Research On The Characteristics Of The Fractional Vortex Vector Light Tightly Focused Focal Field And Its Induced Magnetization Field

The vortex beam are a special beam,which has orbital angular momentum(OAM)and spiral phase wave fronts.Different from the integer-order vortex beam,the fractional vortex beam has fractional topological charges and zero central intensity,but there is a special radial opening in its ring intensity.The fractional vortex phase will lead to an asymmetric focal distribution of a tight focus vector beam,which will provide a new degree of control freedom for applications such as particle transport and screening,and all-optical magnetic recording.Therefore,for the full use of the fractional topological charge of the vortex beam to control the tight focus field,it is necessary to further study the tight focus theory of the fractional vortex vector beam and characteristics of its induced magnetization.The main research of this article is as follows:Firstly,the related theory of the integer-order vortex beam,Richards-wolf vector diffraction theory and the inverse Faraday effect are introduced.The distribution characteristics of the focal field of tight focus integral-order vortex vector beam and its induced magnetization field are numerically simulated.Secondly,the analytical expression of the tight focusing fractional vortex vector beam under approximate conditions is derived,and compared with the calculation method of Fourier transform to verify the correctness of the calculation.The characteristics of asymmetrically distributed focal field are further studied,and the periodic variation of focal field is summarized.Especially when the topological load of the vortex is a half-integer,the number of hot spots and dark points in the focal field has a positive correlation with the number of vortex.Finally,the asymmetric distribution characteristics of magnetization induced by fractional vortex azimuthally polarized beam are studied,and it is found that there are similar laws to the focal field.The discrete magnetization induced by the fractional composite vortex azimuthally polarized beam is further studied.By adjusting the values of the two vortex topological charges,a variety of magnetic spots with special shapes can be realized.