Artificial Electromagnetic Structures To Modulate Polarization Properties And Their Applications

In recent years,metamaterials have set off a research boom in the field of electromagnetics,because of their unique physical properties.It has important application prospects in many fields,such as communication,radar,navigation and medical imaging.Therefore,the development of metamaterials has been accelerated.Polarization is one of the important characteristics of electromagnetic waves.It carries valuable information about electromagnetic waves.Modern communication systems are becoming larger and more complex,and they are very sensitive to the polarization of electromagnetic waves.Many practical applications urgently require complete control of the polarization state of electromagnetic waves.The traditional polarization manipulation is usually realized by utilizing materials with the birefringence effect,such as crystalline solids,liquid crystals.But these materials are limited in practical application systems because of many disadvantages,such as the bulky volume,the narrow band and low efficiency.The emergence of metamaterials has opened up a new technological approach for the polarization manipulation of electromagnetic waves.The research content of this dissertation focuses on the electromagnetic wave polarization manipulation characteristics and asymmetric transmission effects of metamaterials.In the microwave range,we studied the reflection type and transmission type polarization manipulation metasurfaces,including model establishment,optimization simulation,characteristic analysis,mechanism explanation and experimental verification.The goal of the research is to improve several important indicators of polarization manipulation devices:structure size,the operating bandwidth and polarization conversion efficiency.In this paper,we proposed several high performance broadband metasurface-based polarization manipulation structure models,while perfecting the design method and the physical mechanism of polarization control.The major work of this dissertation are as follows:(1)The polarization manipulation principle of metamaterials was studied on the basis of analyzing the influence of structure geometric and arrangement of the unit cell on the electrical and magnetic response characteristics of the metasurfaces.Then,we have studied metasurface-based polarization convertor,including the basic theory of the polarization manipulation,the mechanism analysis,the design method,simulation calculation and experimental verification method.(2)It is proposed that a high efficiency reflective polarization rotator metasurface structure based on the%shape.In order to overcome the disadvantages of traditional polarization controllers such as bulky volume and difficult integration,the polarization rotator adopts an oblique symmetrical pattern multi-resonance structure.The relationship between the electromagnetic wave polarization state and the impedance characteristics of the metasurface is established,which makes different polarization states of electromagnetic waves in the X-band and Ku-band realized.Simulation results and experimental ones show that the design structure can convert 98%the incident linearly polarized wave into its orthogonal linearly polarized reflection wave within a relative bandwidth of 72%,and the corresponding polarization extinction ratio is greater than 30d B.In addition,the thickness of this structure is 0.11?(where?is the free-space wavelength at the center frequency of the operating bandwidth).When the incidence angle is changed,the polarization conversion bandwidth and efficiency are not sensitive.(3)It is proposed that a high performance broadband linear-to-circular polarization reflector metasurface structure.Aiming at the present situation of linear-to-circular polarization converter with mostly working in high frequency band and narrow bandwidth,we improved the classic Jerusalem cross structure and designed a dual diagonal symmetric structure based on equivalent circuit model and the transmission line theory.Simulation results and experimental ones show that the design structure can convert the linearly polarized incident electromagnetic wave into the circular polarized reflected wave,which the bandwidth of axial ratio less than 3d B is 12.4GHz and the fractional bandwidth is 94%.The current distributions and equivalent circuit model are given to analyze the generation of circularly polarized waves.In addition,we analyze the relationship between the electromagnetic resonance of the unit cell structure and the polarization conversion,which obtains the mechanism of the broadband generation of the conversion operation.It provides an important means for the design of ultra-thin broadband linear-to-circular polarization metasurface in the future.This design structure is believed extendable to higher frequency regimes.(4)The asymmetric transmission polarization rotator based on metamaterial structure is studied.Aiming at the current situation of asymmetric transmission devices such as narrow bandwidth and low transmission efficiency,we have studied the construction theory and design method of asymmetric transmission polarization control devices.Based on the principle of Babinet complementarity,we designed a multi-splits square ring structure and established the equivalent circuit model of the complementary structure,which obtains the design method of controlling the electromagnetic wave polarization state through adjusting the structure parameters.Next,for the purpose of simplifying the pattern of the structure,we discussed the electrical and magnetic response characteristics and polarization conversion efficiency of the hexagonal split resonant ring structure.The simulation results show that the structure has unidirectional polarization conversion characteristics in the X-band.Finally,on the basis of the hexagonal split resonant ring structure,we design an ultra-wideband and selective polarization rotator metamaterial structure by adding sub-wavelength metallic gratings.The sub-wavelength metallic gratings is used to reduce the loss of the dielectric substrate and increase the working bandwidth,based on the Fabry-Perot-like cavity theory.Simulation results and experimental ones show that this metasurface structure can achieve polarization rotation with in a relative bandwidth of 115%.In addition,the design structure has extremely stable propagation characteristics when the incident angle is less than 30~o.The band-stop and band-pass characteristics of this structure are of great significance to the application of selective polarization control metasurfaces in microwave switching devices.