兰州大学机构库 >信息科学与工程学院
相位型超表面对正交圆极化波波前调控研究
Alternative TitleRESEARCH ON PHASE-TYPE METASURFACE FOR ORTHOGONAL CIRCULARLY-POLARIZED WAVE WAVEFRONTS MODULATION
陈文琼
Subtype博士
Thesis Advisor梅中磊
2023-09-02
Degree Grantor兰州大学
Place of Conferral兰州
Degree Name理学博士
Degree Discipline无线电物理
Keyword超表面 Metasurface 圆极化电磁波 circularly polarized electromagnetic wave 几何相位 geometric phase 传播相位 propagation phase 波前调控 wavefront manipulation
Abstract

超表面是由一系列人工设计的亚波长电磁散射单元经过周期或准周期的排布组合而成的二维结构。通过精细设计亚波长单元结构,可以改变单元的电磁响应,实现对电磁波极化、相位和幅度等基本特性的灵活调控。近年来,几何相位型超构表面因具有非色散、宽频带和易于实现等优点得到了广泛关注和研究,是实现圆极化电磁波波前调控的主要方法。但是,这种调控模式对正交圆极化波表现出共轭对称的相位响应,限制了两种自旋波极化调控的自由度,导致这类超表面工作性能单一、利用率低。因此,本文以相位理论为依托,分析了正交圆极化波去耦合独立调控的方法,基于相位型超构表面展开了正交圆极化电磁波波前调控研究。本文工作为丰富超表面对圆极化波的调控自由度、扩展超表面的极化复用方式提供了一定的研究基础。主要研究内容概括如下:

首先,基于几何相位理论,设计了一种由H形金属谐振结构组成的高效超宽带单层反射型超表面。该超表面工作在7.3 GHz到21 GHz的超宽频段,相对工作带宽为96.8%,对圆极化入射波的共极化转换效率高于92%。通过构建涡旋超表面阵列,实现了携载2模式轨道角动量(orbital momentum angular, OAM)的涡旋电磁波生成。数值仿真与实验测试结果表明,利用几何相位调控超表面可以实现超宽频段涡旋电磁波的高效发生。此外,由线极化波照射超表面验证了传统几何相位对左旋和右旋圆极化电磁波共轭对称的调控特性。

其次,基于几何相位和传播相位理论,设计了一种由各向异性金属谐振结构组成的单层超薄反射型超表面。由琼斯矩阵理论出发,利用各向异性结构对正交线极化波独立的传播相位响应,打破了传统几何相位对正交圆极化波的共轭对称调控,赋予了正交圆极化波独立的波前相位分布函数,实现了对正交圆极化波的独立去耦合波前调控。在中心工作频点14 GHz,超表面厚度仅为0.07λ0。通过改变单元沿x和y方向的物理尺寸实现所需的传播相位,将单元旋转特定角度实现所需的几何相位,构建了三个两比特编码的相位型超表面阵列,在正交圆极化波作用下分别用于实现不同焦距的汇聚波束、不同模式的汇聚涡旋波束和不同出射方向的偏折波束。仿真分析和实验测试表明这种去耦合调控的设计方法有效提高了超表面调控圆极化波的极化复用率。

第三,基于手性结构的圆二色性和几何相位理论,提出了一种频率复用的几何相位型手性超表面,实现了对入射正交圆极化电磁波的自旋选择性吸收和反射。通过分析圆极化波自旋选择性吸收的工作机理,得到了具有自旋选择吸收功能的超表面所需满足的结构对称性必要条件。结合手性结构的圆二色性,设计了由两个具有不同半径和线宽、相同开口张角的非对称开口谐振环和一个半波片组成的手性超表面单元。引入几何相位理论,分别设计了梯度手性超表面和涡旋手性超表面,数值仿真表明,在谐振频率f1和f3,实现了对右旋圆极化入射波的高效吸收,将入射的左旋圆极化波转换为偏折波束和携载-2模式OAM的涡旋波束;而在谐振频率f2,实现了对左旋圆极化波的高效吸收,同时实现了右旋圆极化入射波的定向出射和+2模式反射涡旋波束的生成。通过加工并测试两个超表面样品,验证了所提出的几何相位型手性超表面对圆极化电磁波自旋选择吸收和反射的工作性能,为相位型超表面实现正交圆极化电磁波的独立波前调控提供了新的思路。

