Multifunctional Magnetization Field Induced By Vector Light Beams

The processing and recording of massive data has become a major demand in the field of information industry.Although the current advanced storage techniques alleviate the pressure to a certain extent brought by such a huge data,they all face several problems in data storage such as low density,high energy consumption and slow speed.Magneto-optical storage technique is based on the interaction between optical fields and magneto-optical materials,which integrates all the advantages of magnetic storage and optical storage.This technique is expected to become one of the effective and reasonable ways to solve the massive information transmission,processing and storage in the era of big data.By regulating amplitude,phase,polarization,angular momentum and time of vector light beams,this thesis is capable of realizing the robust longitudinally polarized superoscillation magnetization fields,polarization-tunable and twisting-controllable subdiffraction magnetization fields,extended super-resolved magnetic vortex cores,as well as ultrafast demagnetization,magnetic switching and magnetically polarized conversion based on vector diffraction theory and inverse Faraday effect,which offers theoretical guidance for the development of high-density,large-capacity,lowenergy-consumption,and high-speed all-optical magnetic storage technique.The thesis mainly carries out the following works:The longitudinal magnetization superoscillation can be induced by the use of azimuthally polarized Laguerre-Gaussian vortex modes under tight focusing to excite the isotropic magneto-optic media.First,we give the criterion of tightly focused light-induced magnetization superoscillation.Furthermore,the effects o f the radial mode coefficient and truncation parameter of the azimuthally polarized Laguerre-Gaussian vortex beams on the focal magnetization fields are analyzed in the high numerical aperture(NA)objective lens focusing system,in which the conditions to yield light-induced magnetization superoscillation are preliminatively determined.Subsequently,by changing the truncation parameters of the first-order,third-order and fifth-order azimuthally polarized Laguerre-Gaussian vortex modes,the variations on the transverse full width at half maximum of the light-induced magnetization fields from the subwavelength size via the fastest Fourier transform regime to the superscillation dimension are studied in detail,and the parameter ranges to generate the superoscillation magnetization fields are defined.Then,from the perspective of the small transverse magnetic domain,the moderate energy conversion efficiency and the low side lobe,we discuss and compare the magnetization field characteristics of different-order azimuhallly polarized Laguerre-Gaussian vortex beams after tight focusing,finding that the high-order Laguerre-Gaussian modes are more suitable to produce the excellent longitudinal magnetization superoscillation.Finally,the physical mechanism of ge nerating light-induced triggered longitudinal superoscillation magnetization field is explained.Based on the coherent coupling of two-crossed azimuthally polarized doughnut Gaussian vortex beams,we are able to realize the magnetization fields with tunabl e polarization states and controllable twising textures in a single high NA objective lens focusing geometry.Fisrt of all,the possibility to trigger polarization conversion and spatial rotation of the light-induced magnetization fields is illustrated in principle.Moreover,the magnetization field distributions and the rotation expressions related to the Gouy phase shift induced by double-beam azimuthally polarized doughnut Gaussian vortex beams are derived theoretically,and the dependence of them on both the incident angle and topological charge is analyzed.Second,when the topological charges of two vector vortex beams remain unchanged,we depict the dependence of magnetic phase diagram and magnetic rotation with incident angle,thus confirming the angle range of magnetic polarization conversion and magnetic twisting.According to theoretical analyses,the flexible conversions among longitudinal and transverse and 3D polarization orientaions of the light-induced subdiffraction twisted magnetization fields determined by certain special angles are demonstrated.At the same time,the relationships the rotation direction,twisting strength and twisting pitch and both the incident angle and the topological charge are elaborated.Finally,for the case of the s ame incident angle,we study the dependence of the rotation direction(including switching)and the distortion degree,the number of twisted domain of the magnetization field on the topological charges of two vectorial vortex beams.The scalable super-resolved magnetic vortex core can be achieved by utilizing tightly focused two counter-propagating radially polarized doughnut Gaussian vortex beams to impinge onto the isotropic magneto-optical materials.Theoretically,the mutual relation of different polarized magnetization field components is clarified,and thus the spin polarization distributions needed to realize the light-induced vortex core is illuminated.In a single high NA objective lens and a 4π focusing system,the effect of wave-front vortex phase on the magnetization characteristics induced by the radially polarized doughnut Gaussian beams is examined.Meanwhile,the relavant machnisms to produce subwavelength magnetic vortices,3D super-resolved magnetic vortex core and their reversal are dissected.We use the particle swarm optimization algorithm to optimize the multi-ring hybrid filters to modulate the radially polarized doughnut Gaussian vortex beams.It is found that we are able to create transverse super-resolved magnetic vortex core needle in the single high NA lens focusing system,whereas we can garner 3D super-resolved magnetic vortex core chain texture in the 4π focusing system.Then,the effects of different filters on the spatial resolution,uniformity and axial length of the light-triggered magnetization vortex core needle(chain)are compared.The ultrafast variation features of magnetization field induced by a time-dependent radially polarized vortex laser pulse focusing through a high NA objective lens is studied.Firstly,a single radially polarized femtosecond laser is employed to excite the isotropic magneto-optical medium,and the regulation rule of vortex order and laser pulse propagation time on the tightly focused light filed and light-induced magnetization field is explored.We further analyze the basic phenomomena on the intensity change,spatial structure,and polarization conversion of the light fields in the focal plane,as well as the strength variation,sign reversal,and polarization conversion of the induced magnetization fileds.We also elucidate the physical mechanism of ultrafast quasi-periodic modulation of light-induced magnetization fields mediated by the laser propagation time.Moreover,the tunable characteristics of the tightly focused light field and related magnetization field of the time-dependent two-beam radially polarized vortex femtosecond lasers are investigated.On the one hand,when the absolute values of topological charges of two vortex beams are equal,the light-induced magnetization field is mainly determined by the transversally polarized component,and the ultrafast quasi-periodic change occurs with the time.On the other hand,for the case of unequal absolute values of topological charges of two vortex beams,it is found that the focal light-induced magnetization field has several phenomenona such as fast demagnetization,magnetic switching,and coversions of different magnetic polarization components.Lastly,the ultrarfast variation of light-induced magnetization field with time in the defocus region is studied,and the relationship between topological charge number of two beams and magnetization field rotation is expounded.We also analyze the factors affecting the rotation direction of light-triggered magnetization fields and uncover the relevant physical reasons.