Design Of Metasurface Structures For Controlling Vortex Electromagnetic Waves And Imaging

Metasurface is a two-dimensional ultra-thin plane structure composed of a series of subwavelength artificial structures.Compared with three-dimensional metamaterials,the manufacturing difficulty of the two-dimensional single-layer structure is significantly reduced,and the existing integrated circuit manufacturing process can be used for production.At the same time,the absorption loss of the metasurface is much smaller than metamaterial.In addition,due to the advantages of volume,the metasurface can be easily integrated and compatible with current electromagnetic or optical systems.Vortex electromagnetic wave is a kind of special electromagnetic wave with axial phase singularity.Its spiral wavefront can generate orbital angular momentum.In recent years,people have begun to try to generate and apply vortex electromagnetic waves by using metasurfaces,because the research of metasurfaces in recent years has demonstrated their excellent wavefront control ability.The main work of this thesis is to design a kind of metal split-square-ring unit cell structure to achieve efficient control of vortex electromagnetic waves in terahertz and microwave bands;In addition,vortex light field control in visible and near-infrared bands is realized by using all-dielectric titanium dioxide and silicon metasurface,and some imaging-related applications are discussed.The main research contents and innovations of this thesis are as follows:1.A kind of metasurface composed of metal split-square-ring elements is designed.Firstly,the principle of phase control of element structure is analyzed.This unit is composed of a metal antenna and a metal wire grating or a metal ground plane.Its quasi-Fabry-Pérot cavity mode can effectively improve the proportion of cross-polarized reflected waves.By changing the size of the unit cell,it can work in terahertz or microwave band.In this thesis,the near-field focusing characteristics of terahertz and microwave metasurfaces are calculated,and the mode decomposition is used to verify that the designed metasurfaces can generate high-purity vortex wave modes.In order to verify the actual performance of this design,the far-field simulation and experimental test of the microwave metasurface are also carried out,and the results are close to the numerical simulation.2.In visible region,a super-resolution imaging method using metasurface assisted super-oscillation is designed.First of all,the thesis shows the element characteristics of the metasurface.Based on Jones matrix,the function of the element is analyzed theoretically,and the phase shift characteristics of the element are analyzed by FDTD simulation.Then,the designed unit structure is used to arrange the entire metasurface.The design of the metasurface array is based on the polarization vector distribution of the radial polarized Laguerre-Gaussian beam electric field.The numerical calculation of the metasurface array is carried out,and the output light intensity of the metasurface is displayed,and compared with the theoretical results.The simulation results are in good agreement with the theoretical results.Next,the paper analyzes the generation of super-oscillation hotspots excited by focused radially polarized Laguerre-Gaussian beams.By changing the size parameters,the super oscillation hot spot was successfully generated.Finally,the paper simulates the point spread function distribution of the confocal imaging system using the designed metasurface,as well as the imaging characteristics of random point objects and strip objects.Using post-processing methods such as deconvolution,we can theoretically achieve nearly twice the lateral resolution.3.Based on all-dielectric silicon metasurface,perfect vortex beams carrying integer and fractional topological charge are realized in near infrared region.In this thesis,a nano-pillar with half-wave plate characteristics is designed by numerical simulation,and then a high-performance phase-only metasurface is designed by combining geometric phase.The metasurface has the characteristics of broadband and high polarization conversion efficiency in the near-infrared range.Different metasurfaces are designed to generate perfect vortex beams with different topological charges and incident wavelengths,and elliptical perfect vortex beams with different topological charges.In addition,the paper also designs a metasurface for generating elliptical fractional perfect vortex beams,and studies the influence of the spiral phase distribution on the intensity distribution.The formation efficiency of the metasurface can reach up to 80% and remains above 50% in a relatively wide wavelength range(780-880 nm).4.An all-dielectric metasurface for generating quasi-ring Airy vortex beams has been proposed.This type of metasurface is based on geometric phase and can operate in the near-infrared band.This thesis designs the unit structure of the metasurface and uses parameter scanning to select the most suitable geometric parameters of the unit structure.Then,using the designed unit structure,based on the phase distribution of the quasi-ring Airy vortex beams,the metasurface were arranged using phase superposition.This thesis designed several metasurfaces,discussed the effects of topological charges and other parameters on the generated quasi-ring Airy vortex beams,and verified the controllability of the focal length of the autofocusing beam generated by the metasurface.The mode purity of the generated quasi-ring Airy vortex beams at their focusing position was calculated,and the calculated mode purity can reach up to 90%.In addition,this thesis also simulated the propagation of quasi-ring Airy vortex beams and perfect vortex beams generated by the metasurface in the presence of obstacles,verifying the self-healing characteristics of quasi-ring Airy vortex beams.