Study About The Generation,Modulation And Propagation Characteristics Of Vortex Based On Metasurface

Vortex beams with the spiral phase wavefront provide more freedoms for the optical field control since they carry the orbital angular momentum,so it has broad applications in optical communication,optical micro manipulation and information coding.Optical vortices are often generated by means of conventional optical components such as spiral phase plates,gratings and optical fibers,the accumulation effect of optical waves including the absorption,optical activity or frequency dispersion along the propagation direction is needed.In recent years,metasurface optical components have attracted wide attention due to their compact and ultrathin structure and the advantages of easier optical integration compared to conventional optical components.The metasurface can manipulate light wave by introducing the abrupt phase to the transmission field through the interaction of ultrathin nanostructures with light.It provides a key technology for the development of various ultrathin optics and advanced planar photonics,and becomes a new idea to generate optical vortex.Therefore,the study about the optical vortex and its transmission characteristics based on metasurface or nanostructure is of great significance.With the help of the optical metasurface,this paper studies the wavefront regulation based on the abrupt phase caused by nanostructures,and it refers to the generation and the modulation of optical vortex and its transmission characteristics depending on designing metasurface structure.The innovative work of this paper includes the following aspects: The first one is we design an optical vortex generator without polarization dependence,the second one is we design a spatially multiplexed super lens and a focus vortex lens,and the third one is we propose optical vortices with variable topological charges are generated based on helical slits.The metasurface vortex generator has the advantages of simple structure and flexible function control,and our work must have an important influence on expanding the applications of vortex beams.The specific content of the paper is organized as follows.The first chapter is the introduction of this thesis.This section introduces the research background of metasurface and optical vortices,and the content to be carried out in this paper.Metasurface is the study foundation of this paper.This chapter first introduces the basicknowledge of metasurface.The matrix description of the polarized light and the polarization elements is given,including the description in circular polarization basis.This chapter also describes the vortex beam,the traditional generation methods of vortex light including spiral phase plate method,laser cavity modulation method and computational holography,etc.,In view that these methods are bulky and inconvenient to optical integration and other problems exist,metasurface vortex generators provides a good road to solve this problem.At the end of this chapter,several kinds of optical vortex generators based on metasurface are introduced,including the vortex generator composed of phase-increasing nano-antenna,the spatially reusable vortex generator designed by using geometrical phase shift of nano-hole,and the variable topological load vortex generator generated by Archimedes' spiral grooves.The second chapter is the study about the optical vortex generator without polarization dependence.In view of the limitation that the vortex generator is strongly dependent on the polarization state of incident light,we designed a plasmonic vortex generator that can generate optical vortices under the illumination of linearly polarized light with arbitrary polarization state.The device is composed of two sets of rectangular holes etched on the silver film,which are uniformly arranged on a ring and rotate according to the set angles and directions.The design principle of the proposed plasmonic vortex generator is given by theoretical analysis,the numerical simulation provides guidance for the design of geometric parameters of vortex generator,and the experimental measurement verifies the performance of the proposed vortex generator.We also give two deformations of vortex generators without polarization dependence.Plasmonic vortex generators have compact structures and flexible lighting conditions and can produce perfect optical vortices.In chapter three,a kind of spatial multiplexing metalenses based on metasurface is proposed,which realize the beam focusing or optical vortex.These lenses consist of the same cross holes etched on the silver film,and each of these cross holes can be equivalent to a half-wave plate,which can achieve complete cross-polarization conversion of circularly polarized light.The rotation of the cross hole induces geometric phase shift,thus the metasurface can shape the light wave front into spherical shape and helix shape.An optical vortex metalens with multiple focal spots or multiple vortiecs with different topological charges is designed by combining several metasurface structures.The proposed spatiallymultiplexing metalens has the advantages of multiple functions,high signal-to-noise ratio and ease to operate.In chapter four,a vortex beam generator based on nanometer spiral slit is proposed,and the evolution law of optical topological charge with propagation distance is explored.The optical vortex generated by the traditional method has a fixed optical topological charge.In comparison,the topological charge of the optical vortex generated by our proposed vortex generator changes with the propagation distance.The topological charge can be controlled by adjusting the opening gap of spiral slit and the propagation distance.Theoretical analysis predicts the change of topological charge of the vortex field generated by the nanometer spiral slit.Numerical simulations and experimental measurement verify the performance of the designed device.The influence of the structure and the shape of the spiral slit on the optical vortex is also discussed.This work provides a theoretical basis for generating vortex fields with variable topological charges.Chapter five gives the summary of this thesis.In this chapter,not only the main content and innovation points of this paper are summarized,but also the deficiencies and unovercome difficulties of this paper are also pointed out.In the end,we also show the following work plan.