Theoretical Study On Generating Method Of Tunable Super-resolved Optical And Magnetization Fields

In recent decades,two-and three-dimensional super-resolved optical and magnetization fields have become hot spots for research,because they have been playing important roles in various fields such as optical storage,particle manipulation,all-optical magnetic recording,and fluorescence imaging.To further expand their applications,it is necessary to get extreme size and make them tunable in terms of size,spin,and polarization.Based on the Richards-Wolf vector diffraction theory,this paper has realized to control the fields in various aspects by choosing the incident vector beams and the focusing systems.The main contribution of this paper are as follows:A magnetization field with an aspect ratio above 2000 was generated by a spherical mirror in this paper.For a focusing system with large aberration,the ray of the incident light at different radial positions can be focused on different positions on the optical axis,thereby generating a ``focal line " instead of a focal point,by which an ultra-long longitudinal full width of half maximum(about 1000 wavelengths)can be realized.If a suitable magneto-optical material is placed in the optical field,a magnetization field can be generated by the inverse Faraday effect.By controlling the position and width of the incident annular beam,the longitudinal full width of half maximum and aspect ratio of the generated field can be tunable.Based on the same principle,this paper also proposes a quasi-parabolic mirror modified from a parabolic mirror.This mirror can also generate such fields.This kind of optical/magnetization fields are two-dimensional super-resolved fields.They have small lateral and large longitudinal sizes.Hence,they are also called``optical/magnetization needles".A single focusing system can produce a needle-like field.If two identical systems are used to form a confocal system,the two needle-like fields can interfere to produce a chain-like field.These chain-like fields are generally called "optical/magnetization chains".Each unit of a chain field can be a two-or three-dimensional super-resolved field.In this paper,two annular parabolic mirrors are used to study this type of field.By controlling the incident beams,each unit of the chain fields can be transformed between the two-and three-dimensional super-resolved field.In this paper,the orbit-spin angular momentum conversion of the incident field to the focused field is studied,which has important guiding significance for generating a super-resolved optical field with controllable spin orientation.First,linearly polarized vortex beam is taken as an example to prove that vortex incident light with orbital angular momentum will break the anti-symmetry of the longitudinal component of the spin angular momentum of the focused optical field,resulting in non-zero longitudinal spin angular momentum near the focus.Then arbitrary parallel incident beams are studied.The results show that the longitudinal spin angular momentum near the focus is derived from the non-zero topological charge of the incident beam;the transverse spin angular momentum near the focus is derived from the interaction between components which possesses adjacent topological charges.Based on the above principles,this paper chooses radially polarized beam whose topological charge is 0 and azimuthally polarized beams whose topological charges are ±1 as incident beams to generate a super-resolved field with tunable spin orientation.By controlling the weight of these three beams,it is possible to control the spin orientation of the focus.In addition,this paper proposes a method to quantitatively evaluate the spin homogeneity focused optical field.The results show that the spin homogeneity of the focused optical field produced in this paper can be larger than0.996.This paper proposes a method to generate a super-resolved optical field with tunable three-dimensional polarization.A radially polarized beam whose topological charge is 0will generate longitudinally polarization at the focus.Azimuthally polarized beams whose topological charges are ±1 can produce left-and right-handed polarization at the focus.These three components constitute a set of orthogonal basis of polarization.Hence,by controlling the weight and phase of these three beams,complete control of the three-dimensional polarization of the focus can be achieved.In addition,this paper proposes a method to quantitatively evaluate the polarized homogeneity near the focus.The results show that the polarized homogeneity near the focus produced by this method is above 0.977.