Terahertz Functional Waveguide-Devices Based On The Non-Bragg Resonances

As a frontier interdisciplinary,terahertz wave technology has a very broad application prospect in fields such as security inspection,medical imaging,broadband communication.With the development of terahertz technology,the issue of transmission and control of terahertz waves has become a research hotspot,and the demand for the easy integration of functional devices is also growing.Based on the non-Bragg resonance,we designed a kind of cylindrical terahertz waveguide with periodic raring cross-sections.The terahertz reflectors and switches of waveguide type have been realized by utilizing the non-Bragg resonance structures and exteral field controlable materials,such as vanadium dioxide?VO₂?and liquid crystals.The main contents of the thesis are as follows:The first chapter introduces the background and significance of the researches on terahertz wave technology,summarizes the studies of different functional devices,and summarizes the main contents of the thesis.In the second chapter,the theory of periodic waveguide was derived theoretically.Moreover,the mechanism of Bragg and non-Bragg resonances is also revealed.We used the COMSOL Multiphysics to simulate the terahertz wave propagation in our designed waveguides and obtained the transmission spectra.In the third chapter,two kinds of waveguide type terahertz broadband reflectors are proposed.One is a kind of single structure periodic waveguide with variable cross-sections.we optimized the different structural parameters to widen the bandgap of the waveguide and achieved a broadband reflection of 240.7 GHz.The other one is combining two different structures to make the Bragg and non-Bragg gaps connected to achieve a broadband reflection near 1 THz.In this chaper,the influence of the periodic structure parameters of the waveguide?ie,the fluctuation parameters and the duty cycle?on the reflection bandwidth and the reflectivity is discussed in detail.Finally,a THz broadband reflector with reflectivity higher than 99%and reflection bandwidth up to 554.5 GHz was obtained.In the fourth chapter,three types of terahertz waveguide switches are realized by external field control.The first is to coat the periodic waveguide with VO₂,which has the phase transition characteristic as the temperature changing.Using the characteristic,the terahertz optical switch in the composite waveguide is realized.The second is to fill the waveguide with liquid crystals,which can be controlled by the external magnetic field.In the hollow cross-sectional waveguide filled with nematic liquid crystal E7,the effective refraction of E7is changed by controlling the direction of the liquid crystal molecules by the external magnetic field,so that the position of the forbidden band edge of the waveguide is changed and the magnetically controlled terahertz waveguide optical switch is realized.The third is to use the non-Bragg defect mode to achieve liquid crystal magnetic switch.In a variant cross-section waveguide with a defect filled with nematic liquid crystal E7,the frequency of the defect mode can be controlled by the magnetically-controlled effect of the liquid crystal,so the terahertz-narrowband waveguide magnetically controlled switch is realized.In summary,a variety of tunable terahertz functional devices are implemented by using single or composite variable-cross-section cylindrical waveguides with periodic structures in combination with different materials.