最后,基于复合几何相位和传播相位理论,提出了一种由轴对称的金属谐振结构组成的宽频带单层反射型超表面,在10-16 GHz实现了对正交圆极化波的独立去耦合波前调控。根据传输矩阵理论可知,当单元的正交线极化相位响应满足180º相位差时,对正交圆极化入射波赋予一致的传播相位和共轭对称的几何相位,可以实现对正交圆极化波的独立去耦合调控。设计了由左右对称的两个开口圆环和一个矩形半波片构成的超表面单元,通过改变两侧开口圆环的开口角度获得所需传播相位,将单元做旋转获得所需几何相位。基于此,构建了两个偏折波束复用型超表面,分别赋予了左、右旋圆极化波45º、90º和30º、60º的梯度相位分布;构建了三比特编码的全息复用型超表面和涡旋波束复用型超表面,在左、右旋圆极化波激励下,分别实现了字母“L”和“R”、“双焦点”和“三焦点”的全息像,以及-3模式和+2模式涡旋电磁波的生成。数值仿真和实验测试表明,所提出的自旋解耦超表面在宽带内有效实现了对正交圆极化波的去耦合独立波前调控,为自旋解耦超表面的实际化应用提供了新的方案。

本文以正交圆极化电磁波波前独立调控为研究目标,结合超表面的几何相位、传播相位和手性超表面的圆二色性等理论,对圆极化波波前解耦调控方法展开了研究。本文的研究结果在极化通道复用、圆极化电磁波探测以及现代无线通信系统等领域具有一定的应用前景。

Other Abstract

Metasurface, a two-dimensional structure, consists of a series of artificially designed subwavelength electromagnetic scatters arranged in a periodic or quasi-periodic manner. Through finely design the subwavelength meta-atom, the electromagnetic response can be changed, and thus enabling flexible manipulation of the electromagnetic wave fundamental characteristics, such as polarization, phase and amplitude. Recently, metasurface based on geometric phase have attracted much attention and been extensively studied, due to its superior features of nondispersive, broadband and easy implementation, that is the main method for achieving wavefront manipulation of circularly-polarized (CP) electromagnetic wave. However, this modulation manner exhibits a conjugate symmetric phase response to orthogonal CP wave, which limits the degree of freedom for polarization modulation of both spin waves and results in a single working performance and low utilization of metasurface. Therefore, according to phase theory, this thesis analyzes the methods of decoupling independent modulation of orthogonal circularly polarized waves, and carries out the study of wavefront modulation of orthogonal circularly polarized electromagnetic waves by using phase-type metasurface. The research provides a certain foundation for enriching the degree of freedom of metasurface modulating CP wave and extending the polarization multiplexing method. The main research contents are summarized as follows:

Firstly, based on the geometric phase theory, a single layer reflective metasurface composed of an H-shaped metal resonant structure is proposed, which can achieve a high co-polarization conversion efficiency over 92% for incident CP waves in the ultra-wide operating band from 7.3 GHz to 21 GHz, the relative working bandwidth is up to 96.8%. A metasurface array is designed to generate vortex waves carrying 2 mode orbital momentum angular (OAM). The research results obtained by numerical simulation and experimental measurement show that the metasurface modulated by geometric phase can realize efficient generation of vortex waves in ultra-broad working band. Additionally, the conjugate symmetry of the conventional geometric phase modulation for left- and right-handed CP waves is verified as linearly polarized incident wave illuminating the metasurface.

