Study On Single Mode Transparency Mechanism In A Terahertz Waveguide

Because Terahertz wave is in the special position of the electromagnetic spectrum and it has many unique advantages, and has potential application prospects in broadband communication, nondestructive detection, electronic countermeasure, medical imaging and security check. In recent years, with the development of ultrafast photonic technology,terahertz science and technology is developing rapidly and has been attracting more and more researchers to focus on this frontier area. Furthermore, in the practical application, terahertz mode controllable devices are very important for transmission and detection of signals with high-power and high-capacity. Therefore, single mode transparency mechanism is very essential for scientific research and practical application. This thesis mainly presents transmission characteristics of terahertz waves in a cylindrical waveguide with periodically undulated walls. The periodical structure causes multi-mode interactions which lead to effectively suppress low-order mode but generate and transmit a single high-order mode in the waveguide. It will provide a new way of theoretical basis for designing of terahertz functional devices. The main content of this thesis is shown below:The first chapter introduces the background and significance of terahertz technology,reviews different kinds of terahertz waveguides, states the necessity and urgency to study on single mode transparency mechanism in a terahertz waveguide . And a brief description of the main contents and arranged structure of this paper is given.The second chapter presents a single mode transparency mechanism in a terahertz waveguide with varying cross-section theoretically. According to Maxwell's equations and multi-mode interactions, a terahertz waveguide with periodically undulated walls which can realize the transmission of a single high-order mode function was proposed. The transmission properties were investigated by FEM software COMSOL Multiphysics. The results confirmed a single mode transparency and proved the validity and efficiency of this mechanism.The third chapter studies the influence of the structure parameters of the terahertz waveguides on the band structures. By introducing a shape factor, the influence of different wall profiles on the band structure of terahertz waveguides was discussed. Considering different vertex positions of triangular wall, high order derivatives of sin function, and rectangular wall with different duty ratios, we also analyzed the transmission characteristics of these terahertz waveguides. When the undulation changed, numerical simulations on terahertz waveguides with sinusoidal and rectangular walls were carried out using COMSOL Multiphysics software. The results revealed the transmission characteristics of TM mode in the waveguides. We obtained a narrow bandwidth of single mode transparency (7.5GHz) in the rectangular wall of terahertz waveguides.The fourth chapter describes a frequency-agile single mode transparency phenomenon in a rectangular walled waveguide by coating different thickness of gold on its inner surface and discusses the essential reasons for frequency manipulation physically.In summary,a single high-order mode transparency in a terahertz waveguide is realized by introducing periodic walls. The results of simulations showed that the suppression of low-order mode and transmission of single high-order mode can be controlled effectively by regulating the structure parameters in the terahertz waveguides.