| Electromagnetic metasurface（Metasurface）is an artificially designed quasi-two-dimensional electromagnetic metamaterial with electromagnetic properties that are very different from natural materials.Through the design of the metasurface unit structure,negative refractive index,asymmetric transmission,negative dielectric can be realized Constant and other characteristics,as well as electromagnetic stealth,polarization conversion and other properties.Devices with the ability to convert the polarization of electromagnetic waves are collectively referred to as polarization converters,which have great application value in the fields of new-generation wireless communications,electromagnetic measurement,and holographic imaging.Terahertz（Terahertz,THz）waves usually refer to electromagnetic waves with frequencies between 0.1–10 THz,which are located between microwaves and infrared in the spectrum.It has a very broad application prospect in the fields of economy and security.In summary,this dissertation focuses on the metasurface-based THz cross-polarization converters,and focuses on the equivalent circuit model method to verify the performance of the polarization converter.The details are as follows:（1）In order to achieve efficient cross-conversion of linearly polarized waves,a reflective ultra-broadband crossed linearly polarized converter based on a metasurface periodic structure is proposed.It consists of a metal resonant split ring,a 0.13λ0 thick dielectric substrate（λ0 is the wavelength corresponding to the center frequency of 1.56THz）and a metal reflector,and the size of the unit（0.42λ0）is much smaller than the traditional metasurface unit.The simulation results show that in the frequency range from 0.92 THz to2.20 THz（with a relative bandwidth of 82%）,the structure can effectively convert the y-polarized incident wave to the x-polarized reflected wave with a conversion rate（PCR）≥90%.Then,the field decomposition method and the equivalent dipole resonance theory are used to clarify the physical mechanism for realizing the cross-polarization conversion,and a method based on the equivalent circuit model is initially proposed to verify the performance of the cross-polarization conversion structure.（2）In order to further study the ability of the proposed equivalent circuit model method to verify the performance of cross-polarization conversion metasurface devices,two terahertz polarization converters are proposed in this paper,and the universality of the equivalent circuit model method is verified by them.The first is a polarization converter based on an obliquely symmetric"3"-like metasurface structure.The size of the unit structure is 0.42λ0×0.42λ0×0.14λ0,which can achieve ultra-wideband（0.92-2.25 THz,relative bandwidth up to 84%）,high-efficiency orthogonal polarization conversion.By extracting the equivalent circuit model generated when the u and v polarized waves are incident on the device,the polarization converter is verified from the circuit point of view,and then the working mechanism of the device is analyzed using the interferometry and resonance theory.The second is an ultra-broadband terahertz polarization converter based on a centrosymmetric"C"-like metasurface structure,which is slightly different from the previous excitation method,and the electric field is incident along the diagonal direction.Simulation results show that efficient cross-polarization conversion can be achieved in the frequency range from 0.89 THz to 2.07 THz,while still maintaining high conversion efficiency at wide-angle incidence.The physical mechanism of the device is explained by using the equivalent transmission line theory,the device is deeply verified by extracting the equivalent circuit model,and the entire equivalent circuit structure is restored by ADS software.The results obtained in the two cases are in good agreement.Then,according to the distribution of the surface current of the device,the resonance characteristics of the device are further analyzed. |