Secondly, based on the geometric phase and propagation phase, a single-layer and ultra-thin reflective metasurface consisting of an anisotropic metal resonator is proposed. According to the Jones matrix theory, the conjugate symmetry of conventional geometric phase can be broken by applying the independent phase response of the anisotropic structure for two orthogonal linearly polarized waves. The decoupled wavefront modulation is realized at a single operating band by endowing independent wavefront phase distribution functions for orthogonal CP waves. The thickness of metasurface is only 0.07λ0 at the central operating frequency of 14 GHz. The desired propagation phase can be achieved by varying the physical sizes along x and y directions, and the desired geometric phase can be achieved by rotating the structure. Additionally, three two-bite encoded phase-type metasurface are constructed and applied to realize convergent beams with different focal lengths, convergent vortex beams with different OAM modes, and deflected beams with different outgoing directions under orthogonal CP incident waves illuminating, respectively. The feasibility of this decoupled modulation design method is demonstrated by simulation analysis and experiments, and the polarization multiplexing rate of metasurface manipulating CP waves is effectively improved.

Thirdly, based on the circular dichroism of the chiral structure and geometric phase theory, a frequency-multiplexing chiral metasurface is proposed to realize spin-selective absorption and reflection functions for dual-orthogonal CP incident waves. The working mechanism for the spin-selective absorption of CP waves is analyzed, and then the necessary conditions of structural symmetry for the metasurface design are obtained. Combined with the circular dichroism of the chiral structure, a chiral metasurface is designed, which is composed of two asymmetrical split-ring resonators with different radii and widths and same asymmetric opening angles, and one half waveplate inclined at a certain angle. Furthermore, a gradient chiral metasurface and a vortex chiral metasurface are constructed. Numerical simulations show that the incident right-handed CP wave is highly absorbed at the resonant frequencies f1 and f3, while the incident left-handed CP wave is reflected and converted into deflected beams or vortex beams with the OAM mode of -2. On the contrary, the incident left-handed CP wave is highly absorbed at the resonant frequency f2, and the incident right-handed CP wave is directionally reflected or vortex wave carrying 2 mode OAM is generated. Through fabricating and measuring two metasurface prototypes, the working performance of spin-selective absorption or reflection for CP electromagnetic waves are verified. The research results provide a promising route for the independent wavefront modulation of orthogonal CP waves with phase-type metasurface.

Lastly, based on the composite geometric and propagation phase theories, a broadband single-layer reflective metasurface composed of an axisymmetric metal resonator is proposed. The decoupled wavefront modulation is realized in the frequency range of 10-16 GHz. In line with the transmission matrix theory, when the phase difference under two orthogonal linearly polarized waves illuminating equals to 180º, the decoupled independent manipulation for orthogonal CP waves in different transmission channels can be easily realized by endowing a consistent propagation phase response and conjugate symmetric geometrical phase response to the orthogonally circularly polarized incident wave. The meta-atom consisting of two rings with symmetry opening angles on the left and right sides and one rectangular half waveplate is designed. The desired propagation phase can be obtained by adjusting the opening angle on both sides, and the rotate of units can obtain the geometric phase. Moreover, two deflected multiplexing metasurfaces are constructed to induce gradient phase distributions of 45º\90º and 30º\60º for left- and right-handed CP waves, respectively.  And 3-bit encoded spin-decoupled holographic multiplexed metasurface and vortex multiplexed metasurface are separately constructed. The effectiveness of the proposed broadband spin-decoupled metasurface for decoupled independent wavefront modulation of orthogonal CP waves is verified by full-wave simulations and experimental measurements. The research results provide a new scheme for its practical application.

In this thesis, the independent wavefront manipulation of orthogonal CP waves as the research objective, through combing the theories of geometric phase, propagation phase and circular dichroism of chiral structure, the spin-decoupled wavefront control methods are investigated. The research results of this thesis have promising application prospects in the fields of phase-type metasurfaces in polarization channel multiplexing, CP electromagnetic waves detection and modern wireless communication systems.

MOST Discipline Catalogue理学 - 物理学 - 无线电物理
URL查看原文
Language中文
Other Code262010_120190907330
Document Type学位论文
Identifierhttps://ir.lzu.edu.cn/handle/262010/538004
Collection信息科学与工程学院
Affiliation
兰州大学信息科学与工程学院
Recommended Citation
GB/T 7714
陈文琼. 相位型超表面对正交圆极化波波前调控研究[D]. 兰州. 兰州大学,2023.
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