Theoretical Study On Elliptically Polarized High-order Harmonics In Intense Laser Fields

With the development of laser technology,the nonlinear physical phenomena generated by the interaction between intense lasers and atoms or molecules have been widely investigated,one of which is the high-order harmonic generation（HHG）.The HHG can obtain coherent extreme ultraviolet light sources,be used for molecular tomography orbital imaging,detect molecular structure and synthesize attosecond pulses.The ultra-high spatial and temporal resolution of the attosecond pulses opens the door to the microscopic world and reveals the ultrafast dynamical processes of electrons.Circularly polarized（CP）light sources have more degree of freedom compared to linearly polarized（LP）light sources,which can be used to investigate chiral molecules and magnetic materials.Thus,the generation of bright and wideband CP HHG has been one of the hot topics in strong-field physics.The paper focuses on the elliptically polarized（EP）HHG from gas medium driven by the intense laser fields through solving the time-dependent Schr?dinger equation for the interaction between laser fields and atoms or molecules.It includes the influence of laser intensity ratios,molecular alignment angles,two-center interference effect and initial electron states on the harmonic ellipticity.In addition,EP resonance-enhanced harmonics can obtain a quasi-monochromatic EUV radiation source,and provide a feasible method for characterizing the polarization of the driving laser.Finally,we obtain nearly CP attosecond pulse with high intensity from the mixed gases.The specific research content of this paper is as follows:（1）We theoretically investigate the generation of EP HHG from hydrogen molecular ions driven by the orthogonally polarized two-color laser fields.The results indicate that when the molecular is placed along the parallel direction,the parallel component of the harmonics is odd-order,while the vertical component of that is even-order,which can be illustrated by the quantum transitions.We can obtain ellipticity-tunable HHG by controlling the alignment angles and the intensity ratio of the laser fields.And the position of the maximum ellipticity almost corresponds to the position of the minimum harmonic intensity.We also investigated the angular distribution of the ellipticity of the odd-and even-order harmonics.The results show that the odd-and even-order harmonics have different polarization directions,and the pointing angles of the ellipticity distribution of the same order is not sensitive to changes in laser intensity ratio.We illustrate physical mechanism of the magnitude of harmonic ellipticity by the phase difference and intensity difference between the two components of the harmonics.In addition,the two-center interference effect of molecules can affect the harmonic ellipticity by affecting harmonic intensity difference of harmonic components.（2）We investigate the effects of initial electronic states（ground state 1sσ_g and first excited state 2pσ_u）of hydrogen molecular ions on EP HHG.The results indicate that it is more conducive to synthesizing isolated EP attosecond pulses for 2pσ_u state.In addition,the ellipticity is more sensitive to changes in alignment angle for 1sσ_g state than that for 2pσ_u state.The position of the maximum ellipticity increases first and then decreases with the increase of molecular alignment angles,which can be illustrated by the two-center interference model and molecular orbital symmetry.We further investigate the influence of the carrier-envelope phase of the laser fields on the harmonic ellipticity.The results indicate that the EP attosecond pulse with high intensity and large ellipticity ε=0.77 is synthesized for 2pσ_u state.（3）We investigate the resonance-enhanced HHG from helium ions driven by the EP laser field.The results indicate that the harmonic spectrum exhibits two orders resonance-enhanced harmonic peaks for different driving laser ellipticities.Analysis shows that the two orders resonance-enhanced harmonics are due to the bound-bound transitions.Through time-frequency analysis of the resonance and non-resonance harmonics,the generation of the resonance harmonics follows the multi-photon transition mechanism,while that of non-resonance harmonics follows the three-step model mechanism.We further adjust the laser intensity and find that the two resonance harmonic intensities decrease gradually and orders do not change with the increase of the laser intensity,which can be illustrated through the Stark effect.In addition,we investigated the effect of the driving laser ellipticity on harmonic ellipticity.The results indicate that the ellipticity of resonant harmonics is almost equal to the driving laser ellipticity,which might provide an accessible route toward characterizing the polarization properties of the laser field.（4）We investigate the HHG from the mixed gases of N₂ and Ar driven by a combined laser field,which is combined with the EP laser field and the LP laser field.The results indicate that the harmonic intensity from the mixed gases is stronger than that from the two single gases within a certain wide spectral range,which is due to the conducive interference effect between the harmonics generated by two single gases.We can find that the bright high-order harmonics with large ellipticity can be obtained from the mixed gases at the specific mixed ratio.The trajectories of the electrons and intensity difference between the two components of the harmonics are investigated to illustrate the origin of large ellipticity.In addition,analysis shows that the left-handed harmonic components of two single gases produce destructive interference,while the right-handed harmonic components produce destructive interference,resulting in the increase in the intensity difference between the left-handed and right-handed harmonics from the mixed gases.Thus,the harmonic ellipticity from the mixed gases is large.The results are conducive to obtaining near CP attosecond pulse with high intensity from the mixed gases in a broad spectral range.