Research On Generating Orbital Angular Momentum Beam Based On Metasurface

Recent years,the vortex electromagnetic wave carrying the helical phase wavefront has attractived great interest from researchers in various countries.Due to the helical phase wavefront distribution characteristics of vortex electromagnetic waves,it has broad application prospects in the fields of communication transmission,quantum coding,particle manipulation,optical imaging,and lithography,especially in the communication.Since the orbital angular momentum is a new dimension to describe the physical properties of electromagnetic waves,the orbital angular momentum(OAM)can greatly improve the spectrum efficiency of the communication system if OAM beam is encoded separately or carried as a carrier combined with traditional wavelength division multiplexing,orthogonal amplitude technology and polarization division multiplexing,etc.In order to push the vortex electromagnetic wave into practical application,it is necessary to conduct research on the generator that generates orbital angular momentum.The most common method is to use spiral phase plates,array antennas and optical devices and other structures to make the wavefront of electromagnetic waves into spiral phase distribution to achieve.In recent years,people have proposed a new method,that is,the use of metasurfaces to control the electromagnetic wavefront.The helical distribution of the electromagnetic wave phase can be obtained by reasonably designing the cell structure form and arrangement of the super surface,so as to realize the vortex electromagnetic wave.On the other hand,the metasurface is a two-dimensional structure,which has the advantages of ultra-thin thickness,simple structure,low loss,low cost,and easy processing.Therefore,it is of great significance to conduct research on the electromagnetic characteristics of metasurfaces and use it to generate orbital angular momentum.The main work and innovations of this article are as follows:1.We deeply study the electromagnetic characteristics of the transmissive multilayer metasurface.From the perspective of the equivalent circuit,then unit cells of metasurface structure are equivalent to a generalized waveguide structure.Using the mode matching method and mathematical derivation,the analytical expressions of the impedance and scattering parameters of metasurface are obtained.By comparing with the full-wave analysis method,the accuracy and efficiency of our proposed equivalent circuit model are verified.At the same time,the proposed equivalent circuit model can effectively analyze the transmission characteristics of multilayer metasurfaces with strong electromagnetic coupling,and provides a new method for design of ultrathin multilayer metasurfaces.2.We deeply study the physical mechanism of vortex beam generated on the metasurface,and the mechanism is combined with the equivalent circuit method to guide the design of the metasurface.At the same time,the metasurface is further modified into OAM generated by using the PB phase principle.Compared with the existing work,the proposed devices have many fascinating characteristics such as ultra-thin thickness,broad bandwidth and high transmission efficiency.3.The electromagnetic transmission characteristics of the bi-layer metasurface are thoroughly studied,and the effect of the coupling characteristics of the bi-layer metasurface on the transmission characteristics are studied by the equivalent circuit method.On this basis,the two bi-layer metasurface structures are designed,one of which improves its working bandwidth by changing the interlayer coupling characteristics of the metasurface,which resulting in an ultra-thin orbital angular momentum generator with high transmittance.And the working performance of the device is verified by simulation and experiment.