Controlling The Propagation Characteristics Of Electromagnetic Waves By Metasurfaces

Metasurfaces,which could be treated as two-dimonsional metamaterials,have smaller thickness,relatively lower losses,easier fabricating technology and easier to integrate to traditional electricmagnetic devices comparing to volume metamaterials.Enormous interests has been attracted by metasurfaces due to these advantages.By introducing abrupt phase changes at the surface,metasurfaces could realize unprecedented controls of electromagnetic wavefronts.This design strategy of controlling electromagnetic waves is simpler,more flexible and completely defferent from metamterials,which has a lot of potential in the design of ultra-thin and high performance electromagnetic devices.Based on these investigations,many applications have been presented,such as mantle surface cloak,ultra-thin and high performace lens,polarization conversion surfaces and ultra-low profile antennas.In this thesis,we focus on the manipulating properties of metasurfaces on electrimagnetic waves,and the the novel resonator-structure designs and their applications in microwave circuits are investigated.The main contributions of this dissertation are listed as follows:(1)Various effective physical parameters to quantitatively describe the interactions of metasurface and electromagnetic waves are investigated.And the advantanges of surface susceptibilities in representing the electromagnetic properties of metasruface are analyzed.The relationship between the surface susceptibilities and the reflection/transmission coefficient of metasurface is obtained by formula derivation.The existing metasurface synthesis techniques are investigated and a simplified solving method of susceptibility tensors synthesis method is proposed.And the relationship between the susceptibility tensors synthesis method and Huygens sysnthesis is also analyzed.(2)An ultrathin,compact Huygens unit cell which could provide extreme controls of phases of the transmitted waves is proposed.Both electric and magnetic currents are supported by the proposed unit cell,thus leading to highly efficient and full controls of phases.The coupling between electric and magnetic responses is negligible,which will significantly reduce the difficulty of design.Based on the proposed Huygens unit cell,a beam-refracting metasurface with low-reflection is designed and manufactured.(3)An ultrathin,anisotropic Huygens metasurface is proposed to manipulate orthogonally polarized transmitted waves independently.In physics,two orthogonal electric dipoles and two orthogonal magnetic dipoles are supported by each unit cell of the metasurface,which enable the complete control of phase distributions in both vertical and horizontal directions.Based on this feature,a polarization beam splitter with large splitting angle is designed and fabricated to demonstrate the capability and flexibility of the proposed Huygens metasurface.And an ultralow profile antenna radome which could refract the orthogonal polarization wave to the same direction is designed.Since there are no complex structures such as vias to be employed in the proposed metasurface,this approach could also be applied to control the orthogonally polarized transmitted waves independently in the infrared and optical regimes.(4)A robust and facile approach for achieving simultaneous phase and amplitude manipulation of reflection or transmission wave by Huygens metasurface is proposed.By altering the electric and magnetic responses of Huygesn metasurface,arbitrary complex reflection or transmission coefficients with modulus values smaller than 0.85 are obtained.Two Huygens metasurfaces capable of controlling the diffraction orders are designed and fabricated by modulating the distributions of the complex transmission coefficients.More complicated functions such as holographic imaging can also be accomplished by using the proposed Huygens metasurfaces.(5)A polarization conversion metasurface composed of I-shaped metallic unit cells is proposed.Four plasmon resonances are generated by electric and magnetic resonances,which lead to bandwidth expansion of cross-polarization reflection.The reflection amplitudes and phases could be independently controlled by changing geometries and orientations of unit cell.Based on the porposed unit cell,two ultrathin metasurface gratings are further proposed to form anomalous reflection with polarization orthogonal to the incident waves.(6)An ultra-wideband and polarization independent metasurface for Radar Cross Section(RCS)reduction is proposed.Combining the concept of coding metasurface and random surface,the I-shaped unit cell is ultized to form the RCS reduction metasurface.More than 10 dB RCS reduction is achieved for normally incidenct in 7.9-20.8 GHz.And the RCS reduction performance mantains well under large incidence angle.