Study On The Transformation And Conservation Of Angular Momentum In High-harmonic Generation

High-harmonic generation is an important nonlinear process in which high intensity laser pulses interact with atoms and molecules.It is a high-frequency radiation source with a frequency that is an integer multiple of the laser frequency and up to hundreds of orders.The angular momentum of transformation and conservation in strong laser field are one of the hot issues that are often discussed recently.The angular momentum transfer in the high-harmonic generation has also attracted a great deal of attention and become one of the frontier topics in the field of strong laser physics.In this paper,the full quantum non-perturbation scattering theory is used to study the transformation mechanism and conservation law of spin angular momentum in the high-harmonic generation of atoms excited by a strong laser field,and reproduce the experimental observations reported in Nature Photonics.The main research contents and results of this paper are as follows:(1)The harmonic spectrum of argon atom in the two-color counter-rotating circular polarization fields with different frequency combinations and different laser intensity ratios is studied.According to a pair of phased generalized Bessel functions,we obtain that the number of photons transformed in two laser modes has the characteristic ofq₁(28)q₂?1,and determine that the harmonic order satisfies the selection law of nq?1.And the nq(10)1 orders harmonics have the same helicity as that of the?-field,while the nq-1 orders harmonics have the same helicity as that of the 2?-field.We find that the nq?1 harmonics depend on the laser intensity of the fundamental frequency field and the laser intensity of the frequency doubling field,respectively,and this property can be used to select the specific harmonics.In addition,by comparing the harmonic spectra of xenon atom and argon atom in the two-color counter-rotating circular polarization fields,it is found that the smaller the atomic beam energy,the higher the harmonic generation rate.(2)High order harmonic generation of argon atom in circular polarization+elliptic polarization two-color laser field is studied.The equilibrium mechanism of spin angular momentum in harmonic generation process is confirmed,and the variation of harmonic production with elliptic polarization degree is reproduced.The elliptically polarized laser field is decomposed into two same frequency,opposite rotation and intensity varying with the degree of polarization.According to the phase of Bessel function phase,we found that the number of photons converted into harmonics by three laser modes satisfiesq_L(28)q_R?1.The spin angular momentum is balanced between the harmonics and left/right circularly polarized photons,which proves that the harmonic generation in elliptically polarized laser field is a parametric process.(3)The analytical expression of harmonic generation in a three-mode laser field is derived and used to study the higher order harmonic generation of argon atom.The variation of harmonic generation in a specific channel with the degree of elliptical polarization is confirmed,and the experimental observation reported in Nature Photonics is reproduced.We find that the harmonics of the same order have multiple generation channels;with the elliptic degree of polarization,the harmonic yield in one channel has multiple maximum value,which presents multiple island structures in the two-dimensional graph.The distribution of the island structure is symmetric about the degree of elliptic polarization 90°,and the region is related to the number of photons converted in the second laser field.The closer it is to 0,the closer the island structure distribution is to the region of 90°.In addition,we find that when the?-field is elliptically polarized field,the distribution of the island structure is V-shaped with the degree of elliptical polarization,and the number of photons absorbed from the first laser mode is fixed;when the 2?-field is elliptically polarized field,the island structure shows an inverted V-shaped structure with the elliptic polarization degree,and the number of photons absorbed from the third laser mode is fixed.This feature is revealed for the first time by our study and is caused by coupling between laser modes.