Study On Mode Control Mechanism In A Terahertz Waveguide

The terahertz wave is between the millimeter wave and the infrared wave in the electromagnetic spectrum,and its properties are different from those in electromagnetic waves of other frequency bands.In the development of terahertz science,the transmission of terahertz waves is the most important part.In addition,the terahertz waveguide mode needs to be controled in some important applications so as to realize some special functions.This requires both solutions to the problems in terahertz wave transmission and an in-depth study about the impact of the waveguide structure on the terahertz wave transmission mode.In this paper,a cylindrical terahertz waveguide with periodic undulation structure is designed for solving the critical and basic problem of mode control in terahertz waveguide.And by ways of defects introduction,waveguide recombination and liquid crystal regulation,this paper explores the mechanisms of mode resonance,localization and mode conversion in the waveguide of the periodically variable cross-section.The thesis includes following aspects:1?Theoretically,the transmission mechanism of terahertz wave in periodically variable cross section waveguide is studied.Firstly,the Bragg resonance in the waveguide and the non-Bragg resonance in the variable cross-section waveguide are analyzed since both can cause the break-up of the band the formation of frequency band gap.By changing the geometric parameters in periodic structure waveguide,the range and location of band and the bandgap can be controlled.Then,the resonance among the three modes in the waveguide is analyzed,and the three-mode resonance can suppress the first order mode and transmit a single higher order mode.Next,it is proposed that the nematic liquid crystal,which is sensitive to the magnetic field,is used to control the center frequency of the single mode transmission band without changing the waveguide parameter.2?Defects are introduced in the periodic tectonic terahertz waveguide,and the defect states in the Bragg and non-Bragg gates are studied.It is found that the two defect states exist simultaneously in the waveguide.Then the spectral structure,field distribution and mode components of the two defect states are analyzed.In the case when the geometrical parameters of the waveguide remain invariable,the effects of the length and radius of the defect on the propagation characteristics of the defect state are discussed respectively.A new method is proposed to obtain the band transmission of defective state by adjusting the numbers of waveguide cycles and the periodic arrangement defect and the waveguide structure.Finally,based on the control of the defect length on the defect state frequency,the influence on the defect state is studied,and it finds the multi-peak penetration in the forbidden band.3?The Bragg and non-Bragg interface states are studied with the composite waveguide structure.The same bandgap range is designed for Bragg waveguides and non-Bragg waveguides under the condition of multimode renosance.Then,the Bragg waveguides are connected to each other to study the Bragg interface state,and the Bragg waveguide and the non-Bragg waveguide are connected to each other to study the non-Bragg interface state.By analyzing the mode components and the field distribution of the interface state,it can be seen that the formation process of the interface state is accompanied by the accumulation of energy and the generation of the local pattern.The follow-up study focuses on the regulation of the interface state,the role of the number of cycles playing in the energy accumulation with various numbers of cycles.The phase adjustment module is added to the composite waveguide connection to realize the regulation of the interface frequency.In conclusion,in the terahertz band,the periodic variable cross-section waveguide designed in this paper can realize the tuning of the forbidden band and the single-mode transmission range.On this basis,through the structural design or composite waveguide to achieve the defect state and interface state tuning.