Achromatic Characteristics Of Multi-layer Metasurfce Microwave Lense

In recent years,planar two-dimensional metamaterials--metasurfaces have gradually been used to design various types of microwave lenses.Due to their compact structure and light weight,metasurfaces with sub-wavelength characteristics provide design flexibility and show unprecedented advantages in electromagnetic wavefront control.However,most metasurface lenses are currently composed of resonance elements with high quality factors,which have shortcomings such as narrow band and dispersion.However,metasurface lenses that consider the group delay characteristics of the unit can solve the above problems.Derivation of f in the transmission phase can be defined the group delay,reflecting the inherent transmission characteristics of the device.Group delay not only indicates the signal propagation time delay,but also directly affects whether the signal will have dispersion during transmission.Therefore,artificially changing the group delay of the metasurface unit can control the chromatic aberration of the microwave lens and broaden the working bandwidth.The specific research work of this article includes::Group delay characteristics of the basic unit are studied.The qualitative analysis of the group delay for two types of structures: metal-dielectric unit and all-dielectric unit.It is found that the value of the group delay is inversely proportional to the frequency response bandwidth,And proportional to the response order N,?r of the dielectric layer,and the thickness of the dielectric layer.Group delay of the band-pass structure is quantitatively analyzed,and it is concluded that a fixed bandwidth corresponds to a fixed group delay value..A generalized method for the equivalent circuit model of MEFSS is proposed.The influence of the coupling effect from the first-order unit to the fourth-order unit on the ECM is analyzed,and a qualitative law is proposed: the coupling effect of the equivalent circuit model of the N-order unit is related to the thickness of the medium.Assuming a fixed thickness of the dielectric layer,the width w of the grid,the overlapping area S between the metal layers,and the relative dielectric constant ?r directly affect the coupling effect.The transmission performance of the fourth-order miniaturized frequency-selective surface element is emphatically analyzed and the factors affecting the transmission coefficient and group delay are discussed.According to the process of arranging MEFSS with specific group delay values in the lens profile,three achromatic focusing lenses with operating frequency of 10 GHz are fabricated and simulated.The first lens is based on a fourth-order patch-wire grid unit,which realizes achromatic function with a fixed focal length of 105 mm within a 2GHz operating bandwidth.Based on the same unit,the second type is met the requirement of the actual processing to realize the achromatic function with a fixed focal length of 110 mm within a 1GHz bandwidth.The third type of lens is based on the fourth-order patch-circular groove unit,and the achromatic function with a fixed focal length of 95 mm is realized within a 2 GHz bandwidth.Finally,a focusing lens with a center frequency of 10 GHz and a function of increasing chromatic aberration within a working bandwidth of 1 GHz is designed by using the method of increasing the group delay in equal areas.Exploring the achromatic characteristics of multi-layer metasurfaces is shown in this paper,the expected performance of the lens is improved,and the application of achromatic supersurface lenses in broadband impulse respons can be further expanded.