Research On The Transformation Mechanism And Conservation Law Of Angular Momentum In Multiphoton Processes

The transformation of angular momentum and its conservation law under strong field is one of the most frequently studied problems in physics.High-order harmonics is a very important high-order nonlinear effect in high-power laser physics,and the conversion of angular momentum during its generation has attracted surge of interest recently,which has rapidly become a leading issue in intense laser field physics.In this paper,the full quantum non-perturbed scattering theory is used to quantize the laser field and the atom at the same time,and the full quantized Volkov state is used to describe the state of electron motion in the laser field,and the analytical expression of harmonic yield is obtained.On this basis,the high-order harmonic energy spectrum and polarization spin generated by high intensity laser excitation are studied,revealing the transformation mechanism of angular momentum and the embodiment of conservation law in multi-photon process under strong field conditions.The paper includes main contents and results:(1)The angular momentum transformation in the process of harmonic generation in circularly polarized single-mode laser field is studied,and a convenient way to study the angular momentum transformation is established.We describe the multi-photon transition of an electron in a high-intensity laser field by using the phase Bessel function,which includes the spin of the photon and the momentum of the electron.According to the phase information of the phase Bessel function,we establish the physical mechanism that harmonics cannot be generated in the circularly-polarized field: the spin angular momentum of circularly-polarized photon is 1,and the requirement of the conservation of angular momentum makes it impossible for multiple pumped photons to synthesize a harmonic photon in the single-mode laser field,so the higher harmonics cannot be generated.(2)The conversion of angular momentum in harmonic generation is studied by simulating the elliptically polarized laser field with an opponent,homo-chromatic two-mode laser field whose intensity varies with the degree of elliptical polarization,and the main characteristics of harmonic generation in the laser driven field with different polarization are reconstructed.It is found that the number of photons converted into harmonics by two laser modes must be 1 apart in order to achieve the balance of spin angular momentum,so the harmonic generated only contains odd order.The numerical results show that the harmonic generated in the linearly polarized laser field is linearly polarized and the spin angular momentum is 0.Because the intensity of left-handed and right-handed components is equal.The harmonic generated in the elliptically polarized laser field is elliptically polarized,and the spin angular momentum and yield vary significantly with the degree of elliptically polarized laser field.These characteristics are consistent with the existing experimental observations.(3)The angular momentum transformation during the generation of high-order harmonics in the two-color circular polarization laser field is studied,and the balance mechanism of angular momentum in the harmonic generation in the laser driven field with different curl is established.It is found that the spin angular momentum can be balanced and harmonics can be generated as long as the number of photons converted into harmonics in different laser modes of different curl is 1,but the intensity depends on the value of Bessel function.Therefore,harmonics can be generated in the laser field of circular polarization with continuous changes of order,and the intensity of high harmonic is low and fast attenuation.The plateau and cut-off of high harmonic spectrum are not obvious.In the dual mode circular polarization field of counter-rotation,the high order harmonics,the high order harmonics order discontinuous change,left-handed harmonics and right-handed harmonics appear alternately.The high harmonic intensity is higher,and the plateau and cut-off of the high harmonic spectrum are obvious.(4)Three circularly polarized laser modes are used to simulate the circularly polarized elliptically polarized two-color field.The conversion and balance mechanism of angular momentum during harmonic generation is established,and the characteristics of harmonic generation with the degree of elliptical polarization are reconstructed.According to the phase relation of the phase Bessel function.We draw a conclusion the spin angular momentum conservation mechanism of harmonic generation in the three-mode laser field:the distinction between the number of left and right photons converted to harmonic is 1,and the net spin angular momentum is converted to the spin angular momentum of harmonic photons.When the fundamental frequency is elliptically polarized,the maximum harmonic output is V-shaped.When the frequency is doubled elliptically polarized,the maximum harmonic output presents an inverted V-shape distribution with order.This is consistent with experimental observations in Nature Photonics.