Controlling The Scattering Characteristics Of Electromagnetic Wave By Metasurfaces

Electromagnetic metamaterials,are defined as artificial composite structures or composite material formed by periodically arranged subwavelength structures.Because metamaterials can produce exotic electromagnetic characteristics that are unavailable in naturally occurring materials,which can be used to generate new physical phenomena,such as abnormal reflection and refraction,they have gradually become a hotspot of research in the field of electromagnetic wave in recent years.Metasurface is a two-dimensional equivalence of metamaterials,which has the advantages of light weight,small size,and easy to design.The metasurface introduces a resonation element with a phase abrupt change characteristic across the interface,so that the electromagnetic wave propagation characteristics can be flexibly controlled according to the designed spatial phase distribution.In this dissertation,the research work on the control of the scattering properties of free-space electromagnetic waves using metasurface is studied.By designing the unit cell of the metasurface with specific phase and amplitude response and designing the spatial phase distribution of the metasurface according to the pre-designed scattering aim,we have controlled the scattering characteristics of the electromagnetic wave in a wideband.The main contents of this article are as follows:1.Based on the anisotropic metasurface,multi-functional flat parabolic antennas are designed,including a polarization-switchable planar parabolic antenna and a flat parabolic antenna with low scattering characteristics.The polarization-switchable flat parabolic antenna operates in the X-band,which is capable of switching between the linear polarization of the electromagnetic wave,the left-handed circular polarization,and the right-handed circular polarization by changing the feed rotation angle.The low scattering planar parabolic antenna consists of a basic resonant element(orthogonal "I",shaped structure)with anisotropic electromagnetic properties.By changing the geometrical parameters of the unit cell structure,the overall control of the phase response of the unit cell can be achieved.By designing the spatial phase distribution of the reflective metasurface in the x-and y-polarization directions respectively,the metasurface achieves a high directivity parabolic antenna function in the x-polarization,while at the same time a random scattering function in the y-polarization direction to reduce the backward radar cross section of the parabolic antenna,2.Based on the principle of random surface design,we have developed a variety of wideband coding low-scattering metasurfaces for the backward radar cross section reduction of target objects.The unit cell with a wideband electromagnetic response is sequentially distributed in a finite-size plane according to a pseudo-random sequence generated by the computer.A backward scattering reduction of more than 10 dB can be achieved within a wideband.In addition,for specific visual window application,such as glass windows on ships and military vehicles,we also designed a microwave broadband random scattering metasurface with optical transparency that can be used for both visual observation and electromagnetic stealth technology.In this type of metasurface,we replaced the traditional copper material with a transparent conductive oxide,and replaced the ordinary circuit board with a flexible transparent dielectric substrate,thus simultaneously achieving the functions of optical transparency and electromagnetic scattering control.Both the simulated and the experimental results verify the wideband scattering suppression ability of the metasurface.