Electromagnetic Wave Manipulation And Novel Designs Based On Metasurfaces

Recently,due to the development of the concept of metamaterials,it became widely understood that a more profound control of electromagnetic waves by planer structures is granted by a design of subwavelength inclusions because they can be densely arranged and allow the fields to be engineered with much better spatial resolution.In general,such inclusions possess both electric and magnetic polarizations,and they allow a full control over reflected and transmitted waves with great opportunities for novel generation of microwave and optical devices.A few years ago,all such planar metamaterial structures were conventionally termed as metasurfaces.Metasurfaces(MSs)are two-dimensional analogues of metamaterials defined as optically thin and dense two-dimensional arrays of structural elements possessing unusual and useful properties granted by their constitutive elements with usually resonant behavior.The element size and periodicity of MSs are subwavelength and,therefore,homogeneous or nearly homogeneous MSs reflect and transmit plane waves like homogeneously polarizable sheets,in contrast to diffraction gratings which produce multiple diffraction lobes.The currently known MSs can be divided into the following categories based on their functionalities: frequency selective surfaces,high-impedance surfaces,perfect absorbers,polarization transformers,leaky-wave surfaces,wavefront shaping surfaces,two-dimensional flat lenses and also nonlinear metasurfaces.In this dissertation,the purpose is to control the polarization states of the electromagnetic wave by MSs,particularly in the linear-to-circular polarization transparent MSs.A series of work has been carried out on the theory,modeling analysis,parameter design,and experimental verification of MSs in reflection and transmission modes.Four types of linear-to-circular polarizers by MSs have been proposed,and the causes of conversion effect are analyzed one by one.(1)An ultra-wideband linear-to-circular polarization converter by MSs in reflection mode is proposed.First,on the basis of the classic reflection-type resonant conversion dipole,a parallel-splitwire hexagonal metallic unit cell is used,which greatly improves the conversion bandwidth.Numerical simulations show that the MS can convert incident linear polarized waves in the 2.37-4.61 THz band into circularly polarized reflected waves with the axial ratio of 3dB or less,and the fractional bandwidth is as high as 64%.There are few related contemporary literatures which exceed this relative bandwidth and performance.In addition,the converter can still maintain 85% conversion efficiency when the incident angle deviates from 40 °,so it has respectable performance tolerance against the angle of incidence.This converter has a simple structure and is easy to implement,and has a wide range of functions in terahertz imaging,electromagnetic stealth,miniaturized communication and sensing equipment.(2)A chiral dual-band transparent linear-to-circular polarization converter is designed.On the one hand,the MS can convert an incident orthogonal linearly polarized wave into a circularly polarized wave with an axial ratio of less than 3dB in dual bands of 8.5-10.9 GHz and 18.1-22.5 GHz.The transmitted circularly polarized waves in dual bands have opposite rotation directions.On the other hand,the converter sacrifices the cross-linearly polarized wave corresponding to the incident linearly polarized wave and obtains a theoretical 100% transmission efficiency.Both simulation and experimental results show that the peaks of transmission efficiency in both bands are up to 90% and the polarization extinction ratio is above 18 dB.The traditional transparent linear-to-circular converters with symmetrical poles have conversion efficiency of no more than 50%.This new viewpoint provides an important means for designing other high-efficiency dual-band linear-to-circular polarization MSs.(3)A chiral-like MS based on Fabry-perot cavity was proposed for the realization of transparent tri-band linear-to-circular polarization.On the one hand,similar to the dual-band chiral linear-to-circular polarization converter,polarization selection recognition is introduced,so that in three frequency bands with an axial ratio below 3dB,The rotation directions are different from each other,and the peaks of in-band transmission efficiency all reach more than 90%.On the other hand,the traditional Fabry-perot cavity for linearly polarization cross conversion is improved to make it applicable to linear-to-circular polarization conversion,and the physical mechanism is analyzed for symmetry breaking or similar chirality symmetry enable the high conversion efficiency and transmission efficiency.(4)A series of theoretical models of linear-to-circular polarization beam splitters with self-complementary structures are proposed.First of all,it is inferred from the strict self-complementary model and Babinet principle that when the incident wave is linearly polarized,the reflected and transmitted waves will be circularly polarized at some frequencies,and the reflected and transmitted energy each half.The most important thing is that,in theory,this type of converter has infinitely thin thickness,and the number of circularly polarized frequency bands is completely controllable,which is related to the fractal times of the model.This function cannot be achieved by other related literatures.In actual design,the model can be fractal up to 4 times to obtain an eight-band linear-to-circular polarization converter.This provides a new theoretical basis for designing multi-band and even ultra-multi-band converters in the future.This paper studies how to use MSs to achieve the linear-to-circular polarization conversion with high transmission efficiency and high conversion performance.Based on traditional conversion dipoles,the electromagnetic/optical characteristics of the unique optical activity,circular dichroism and asymmetric transmission of chiral MSs are used to manipulate electromagnetic polarization.It has also proposed corresponding solutions in the field of multi-frequency band,linear-to-circular polarization beam splitter with both reflection and transmission.Related research results have extensive application value in the design of communication equipment and optical devices with excellent performance such as polarization control and electromagnetic cloaking.