Theoretical Study On Generation And Regulation Of Circularly Polarized High-order Harmonic

High-order harmonic generation?HHG?from atoms and molecules in an intense laser pulse is an effective method to obtain low-cost desktop extreme ultraviolet?XUV?and even soft X-ray band coherent light sources.Owing to its wide spectrum,HHG is the most effective way to produce optical pulses with an attosecond duration(10^-18 s).As HHG is generated by the collision of the tunneling ionized electron with its parent ion in the laser electric field,the harmonic spectrum carries the structural properties of the parent ion.Therefore,HHG can be used as a detector for a static and dynamic molecular structure.Owing to the wide application of harmonics,in recent years,the study of harmonic characteristics has become a hot topic in the field of strong fields.In this paper,by adopting the numerical solution of the time-dependent Schr?dinger equation?TDSE?,we simulated the single-electron response process of H₂⁺molecular ion and He atom under a combined two-color field.In theory,we studied the process of generating high-intensity elliptically polarized harmonics from H₂⁺irradiated by a co-rotating two-color circularly polarized?CRTCCP?laser field,and the influence of relative intensity ratio of two circularly polarized pulses in the CRTCCP field,the molecular internuclear distance,and the alignment angle on the intensity and ellipticity of the high harmonic spectrum.The theoretical studies show that a bicircular field consists of two coplanar co-rotating circularly polarized fields and a frequency ratio of 1:2,H₂⁺can generate even and odd ordered harmonics under the laser field.When the electric field amplitude ratio of the fundamental frequency and the second one is approximately 1:2.3,the harmonic intensity of H₂⁺is much greater than that of atom,and it is close to the harmonic intensity of the CRTCCP field under the same parameters.The enhancement of harmonic intensity for H₂⁺under CRTCCP laser field can be explained by the evolution of the time-dependent wave packets and the classical trajectory.We also found that the intensity of specific harmonics can be enhanced or suppressed by adjusting the electric field amplitude ratio and the molecular internuclear distance in the CRTCCP laser field.Finally,we found that the harmonic ellipticity is significantly affected by the molecular alignment angle.As the molecular alignment angle is changed,the high-order harmonics are converted among the left circular,linear,and right circular polarizations.In addition,we also studied the modulation of the harmonic ellipticity form He under the orthogonally polarizeod two-color?OTC?driving fields with a frequency ratio of 1:3.Normally,the harmonics generated by the laser field are elliptically polarized.We found that by changing the relative phase and the amplitude ratio of the laser field.Under certain conditions,we can get the harmonics of pure circular and nearly pure circular polarization.The amplitude angle and intensity of different polarization direction components of harmonics can be changed,thereby regulating the ellipticity of harmonics.This systematic research work on circularly polarized high harmonic generation in CRTCCP and OTC field has deepened people's understanding for the new characteristics and laws of atomic and molecular high harmonics driven by complex laser field.It is possible in experiments to use this scheme to produce a more intense and polarization controllable coherent coherent high-frequency light source,which provides a ways for measuring the circular dichroism of magnetic materials and chiral molecules.