论非互易电磁表面波的传输及耦合

发布时间：2018-11-12 12:44

【摘要】：电磁波传播通常是互易的。但是在磁光介质和旋磁介质中,电磁波的传播可以是非互易的。这个非互易性是通信系统中一些重要器件的基础,例如隔离器和环形器。最近,人们发现在磁光介质和旋磁介质的表面,电磁波甚至可以单向地传播。这个独特的性质提供给我们一个独特的系统去研究最基本的电磁波传播与耦合的问题。这篇博士论文的目标就是揭示非互易电磁表面波的传播与耦合的基本性质,并用这些性质去设计新器件。我们在第一章简要介绍了背景。然后,在第二章我们给出了电磁波非互易性的概念,讨论了最主要发展的计算方法,针对旋磁光子晶体的平面波展开法,并介绍了能带拓扑数—陈省身数,同时介绍了微波旋磁介质铁氧体的材料模型。第三章,我们首次证明了,在外加磁场下,在空气中的旋磁光子晶体平板的边缘可以支持单向电磁波。此外加磁场打破了结构的时间反演对称性,打开原来的简并点,创造了新的禁带。由于水平向的禁带效应及垂直向的介电常数差,使得单向电磁波可以束缚在结构边缘。然后,我们研究了两个单向波导间的耦合。我们发现当两波导中的模式传播方向相同时,他们间可存在正向耦合。当两波导中的模式传播方向相反时,他们间可以在传播常数很接近的窄带宽里发生反向耦合。这个反向耦合的效应与‘"trapped rainbo w"概念相关。在第四章,我们讨论了“trapped rainbow"的概念。它试图将不同频率的电磁波永久地停驻在不同位置。前人提出的结构,由于前向模式和后向模式间的反向耦合,所有的入射波都被反射了,而非停驻在特定位置,它们均不能真正实现‘"trapped rainbow"效应。我们提出采用梯度磁场下非互易波导来克服这个根本性难点,在微波段实现真正的“trapped rainbow"式电磁波存储。我们通过色散关系揭示了其背后的物理原理,并通过频域和时域的仿真证明了电磁波停驻的效应,并且发现在临界位置附件,电磁场得到增强,电磁’波脉冲也可以停驻较长时间。而且这些效应在非互易波导中对障碍物具有鲁棒性。在第五章,我们首次提出单向腔的概念。在太赫兹频段,单向腔可由表面磁等离子体构成。我们发现外加磁场可以使得腔中的顺时针模式和逆时针模式完全分开在两个不同的频率范围。这为我们在单向波的方向和频率范围上提供了更多的选择。我们也对单向腔与波导间的耦合进行了研究,并基于此设计了一个四端口环形器。最后,在第六章,我们做出结论,并讨论了未来研究的方向。
[Abstract]:Electromagnetic wave propagation is usually reciprocal. However, the propagation of electromagnetic wave can be non-reciprocal in magneto-optic medium and gyromagnetic medium. This non-reciprocity is the basis of some important devices in communication systems, such as isolators and annulators. Recently, it has been found that electromagnetic waves can even propagate in one direction on the surface of magneto-optic and gyromagnetic media. This unique property provides us with a unique system to study the most fundamental problem of electromagnetic wave propagation and coupling. The purpose of this doctoral thesis is to reveal the basic properties of the propagation and coupling of non-reciprocal electromagnetic surface waves and to design new devices with these properties. In the first chapter, we briefly introduce the background. Then, in the second chapter, we give the concept of non-reciprocity of electromagnetic wave, discuss the most developed calculation method, and introduce the topological number of energy band-Shiing-Shen Chern number for the plane wave expansion method of gyromagnetic photonic crystal. At the same time, the material model of microwave magnetic medium ferrite is introduced. In chapter 3, we prove for the first time that the edge of a plate of magnetomagnetic photonic crystals in air can support unidirectional electromagnetic waves under an external magnetic field. In addition, the magnetic field breaks the time inversion symmetry of the structure, opens the original degenerate point, and creates a new forbidden band. Because of the horizontal bandgap effect and the permittivity difference in the vertical direction, the unidirectional electromagnetic wave can be bound to the edge of the structure. Then, we study the coupling between two unidirectional waveguides. We find that there is a positive coupling between the two waveguides when the modes propagate in the same direction. When the modes in the two waveguides propagate in the opposite direction, they can be coupled in a narrow bandwidth where the propagation constant is very close. This reverse coupling effect is related to the 'trapped rainbo w' concept. In Chapter 4, we discuss the concept of "trapped rainbow". It tries to stop electromagnetic waves of different frequencies permanently in different places. Because of the backward coupling between the forward mode and the backward mode, all incident waves are reflected instead of stopping at a specific position, so they can not really realize the 'trapped rainbow' effect. We propose to use nonreciprocal waveguides in gradient magnetic field to overcome this fundamental difficulty and realize real "trapped rainbow" electromagnetic wave storage in microwave field. We reveal the physical principle behind it by dispersion relation, prove the effect of electromagnetic wave stopping by simulation in frequency domain and time domain, and find that the electromagnetic field is enhanced at the critical position. Electromagnetic'- wave pulses can also stay for longer periods of time. Moreover, these effects are robust to obstacles in non-reciprocal waveguides. In the fifth chapter, we propose the concept of unidirectional cavity for the first time. At terahertz, a unidirectional cavity can be made up of a surface magnetic plasma. We find that the applied magnetic field can separate the clockwise mode and the counterclockwise mode completely in two different frequency ranges. This provides us with more choices in the direction and frequency range of unidirectional waves. We also study the coupling between the unidirectional cavity and the waveguide, and design a four-port loop based on this. Finally, in chapter 6, we draw a conclusion and discuss the direction of future research.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O441

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