale heat flow,track the dynamics of specific elements in magnetic materials,and nanometer resolution coherent diffraction imaging,etc.Due to its broad spectrum,it is one of the primary methods for obtaining attosecond pulses.Also it has a wide range of applications for ultrafast holography and tracking ultrafast dynamics of an electron in atoms.In this thesis,the harmonic emission of helium atoms caused by a combined laser pulse was deeply studied.We concentrated on analyzing and managing the creation of circularly polarized high-order harmonics and isolated attosecond pulses without chirps.The main works of this thesis include:Firstly,by employing the numerical solution of the time-dependent Schr(?)dinger equation,we simulated the single-electron response process of He under the action of orthogonal two-color combined pulses.In theory,we systematically studied the process of high-order harmonics generation from He irradiated by using orthogonal polarization two-color fields.It is found that the high-energy part of its harmonic emission exhibits small chirp characteristics at a specific phase.Through the analysis of the wave packets evolution and the classical motion trajectories of the electron,it is demonstrated that the quasi-chirp-free harmonic can be attributed to the simultaneous return of electrons ionized at different times to the parent ion.The impact of the two pulses’ relative phase on the harmonics is further examined,when it is discovered that while this phenomenon can produce isolated attosecond pulses inside a certain phase,it is sensitive to the relative phase.Secondly,by numerically solving the time-dependent Schr(?)dinger equation,we studied the control of the harmonic ellipticity from He under the two-color laser fields driving fields.Only linearly polarized harmonics are discovered to be generated when orthogonal linearly polarized pulses with a frequency ratio of 1:2 are applied.However,elliptic polarization harmonics can be generated in the case of frequency ratio of 1:3.We further investigate the relationship between harmonic ellipticity and the incoming laser field’s ellipticity and relative phase difference.The results show that the harmonic ellipticity of He atom changes alternately with the relative phase of the two-color laser field.Left-,right-handed elliptically polarized light,and linearly polarized light are all successively selected in our harmonic spectra.It is found that the amplitude angle and intensity of hamonics can be changed by changing the relative phase and the amplitude ratio of the two-color elliptical laser fields.The phase and intensity of harmonics can be changed by changing the relative phase of the two color ellipsometry laser pulse.Thus the harmonic ellipticity can be modulated.By adjusting harmonic ellipticity can generate high-intensity and high-order harmonics with controllable ellipticity,and then attosecond pulses with polarization control are obtained.This provides a means for the measurement of magnetic circular dichroism of magnetic materials and chiral molecules.