Investigation On Propagation And Coupling Of Nonreciprocal Electromagnetic Surface Waves

Electromagnetic wave propagation is usually reciprocal. However, in magneto-optical or gyromagnetic media, the propagation can be nonreciprocal. This effect has contributed to many important components in telecommunication systems, such as isolators and circulators. Recently, it has even been demonstrated that on the surface of magneto-optical or gyromagnetic media, electromagnetic waves can propagate in only one direction. This unique property provides us with a very feasible system to observe fundamental effects in wave guiding and coupling. The aim of this thesis is to reveal the fundamental guiding and coupling properties of nonreciprocal electromagnetic surface waves and to design novel applications based on the properties.We introduce the background in the first chapter. In the second chapter, we describe the concept of nonreciprocity. We also discuss the main calculation method we developed, the plane wave expansion method for gyromagnetic photonic crystals. In addition, we introduce one topological invariant of photonic band-Chern number and discuss the material model for ferrie, which is a gyromagnetic material at microwave frequencies. In the third chapter, we demonstrate that one-way waves can be sustained at the edge of a gyromagnetic photonic crystal slab under an external magnetic field. The magnetic field breaks the time-reversal symmetry, separates a degenerate point and opens a bandgap. In the bandgap, this one-way wave is localized horizontally to the slab edge and confined by the index contrast in the vertical direction. Next, we study the coupling between two parallel one-way waveguides. We find that when the waveguides support modes propagating in the same directions, there exists a co-directional coupling between them. When they support modes propagating in opposite directions, a contra-directional coupling between them can effectively occur in a narrow window where the propagation constants of the two modes are very close. This contra-directional coupling is related to the concept of a "trapped rainbow".In the fourth chapter, we address the concept of a "trapped rainbow". It aims at trapping different frequency components of the electromagnetic wave packet at different positions in space permanently. In previously proposed structures, the entire incident wave is reflected rather than trapped due to the strong contra-directional coupling between forward and backward modes. To overcome this difficulty, we show that utilizing nonreciprocal waveguides under a tapered external magnetic field can achieve a truly "trapped rainbow" effect at microwave frequencies. We reveal the physical mechanism by dispersion relations, and show "trapped rainbow" effect through frequency domain and time domain simulations. We observe hot spots and relatively long duration times around critical positions and find that such a trapping effect is robust against disorders.In the fifth chapter, we show a one-way cavity formed by surface magnetoplasmon at terahertz frequencies. We find that the external magnetic field can separate the clockwise and anti-clockwise cavity modes into two totally different frequency ranges. This offers us more choices, both in the frequency ranges and in the one-way directions, for realizing one-way components. We also investigate the waveguide-cavity coupling by designing a circulator, which establishes the foundation for potential applications. Lastly, in the sixth chapter, we make the conclusion and discuss about the future work.