| Light is a transverse wave,meaning that the polarization direction of light is perpendicular to the propagation direction defined by the Poynting flux.Usually,the polarization of light does not need to be considered when studying the diffraction propagation of light.However,in recent years,in many practical applications such as microscopy and data storage,close focusing is required,and the polarization properties of light need to be considered in these optical systems.Near the closely focused focal point,in addition to the traditional transverse polarization component,there is also a longitudinal polarization component,whose vibration direction is consistent with the propagation direction of light.With the development of research,more and more application fields such as surface plasmon excitation,scanning microscopy imaging,laser lithography,light capture and manipulation involve the longitudinal component of the light field.However,the longitudinal polarized light field generated by traditional compact focusing systems is very weak and difficult to meet application requirements.This thesis focuses on the application requirements of longitudinally polarized light fields and conducts relevant research on how to generate longitudinally polarized light fields that meet specific requirements.By inserting a phase mask element in a high numerical aperture focusing system,a uniform longitudinal polarized light field with a specific intensity distribution required for the above application is generated in the focusing area.This thesis mainly proposes two regulatory methods:1.By inserting a loop phase mask into a tight focusing system,designing the phase mask with an optimization algorithm,and modulating the phase of the incident radially polarized Gaussian beam,a uniform "optical needle" with a long focal depth and high energy conversion efficiency can be obtained on the optical axis of the system.2.The design of annular phase mask is expanded to a two-dimensional phase mask case,and the optimized design scheme is changed accordingly to obtain a longitudinally polarized light field with a specific pattern distribution in the output focus plane.In this thesis,a quarter wave plate is used to adjust the Gaussian beam output from a common laser to circularly polarized light,and then the radially polarized Gaussian beam is obtained through a spiral phase plate and a combined polarizer in turn;A multi-objective and multi-constraint optimization objective was established based on the experimental instrument parameters,and the optimization design of diffractive optical elements was completed using simulated annealing algorithm;Efficient generation of longitudinally polarized light needles and longitudinally polarized light fields with specific distributions using diffractive optical elements and tight focusing systems;Formulas were derived based on vector diffraction theory,and numerical calculations and feature analysis of the focused vector light field were completed.Compared to other methods,the optical system of this method is simple,and the incident beam used is the ordinary output mode of the laser.The output longitudinally polarized light field has high energy conversion efficiency,high uniformity of electric field intensity,and is convenient for practical application.The method of generating longitudinally polarized light field in this thesis can also be applied to the generation of other structured light. |
