| As a synthetic structural material,metamaterials haven many exotic properties which are not possessed by natural materials.Over the past ten years,metamaterials have been used in many fields,such as enhanced antenna performance,electromagnetic stealth,frequency selection,beam focusing,etc.Polarization is an important feature of electromagnetic wave,the traditional polarization control device of wave plate can be utilized to achieve,for example,crystalline solid and liquid crystal,etc.,however,these materials have strong birefringence characteristics and larger phase delay.In microwave band,ferrite phase and multilayer grating are usually used to realize the polarization control,but,these materials cannot be integrated into the micro optical system because of the larger geometry and narrow workdng frequency band.As metamaterials research increasingly deepening,the polarization control can be achieved by anisotropy or chiral metamaterials,which provides a new technical way to control the polarization states of the electromagnetic wave.In this dissertation,the purpose is to control the polarization states of the electromagnetic wave by metamaterials.We preset the model design,optimization,simulation and experimental validation of metamaterials for controlling the polarization states of reflected and transmitted waves in the microwaves and visible light band,and a series of metamaterial structures are designed for controlling the linearly and circularly polarized wave working on broadband with high efficiency;the physical mechanism of the polarization control is analyzed and discussed in detail.The main work of this dissertation is as follows:(1)A simple anisotropic metamaterial is presented to realize the polarization manipulation of a linearly polarized wave with high conversion efficiency in a wide frequency band.The presented simple metamaterial can convert the linearly polarized incident wave into its orthogonal polarized reflected wave at the frequency range of 8.16-15.32 GHz.Both the simulated and experimental results demonstrate that the bandwidths of the polarization conversion ratio of more than 90%with the relative bandwidths of 60%.The conversion mechanism of the polarization is discussed with the theoretical calculations and surface current distribution.Furthermore,the presented simple metamaterial can keep the handedness of the reflected waves for circularly polarized incident waves.Both the simulated and experimental results is shown that the bandwidth of the polarization conversion ratio more than 90%is 7.16 GHz(8.16-15.32 GHz),and the polarization extinction ratio is larger than 15 dB in this frequency band.Because of the simple design and easy fabrication,the designed metamaterial can be a very good candidate for reflection circular polarization conversion.(2)A dual-band metamaterial based on a concentric rectangular arrangement is presented to control the polarizations of both linearly and circularly polarized electromagnetic waves.We show that,on the one hand,the proposed metamaterial structure can convert the polarization of linearly polarized waves to the cross direction of two broad frequency bands 4.40-5.30 GHz and 9.45-13.60 GHz.On the other hand,this design can also reflect circularly polarized waves without changing the handedness in two frequency bands 4.47-5.35 GHz and 9.57-13.57 GHz.For both cases,average polarization conversion efficiency larger than 86%was achieved in our experiment.Numerical simulations reveal that the dual-band cross-polarization coupling results from the strong electric dipole interaction between the inner and outer rectangles,which are inherent to the concentric arrangement.The ultrathin polarization converter is experimentally realized in this work,provides an important stepping stone for future design of other dual-band metasurface devices and is believed extendable to higher frequency regimes.(3)A few-layer metamaterial based on split ring resonators is presented for simultaneously controlling polarization of reflection and refraction waves.It is shown that either x-and y-polarized incident wave was vertically reflected or transmitted through the designed metamaterial with polarization conversion ratio of more than 90%in some specific frequencies.Moreover,the designed metamaterial has respectable performance tolerance against the angle of incidence both in x-and y-polarization.All the simulated results are confirmed in microwave experiment,and the experimental results are agreement with the simulation.The proposed metamaterial will be beneficial for designing polarization-controlled and selective transmission converter and has great potential as a polarization manipulation device in polarization insensitive devices and 90° polarization rotator and related applications.(4)A three-layer bi-anisotropic metamaterial is proposed to realize a 90° polarization rotator in an ultra-broadband frequency range of 5.8-11.8 GHz,which can convert the linearly polarized wave into its cross-polarized wave for propagating directions.Simulated and measured results show that the ratio of polarization conversion of the three-layer metamaterial is nearly 90%and FWHM is more than 68%.In addition,the three-layer metamaterial can achieve asymmetric transmission for forward and backward propagation of linearly polarized waves.The proposed three-layer metamaterial has simple geometry and wider operating frequency bands comparing to the previous designs.The physical mechanism of the ultra-broadband linear polarization conversion effect is illustrated by simulated surface current distributions of the structure.(5)A simple structure is designed to realize a broadband polarization modulation in visible light with high efficiency of polarization conversion ratio more than 90%to simultaneously control the polarization state of linearly polarized light and circularly polarized light The designed structure can convert the visible light of 400-800 nm ranges into its cross polarized light when the linear polarization incidence;and the reflection circularly polarized waves without changing the handedness in the visible light of 400-800 nm ranges.The experimental results are coincided with the simulated results,and the error between simulated and experimental results is discussed in detail.In addition,the anomalous reflection of the circularly polarized wave is realized by using the phase gradient supersurface,and the reflection angle of the anomalous reflection is calculated based on the generalized Snell law.Based on the modeling,optimization and experimental verification,the reflected and transmitted polarization control metamaterials are studied,and the physical mechanism of polarization manipulation is carried on the detailed analysis,which has the important guiding significance on designing a variety of metamaterial structures for controlling the polarization states in broadband working frequency with high efficiency.The designed metamaterials can be used in radar technology,antenna technology of manipulating the polarization state of electromagnetic waves,also can be implemented using electromagnetic wave polarization state sensing,detection and display,and other functions. |
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