High-order Harmonic Generation From He Atom Irradiated By Bichromatic Counter-rotating Circularly Polarized Laser Fields

The laser provides a good tool for exploring and manipulating the motion of atoms and molecules.In order to realize the manipulation of the motion of electron in the atoms and molecules,it is necessary to obtain a coherent light source with higher frequency and adjustable polarization.The emission of high-order harmonic generated from the interaction between the bichromatic counter-rotating circularly polarized laser field and the atom meets this need.But at present,the harmonic intensity is low.To enhance the high harmonic intensity,it is necessary to deeply analyze the process and mechanism of the high harmonic generation.For this purpose,this thesis systematically studied the harmonic emission process of helium atoms under the action of the bichromatic counter-rotating circularly polarized laser field,and analyzed the influence of the laser intensity and the multiple re-collisions of electrons on the intensity of harmonic emission.The main works of this thesis include:(1)By numerically solving the time-dependent Schr(?)dinger equation,the single-electron response process of helium atoms irradiated by the anti-rotating two-color circularly polarized laser pulses is simulated.The influence of different potential function models on higher harmonics has been systematically studied.The energy position of the minimum value of the harmonic spectrum is related to the selected potential function model.By analyzing the quantum trajectories in one optical cycle,it is found that the origin of the intensity minima in the harmonic spectrum is generated from the interference between different quantum trajectories.On this basis,we systematically studied the effect of potential on the polarization of the high harmonic along with the change of the driving laser intensity.It turns out that the harmonic intensity changes largely when the driving laser intensity is closed to the intensity of atomic ionization need addiational photon.By optimizing atomic and molecular targets,it is possible to generate higher-intensity circularly polarized harmonic radiation.(2)The effect of multiple re-collisions of ionized electrons of atoms on the high-order harmonics generated from atom irridiated by the reverse two-color circularly polarized laser is studied.By recombining the dipole moments in the x and y directions,the left and right polarization in the harmonic emission spectrum are separated.Then,the harmonic emission of helium atom driven by the reverse rotation dual color circularly polarized laser pulse is studied by the laser pulse with the electric field peak amplitude of 0.096 a.u.,and the fundamental frequency of 1064 nm.It is found that the harmonic intensity has enhanced in the energy range of from 43 rd to 50 th harmonic orders.Through the time-frequency analysis of the harmonic emission behavior,within this spectrum range,it is found that within this spectrum range,an ‘extra' new emission trajectory will be generated near each main emission trajectory.The analysis of the wave packet evolution,electron probability flow and the classical trajectory of ionized electrons within an optical period reveals that this "extra" emission trajectory originates from multiple re-collisions of ionized electron.This study shows that is possible to optimized the intensity of high-order harmonic by changing the laser parameters to control multiple electronic re-collisions.