【作者】 张磊；

【导师】 崔铁军；

【作者基本信息】 东南大学 ， 电磁场与微波技术， 2020， 博士

【摘要】 超表面是由亚波长尺寸单元在二维平面或曲面上进行周期或非周期性排列而成的一种人工结构,具有厚度小、损耗低、集成度高等优点。近些年,超表面被广泛地用于调控电磁波,并产生了很多新奇的物理现象和应用。作为超表面的数字形式,数字编码与可编程超表面从信息科学的角度来研究超表面,创新性地用二进制编码来表征超表面,通过改变编码序列来实时操控电磁波。数字化的表征也使超表面成为物理世界和数字世界的桥梁,方便与信息科学中的相关理论相结合。本文系统地研究了空间编码、时间编码和时空联合编码数字超表面的一般性理论及应用,在空间域和时间域对超表面的表征参数进行编码,可在空间域和频率域调控电磁波的辐射和散射特性,最终实现编码超表面在空、时、频域对电磁波进行多维度调控。主要研究内容和贡献概括如下:1.提出了一种基于空间编码超表面的法布里-珀罗谐振腔天线,特殊的编码超表面结构使天线获得了高增益,同时也降低了雷达散射截面。这种新型天线结构紧凑、剖面较低,在宽频带内实现了低散射的隐身效果。2.利用几何相位原理设计空间编码超表面,即利用结构相同但旋转角度不同的单元来构造多比特相位编码。结合散射方向图的卷积运算,空间编码超表面可实现对圆极化波的灵活调控,在自由空间产生旋性可控的多波束,包括携带轨道角动量的涡旋波束和高定向性的笔形波束等。3.提出了一种多功能集成的反射透射一体化空间编码超表面,通过设计多层各向异性单元结构,当改变入射波的极化和传播方向,一款编码固定的超表面可实现多种不同功能,包括异常反射、随机漫散射以及涡旋波束生成。这种集成设计的空间编码超表面既可工作于反射模式,也可工作于透射模式,能在同一个超表面口径上同时控制反射和透射波前。4.提出了时空编码数字超表面的概念及一般性理论,在空间域和时间域对超表面的表征参数进行联合编码,可同时在空间域和频率域调控电磁波的辐射和散射特性,即控制电磁波的传播方向和谐波频谱,最终实现了超表面在空、时和频域对电磁波进行多维度调控。基于时空编码调制理论,设计了谐波波束扫描、波束赋形和散射缩减三个示例来展示时空编码超表面的特性与优势。5.基于时空编码超表面理论,通过设计合适的时空编码矩阵,时空编码超表面可用于打破互易性和时间反演对称,结合时空编码超表面的动态可编程特性,最终实现了动态可控的非互易效应和谐波频率转换。6.基于时空编码超表面理论,提出了一种利用物理层2比特可编程超表面来实现任意多比特等效相位的通用方法,并给出一种矢量合成分析法来设计特定谐波频率处所需的目标等效相位值。具体地,通过设计2比特时间编码序列综合出4比特及更高比特的相位值,利用4比特等效相位在基波频率和正一阶谐波频率处实现了波束偏折,展现出了高比特编码的优势。7.基于时空编码超表面理论,提出了一种新型信息编码方案,并利用时空编码超表面构造了一种具有空分和频分复用特性的双通道无线通信系统,在超表面直接编码信息,可同时向空间不同位置的两个用户独立地传输两幅图片,省去了传统发射机中的数模转换和混频过程。时空编码超表面可以同时实现信息调制和能量辐射的功能,能调控电磁波的空间谱和频率谱特性,极大地拓展了超表面的应用范围。更多还原

