Research On Focal Characteristics Of Double-mode Fractional-order Vector Fields

Spin angular momentum provides a new degree of freedom for optical manipulation by transferring the anmular momentum to the particle.In theory,the interaction between light and nano particles will be affected by the spatial distribution of physical parameters such as energy flux and angular momentum.Therefore,in micro-nano optics,vectorial optical fields with spicific spatial distribution of state of polarization have important research significance and application prospects.At the same time,the novel physical properties caused by the complex beams have promoted the study of the properties of the beam itself in turn.According to the generation technology of polarization control of vectorial optical fields,a type of vectorial optical fields have been proposed including fractional-order vector fields,hybridly polarized vector fields.These beams expand the range of vectorial optical fields and provide more options for optical manipulation and particle trapping.In recent years,the transverse spin angular momentum in the focal region has attracted the attention of scholars.The transverse spin angular momentum perpendicular to the propagation direction will cause the particle to generate a lateral spin force under the torque.The main research innovations of this thesis are as follows:(1)The evolution characteristics of the spin angular momentum of the double-mode fractional vectorial optical fields in the focal field are studied;(2)A generalized hybrid polarized vectorial optical field is proposed,and a focal field with pure transverse spin angular momentum is generated by adjusting the phase difference.The main contents of this thesis are as follows:1.Several typical generation techniques of vectorial optical fields are introduced,and the generation principle and the detection methods of state of polarization are explained.The 4f system based on the beam coherent principle and its advantages are emphatically introduced,and the fractional-order vectorial optical fields with different topological coefficients generated by this method are listed.In addition,we also introduce the focal evolution characteristics of the fractional-order vectorial optical fields and discover the separation of spin angular momentum at focus.2.The evolution characteristics of weakly focused annular fractional-order vectorial optical fields based on double-mode superposition are studied.Firstly,based on the fractionalorder modal superposition theory,the superposition mode and parameter influence of the double-mode fractional topological coefficients are analyzed,and the key vector parameters are screened out.The expression of the focal electric field is derived theoretically.We experimentally proved that the spin angular momentum of the annular vectorial optical fields has the characteristic of separation and rotation under the influence of double-mode.The influence of key parameters on the distribution of spin angular momentum density is analyzed.In addition,both the ring radius and the position of the singularity have strongly effects on the distributions of both light intensity and spin angular momentum.3.A generalized hybridly polarized vectorial optical field combining dual modes is proposed.The dependence of evolution characterisitcs of its focal spin angular momentum on the state of polarization of the incident field is studied.Firstly,based on the principle of MachZehnder interference,an experimental generation method for generalized hybrid polarized vectorial optical fields is proposed.The effect of the phase to the topological coefficient at focus is studied.The evolution characteristics of its polarization under weak focusing are verified in the experiments.Secondly,the three-dimensional transfering of the spin angular momentum density of generalized hybrid polarized vectorial optical fields under tight focusing is explored.It is found that the stable topological coefficient difference can realize the generation of pure transverse spin angular momentum under tight focusing.