A Study Of Atomic And Molecular Harmonic Radiation In Elliptically Polarized Laser Fields

In recent years,the research on high-order harmonic generation(HHG)of various systems in strong laser fields has become a hot topic in the field of strong field physics,and it has important significance in the study of strong field physics.Thanks to the rapid improvement of computer performance,the computational efficiency of the multidimensional system has been greatly improved,which greatly shortens the period of theoretical research;at the same time,with the continuous development of strong field experimental technology,the manipulation of complex and meticulous molecules and the acquisition of specific light sources have also make it possible,such as the alignment of symmetrical molecules and the generation of elliptically polarized lasers.Studies have shown that the high-order harmonics of the atomic and molecular systems driven by the elliptically polarized laser field are different from those when driven by the linearly polarized laser field.These phenomena have attracted widespread attention.In this paper,starting from the atomic system,the mechanism and characteristics of the high harmonic generation in the elliptically polarized laser field are studied,and then the various properties of the higher harmonic generation of the symmetrical diatomic molecules in the elliptically polarized laser field are investigated.The dynamic process and the mechanism of high-order harmonic generation are analyzed in detail.First,we studied the harmonic generation of the atomic sy stem.Based on the singleelectron approximation and numerically solving the two-dimensional time-dependent Schrodinger equation(TDSE),we simulated the generation process of high-order harmonics from the atomic in linearly polarized laser field.The calculation is extended to the case of elliptically polarized laser fields as the driving laser field,and the high harmonics generated by the atoms under the two light driving lasers are analyzed and compared.The results show that in the elliptically polarized laser field,high-order harmonics perpendicular to the main field are generated,in addition,different from that the contribution of the high-order harmonics in the linearly polarized laser field is dominated by the short orbit,for the high-order harmonics in elliptically polarized laser fields,the contribution of multiple returns has increased significantly.Through the strong field approximation(SFA)model,we have explained the reasons behind this phenomenon.Secondly,we studied the high-order harmonic generation mechanism of symmetric diatomic molecules.We have studied the parallel high-order harmonics and perpendicular high-order harmonics from the H2+ and N2+ in linearly polarized and elliptically polarized laser fields in detail,and the polarizability of the harmonics has been studied at the same time.The numerical calculation results also show that the symmetrical diatomic molecules can also generate perpendicular high-order harmonics in the elliptically polarized laser field.At the same time,we also found that at different molecular axis orientation angles,the harmonic generation field and structure of the molecules are different.By comparing the polarizability at different orientation angles,we notice that as the orientation angle increases,the peak value of the HHGs' ellipticity shifts to the region with higher harmonics.In addition,our research results show that in the case of parallel alignment,compared with model atoms with the same ionization energy,the ellipticity of symmetric diatomic molecules produces a higher and more stable platform in the below-threshold(BTH)and near-threshold(NTH)region.Through analytical model analysis,we found that the "two-center interference" effect of molecular plays an important role in it.At the same time,different from the one-dimensional movement driven by the linearly polarized laser field,the collision angle in the re-collision for the two-dimensional movement of the electron in the elliptically polarized laser field is also very important in the process of harmonic generation.Since the BTH and NTH have the properties of both high intensity and high ellipticity,we can use this part of the harmonics to produce extreme ultraviolet(EUV)pulses with high ellipticity and high intensity.The above calculation results will provide a certain theoretical basis for understanding the generation of atomic and molecular high-order harmonics in the elliptically polarized laser field,and at the same time have inspiring significance for the control of optical field and the generation of ultrashort pulses in the experiment.