Near-field Modulation Based On Holographic Impedance Surface

In this paper,holographic impedance surfaces for near-field tuning are investigated to address the shortcomings of large profiles in metasurface systems,with a focus on how to use holographic impedance surfaces for near-field tuning such as focusing,vortex beams and other functions.This paper focuses on two aspects,the theoretical design of scalar and tensor holographic impedance surfaces,the creation of unit databases and the implementation of different functions,both of these holographic impedance surfaces can achieve near-field tuning,but the two metasurfaces have their own advantages and disadvantages,and can be selected according to different needs of different surfaces.Depending on the functional requirements,various types of unit structure are used in this paper.For the scalar unit,the square patch unit was analyzed as an example,and its patch impedance was obtained by the transverse resonance method and the eigenmode solution method,respectively.Although the scalar impedance obtained by the two methods is somewhat different in value,the effect of the difference is negligible when the focus function is achieved.Changing its scalar impedance by varying the edge length of the square patch,we established a scalar cell impedance database at 17 GHz.Also,the principle of surface wave modulation by scalar holographic impedance surface is investigated,combining the traditional leakage theory and the concept of optical holography to establish the Bessel beam holographic impedance surface,one-dimensional focused holographic impedance surface,two-dimensional focused holographic impedance surface,and bifocal holographic impedance surface,Moreover,the frequency scanning properties of this scalar holographic impedance surface are analyzed.For the tensor unit,the impedance databases of the circular slit unit and the cross-shaped unit are established,and the performances of the two units are compared.Although the circular slit cell can perform functions such as focusing and vortex beam,it introduces errors because the circular slit cell has only two physical quantities,radius and gap angle,but three impedance components to control.and the circular slit unit cannot regulate the individual components of the impedance individually,so the unit will be less effective in achieving its function.The cross-shaped unit solves this problem perfectly by changing the length of the two strips of the cross-shaped unit to control the two components of the 2D tensor,and by using the rotational characteristics of the 2D impedance tensor to control each component of the 2D tensor.Two design methods for establishing tensor impedance surfaces are also analyzed.The first design theory would introduce an antiHermitian matrix impedance component that would lead to inaccurate surface tensor impedance,resulting in unsatisfactory results.The second method,on the other hand,works better by introducing a new coordinate system and a rotation matrix that does not produce an anti-Hermitian matrix,And the vortex beam tensor holographic impedance surface and the focused tensor holographic impedance surface were established.Both the scalar holographic impedance surface and the tensor holographic impedance surface are simulated by CST2019.the results show that the scalar holographic impedance surface and the tensor holographic impedance surface are consistent with the theoretical designs.The scalar holographic impedance surface has a wider bandwidth and frequency sweep properties,while the tensor holographic impedance surface is able to modulate the polarization of the electric field for more complex functions,but its bandwidth is narrower.