Theoretical 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, and also brings us more sophisticated ways to control the electromagnetic waves and obtain novel devices. 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. We then describe the concept of nonreciprocity and the main calculation method used in our papers in the second chapter. 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 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 oneway 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 these 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