High-order Harmonic Generation Of Ar Atoms In An Elliptically Polarized Laser Field

The development of ultra-short and ultra-intense laser pulse has enabled the study of many nonlinear interactions and phenomena.High harmonic generation(HHG)as a strong-field physical phenomenon has been widely studied.HHG is an important way to obtain table-top short-wave coherent light sources and attosecond pulses.The semiclassical picture of the HHG process,"ionization-acceleration-recombination," provides the physical basis for studying the quantum state and evolution dynamics of atoms and molecules.By combining molecular orientation and obtaining harmonic spectra information,molecular orbital imaging can be achieved.Various physical effects can be produced in the HHG process by changing laser parameters,such as polarization and phase.In this paper,the high harmonic spectrum of Ar atoms in an elliptically polarized optical field is experimentally measured,and the ellipticity dependence phenomenon of the plateau region and the near-threshold region harmonic are analyzed in detail.Firstly,based on the non-adiabatic semi-classical theory,the normalized ratio of the intensity of high-order harmonics to that of low-order harmonics under long and short trajectories is compared.It is found that the harmonic ratio under short trajectories increases more significantly than that under long trajectories with the increase of laser ellipticity.The theoretical analysis reveals that non-adiabatic effects play a crucial role in the HHG process,affecting the initial momentum of electrons and trajectory control.The "threestep model" picture implies that the probability of electronic and nuclear combination during the HHG process is closely related to the polarization of the optical field and decreases rapidly as the ellipticity increases.By changing the polarization and laser intensity,it is found that the HHG yield of Ar decreases with the increase of laser ellipticity in general,but the near-threshold 13 th harmonic shows a significant abnormal ellipticity dependence at a certain level of the intensity.Moreover,the maximum yield at nonlinear polarized light is also observed in the plateau region at a certain optical intensity.The experimental results analysis indicates that this abnormal dependence may come from multiple physical factors,especially Rydberg state excitation.The abnormal ellipticity dependence near the threshold contains rich physical mechanisms that require more in-depth theoretical calculations.