Manipulation On Spatial Structure And Orientation Of Focused Light Induced Magnetization

Interaction between light and magneto-optic(MO)material attracted intensive research interests,particularly in tight focusing region.On a scientific level,studying the physical mechnism between them can help us understand their relationship.From a technological perspective,understanding the interaction between them has tremendously practical value in ultrahigh density magnetic storage,super-resolved confocal and magnetic resonance microscopy as well as atoms or magnetic particle trapping and transportation.Therefore,it is of great theoretical and practical significance to explore the interaction between light and magnetic materials in far-field small-size.In this thesis,based on the vector diffraction theory and the inverse Faraday effect,via tightly focusing the amplitude,phase and polarization modulated incident light,we systematically study the magnetization structure shaping and polarization control,for example ultralong magnetization needle,magnetization chain,transverse magnetization arrays and magnetization spot with three-dimensional(3D)polarization.The main research contents and results are as follows:We have theoretically studied magnetization spatial structure generated by tight focusing of the amplitude and phase modulated incident light.By selecting optimized parameters of multi-Gaussian beam and topological charge of spiral phase plate,not only a super-long and sub-wavelength longitudinal magnetization needle with single/dual channels for a single lens high numerical aperture(NA)focusing system,but also an extra-long and 3D super-resolution longitudinal magnetization chain with single/dual channels for a4? high NA focusing system is achieved in the focal region.Further more,we also study the relationship between axial shift of magnetization chain and phase difference of two arms in 4? system.To extend the length and improve the axial homogeneity,the lightinduced magnetization distributions,for a high NA focusing configuration with an azimuthally polarized beam modulated by an optimized pure multi-zone plate(MZP)phase filter,are investigated.By making use of the compeletely destructive interference of its inter circle with ? phase shift between adjacent sub-annuli,and the capability to extend the constructive interference in the propagating direction through its narrow outer annulus modulated by three misplaced helical phases,an ultralong(107?)magnetization needlewith both transverse super-resolution(0.37?)and uniform axial field strength is achieved in the focal region.Considering an azimuthally polarized vortex beam with a Gaussian annulus as an incoming light,light induced magnetization fields for both a single high NA lens and a pair of high NA lenses are investigated theoretically.We deduce analytical formulas for the parameters of a magnetization needle and a magnetization chain when the angular width of the incident beam is far less than its central angular position.Through these analytical formulas,the properties of the magnetization needle and the magnetization chain are very clear and distinct.Compared with parameter optimizing to produce an ultralong magnetization needle with lateral sub-wavelength scale and a super-long spherical magnetization chain with 3D super resolution,the analytical method is direct and has a theoretical guideline.The validity of these formulas is proved,compared to numerical solutions.We propose a raytracing model for firstly constructing diffraction-limited light-induced magnetization spot arrays capable of dynamically controlling over transverse orientation of each spot.To achieve this goal,we subtly design a tailored incident light comprised of radially polarized beam and radially polarized beam imposed with ?-phase-step filter,sufficiently demonstrate to produce it through phase modulation of a radially polarized beam.Via tightly focusing counter-propagating composite illuminating beams in 4? optical microscopic configuration,two orthogonally polarized focal fields with ?/2 phase difference between them are formed,inducing a 3D super-resolved transverse magnetization spot in the MO film.Exploiting the ideal of the MZP filter,we further achieve versatile magnetization spot arrays with controllable in-plane direction in each spot.In order to gain more storage states,we need to achieve a robust magnetization spot bearing both super resolution and arbitrary orientation.Toward this aim,we meticulously devise a structured incident light consisting of radially polarized beam,azimuthally polarized beam imposed with ?-phase-step filter along the x-axis and azimuthally polarized beam imposed with ?-phase-step filter along the y-axis.Such newly configurable beams are tightly focused and isotropically interfered in a 4? microscopic configuration to create three polarized field components perpendicular to each other beyond the diffraction limit,thus enabling to yield a super-resolved magnetization spot possessing spatial axis.We further demonstrate the relationship between the amplitude factors of the reconstituent fields and the orientation of the magnetization spot.To facilitate the confocal and magnetic res-onance microscopy,we present a rotating incident beam after the entrance plane to firstly construct light-induced spherical super-resolved magnetization along with steerable orientations and trivial side-lobe levels.In ideal 4? optical microscopic configuration,we discuss the effect of the scaling parameter on the spherical magnetization spot and the influence of the rotatable azimuth angle on orientation of the magnetization filed.