Research On Polarization Converter Based On Multilayer Metasurfaces With Subwavelength Spacing

Metasurfaces are periodic planar arrays of sub-wavelength elements.Compared with3 D electromagnetic materials,it has an ultra-thin thickness,low-lossy,and easy integration with other devices.Also,metasurfaces are more freedom in manipulating electromagnetic(EM)waves by designing the geometry or arrangement of unit cells,which activates its extensive use in the EM domain.In this paper,we propose two high-performance transmissive linear-to-circular polarization converters.Combined with Floquet theory and transmission line theory,an accurate equivalent circuit model with interlayer EM coupling has been studied and presents the analytical expressions of the equivalent impedance and scattering parameters for the proposed polarization converter.Based on the equivalent circuit model,the coupling mechanism,and the influence of EM coupling on the device performance have been studied.Furthermore,this model gives a guideline to obtain an ultrathin linear-to-circular polarization converter with wideband.The main contents and innovations are as follows:1.In the equivalent circuit model of multi-layer metasurfaces,early research shows that each metallic layer is represented by a shunt impedance and the dielectric layers are represented by a transmission line.This model has a high accuracy when the thickness of a dielectric layer is relatively thick,however,when the thickness is in the subwavelength range,the high order harmonics will act on the adjacent metasurface,thus leading to significant inaccuracy.To overcome those issues,we propose a universal coupled equivalent circuit model with high accuracy based on Floquet theory and transmission line theory.From the perspective of the equivalent circuit,metasurfaces are equivalent to a generalized waveguide structure.Using the pattern matching method and mathematical derivation,the equivalent impedance,and scattering parameters of the multi-layer metasurfaces are obtained.By comparing with the full-wave analysis,the accuracy and efficiency of the proposed equivalent circuit model have been successfully verified.Also,the proposed equivalent circuit model can effectively study the EM coupling phenomenon between the metasurface layers,which provides a new method for the analysis and design of multi-layer metasurfaces functional devices with ultrathin thickness.2.We propose a transmissive circular polarization converter with reduced thickness and high performance.The proposed device is composed of a three-layer metasurface spaced by two dielectric plates.The top and bottom layers are formed by meander lines,and the middle metasurface is a modified fishnet structure etched periodically with gaps along the x-direction.By employing these periodic gaps,the bandwidth of the proposed device is significantly extended.Meanwhile,two equivalent circuit models for two orthogonal polarizations are accurately extracted to better understand the physical mechanism of bandwidth expansion.The simulation results reveal that the device can convert a linearly polarized incident wave into a circularly polarized transmissive wave in wideband from 10 to 16 GHz,with relative bandwidth of 46%.Fortunately,the device’s bandwidth can still cover the range of 10 to 13.8 GHz even when the incident angle increases to 40°.An experimental sample is fabricated to verify the device performance.The experimental results are in reasonable agreement with simulations.3.The aforementioned work is based on the traditional equivalent circuit model,all higher harmonics have been ignored owing to the thick dielectric layer.To obtain ultrathin and ultra-wideband polarization converter,we propose a bi-layer transmissive metasurfaces to obtain a linear-to-circular polarization converter in wideband.The unit cell of each metasurfaces layer has the same configuration and consists of a splited Jerusalem-cross-like resonator and a metal strip.An accurate equivalent circuit model is developed to study the interlayer coupling of bilayer metasurfaces.Based on the circuit model,four metal strips are inserted along the y-direction in each unit cell to adjust the impedance of y-direction,so that two orthogonally polarized waves can meet the amplitude and phase conditions of the circularly polarized wave in ultra-wideband,thus leading to significant bandwidth expansion of linear-to-circular polarization conversion.Numerical and experimental results show that the proposed metasurfaces can converter a linearly polarized incident wave into a circularly polarized transmission wave in a wideband from6.1-12.6 GHz,with a fractional bandwidth of 69.5%.