Research On The Generation Method Of High-order Harmonics With Controllable Polarization State

High-order harmonics are a kind of high-energy coherent electromagnetic radiation generated by a strong laser field acting on atoms,molecules,and solids.Its waveband covers extreme ultraviolet to soft X-rays.Due to the ultra-short time scale(femtosecond order)and superior coherence,high-order harmonics have important applications in diffraction imaging,detection and control of electronic ultrafast dynamics,and measurement of the circular dichroism of molecules and magnetic materials.Limited by its generation mechanism,laboratories are more likely to generate linear polarization high-order harmonics.How to generate elliptically polarized,circularly polarized high-order harmonics and attosecond pulses has become a new hot research issue in strong field physics in recent years.Based on the numerical solution of the two-dimensional time-dependent Schrodinger equation,this paper proposes and verifies a method for generating high-order harmonics with a controllable polarization state.We use a counter-rotating two-color combination laser field consisting of an elliptically polarized fundamental frequency field(800 nm)and a circular-polarized triplex frequency field(267 nm)to drive He atoms.It is found that the polarization states of the harmonic radiation(including the ellipsicity,polarization direction and the direction of electric vector rotation)strongly depend on the laser parameters of the fundamental frequency field.By changing the ellipticity,intensity and phase of the fundamental frequency field,the complete control of the polarization state of the higher harmonics from circular polarization to elliptical polarization and linear polarization is achieved.By analyzing the dependence of the intensity ratio and phase difference of the two orthogonal directions of the harmonics on the laser parameters,the reasons for the modulation of the harmonic polarization state are clarified.This research provides a new method for the laboratory to produce ultraviolet-soft X-ray light sources with controllable polarization states.