【Abstract】 Electromagnetic(EM)metasurfaces are artificial composite materials formed by periodic or aperiodic arrangement of sub-wavelength unit cells on a two-dimensional plane or curved surface.They have the advantages of ultra-thin thickness,extremely low loss,and high integration.Therefore,in recent years,metasurfaces have been extensively studied and used to control EM waves,and many novel physical phenomena and applications have been produced.As the digital version of metasurfaces,digital coding and programmable metasurfaces make it possible to study metasurfaces from the perspective of information science,in which binary codes are innovatively used to characterize the metasurfaces.Coding metasurfaces could greatly simplify the design and optimization procedures,and manipulate EM wave in real time by changing the coding sequences.The digital representation of coding metasurfaces also builds a bridge between the physical world and the digital world,which is convenient to be combined with related theories in the information science.This thesis systematically studies the general theory and practical applications of space-coding,time-coding and space-time-coding digital metasurfaces.The characterization parameters of the coding metasurfaces can be encoded in the space and time domains,which can manipulate the radiation or scattering characteristics of EM wave in the both space and frequency domains.Hence,coding metasurfaces can finally achieve multi-dimensional control of EM waves in the space,time and frequency domain.The main research contents and contributions of this thesis are summarized as follows:1)A Fabry-Perot resonant cavity antenna based on space-coding metasurface is proposed.The special structural design of coding metasurface makes the antenna obtain high gain and keep low radar cross section simultaneously.This new antenna has a compact structure and a low profile,and can achieve a low-scattering stealth effect in a wide frequency band.2)The principle of geometric phase in optics is used to design the space-coding metasurface,that is,the unit cells with the same structure but different rotation angles are used to construct multi-bit phase coding.Combined with the convolution operation of the scattering patterns,the space-coding metasurface based on geometric phase can realize flexible controls of circularly polarized waves,forming spin-controlled multibeams in free space,including vortex beams carrying orbital angular momentum and pencil beams with high-directivity.3)A transmission-reflection-integrated multifunctional space-coding metasurface is proposed.Thanks to the special design of multilayered anisotropic unit cells,this fixed space-coding metasurface can realize multiple different functionalities by changing polarization and propagation direction of the incident waves,including anomalous reflection,diffuse scattering,and vortex beam generation.This integrated design of space-coding metasurface can work in both reflective mode and transmissive mode,and hence simultaneously control the reflected and transmitted wavefronts on the same aperture.4)The concept and general theory of space-time-coding(STC)digital metasurfaces are proposed.The characterization parameters of the coding metasurfaces are encoded in both space and time domains,which can manipulate the radiation and scattering characteristics of EM waves in both space and frequency domains,i.e.to control propagation direction and harmonic spectrum distribution simultaneously.Hence,STC digital metasurfaces can finally achieve multi-dimensional control of EM waves in the space,time and frequency domains.Based on the proposed concept and theory,three illustrative examples of harmonic beam steering,beam shaping and scattering reduction are presented to demonstrate the characteristics and advantages of STC digital metasurfaces.5)Based on the theory of STC digital metasurfaces,Lorentz reciprocity and time reversal symmetry is broken by designing a suitable STC matrix.Combined with the dynamic programmability of digital metasurfaces,such reciprocity effects can be controlled dynamically and harmonic frequency conversion is also realized.6)Based on the theory of STC digital metasurfaces,a general method for realizing arbitrary multi-bit equivalent programmable phase based on a physical layer 2-bit programmable metasurface is proposed.Besides,an analysis method of vector synthesis is also presented to design the required phase value at specific harmonic frequencies.Specifically,the 4-bit and arbitrarily higher-bit coding phases are synthesized by designing 2-bit time-coding sequences.Beam deflection is realized at the fundamental frequency and positive first harmonic frequency by using the 4-bit equivalent phase,which shows the advantage of high-bit digital coding.7)Based on the theory of STC digital metasurfaces,a new information encoding scheme is proposed to construct a two-channel wireless communication system with space-and frequency-division multiplexing characteristics,in which the original information can be directly encoded at the metasurface level.Two different pictures are directly transmitted to two designated users located at different positions simultaneously and independently,without the need of digital-analog conversion and mixing processes in traditional technologies.The STC digital metasurface can realize the functions of information modulation and energy radiation at the same time,and can manipulate the spatial and frequency spectrum characteristics of electromagnetic waves,which greatly expands the application range of the metasurfaces.更多还原