Manipulations Of Electromagnetic Waves By Transmission-type Metamaterials

Metamaterials are artificial composite materials that are composed of periodic or non-periodic unit cells,which have exotic electromagnetic properties that can be used to flexibly tailor the amplitude or phase of electromagnetic wave.Metamaterials provides a new freedom for the design and optimization of microwave devices.Electromagnetic devices based on metamaterials have been widely used in many application,in which metamaterials and metasurface are two typical types.This thesis mainly focus on manipulations of electromagnetic waves based on metamaterial lenses and metasurfaces.The main contributions and contents of this thesis are listed as follows:A new method is proposed to design broadband metamaterial lens antennas by controlling both the refractive-index distribution of the lens and the radiation pattern of the feed.As a consequence,the phase and amplitude distribution of the broadband metamaterial lens are controlled simultaneously.Firstly,by means of particle swarm optimization algorithm,the refractive index distribution is optimized to realize the uniform phase distribution.Secondly,the specific amplitude distribution is realized by optimizing the radiation pattern of the feed.A novel planar-spiral phase plate(planar-SPP)based on ultrathin transmission-type metasurface is designed for generating the orbital angular momentum(OAM)radio waves.The proposed three-layer unit cell is compact with a total thickness of ?0/20 at the frequency of 10 GHz,which achieves a full transmission phase range and transmission magnitude equal to or greater than 0.9.The simulated far-field radiation patterns and near-field phase distributions clearly show the generation of OAM-carrying radio waves.A novel transmission-type coding metasurface is designed for generating multiple OAM beams.According to the principle of coding metamaterials,by arranging coding particles on a metasurface with specific coding sequences,different electromagnetic responses can be obtained.The coding sequence of M1 for coding metasurface generates one OAM beam(mode of l=1),and the coding sequence of M2 for coding metasurface generates the other OAM beam(mode of l=-1).According to the principle of the holographic technique,the coding sequences Mi and M2 are superimposed on the coding metasurface,and the final coding sequence M is obtained.The simulated results verify that multiple OAM beams can be simultaneously generated by using a single coding metasurface with the coding sequence distribution M.