High-order Harmonic Generation From Ar And N₂ In Elliptically Polarized Laser Fields

High-order harmonic generation(HHG)through the interaction of femtosecond laser pulses with materials is by far the most important way to obtain desktop coherent extreme ultraviolet light sources and attosecond pulses.The investigation of harmonic spectrum can not only deepen our understanding of the process of light modulation,but also help us to obtain important information about the orbital structure and quantum state of atoms or molecules.The process of HHG is closely related to the electronic dynamics.The probability of electron back to nucleus determines the yield of harmonic radiation.Extracting the intensity of harmonic spectrum can effectively reflect the various effects of the electrons in the process of contributing to HHG signals and the Coulomb effect of the electron affected by the nucleus.In this dissertation,we have investigated the radiation characteristics of high-order harmonics from both experimental and theoretical perspectives.In the experiments,we compared the ellipticity dependence of HHG yield from Ar and N₂,and reported that the ratios of lower order harmonic yield to the case of higher order rise with the ellipticity increasing for both gaseous media,and the increment of ratios for short trajectory is more obvious.In view of the interesting phenomenon found in the experiment,we have simulated the relationship between the harmonic ratio of Ar and N₂and the ellipticity by using the nonadiabatic semi-classical theoretical calculation approach,and the results can well reproduce the ratios measured in the experiment.Finally,the phenomena shown in the experiments and theories were analyzed and explained based on the electronic trajectories.In addition,we reported that increasing the nuclear distance of N₂molecule can change the Coulomb potential,which makes the harmonic ratio of different orders change regularly with the change of laser ellipticity.Our results show that the nonadiabatic effect plays an important role in dealing with the spectral information of high-order harmonics,and the information of molecular nuclear distance is encoded into the ellipticity dependence of HHG.