Guided Wave Mode Resonances Of Terahertz Waveguides And Their Modulation

Terahertz technology is a promising research field with considerable application prospects in communications,defense industry,biomedicine,astronomy and other fields.Compared with terahertz signal source and detection technology,the development of terahertz functional devices is still relatively backward,which is an important factor restricting the development of terahertz technology.The development of terahertz devices has become a key issue to be solved urgently,especially filters,mode selectors,and so on,with the development of terahertz technology,particularly apply in 6G communication technology in the future.The terahertz waveguide structures have the advantages of low transmission loss,small size,compact structure and easy integration,and have been widely studied and applied in the transmission control of terahertz waves and the functional devices.Therefore,four terahertz waveguide structures are designed based on a kind of periodic circular waveguide in this thesis.The guided wave mode resonances and band gap characteristics are studied.Liquid crystal materials are filled in the waveguide cavities,and the tunable terahertz waveguide type device functions are realized by using the change of external magnetic field and temperature field.The functions are realized which include tunable terahertz mode selection,filtering,sensing and so on.The main research contents of this thesis are as follows:(1)The four-mode resonances and their regulation characteristics at different positions of the Brillouin zone in the terahertz waveguide are studied,and the functions of terahertz mode selection and mode filtering are realized.It is found that all can generate two passbands when the resonance occurs at the center and the boundary of the Brillouin zone.The analysis of the passband modes shows that one passband is a multimode passband,and the other one is a single-second-order mode passband in the case of four-mode resonance at the center of the Brillouin zone.Both two are multimode passbands in the case of four-mode resonance at the boundary of the Brillouin zone.The waveguide cavity is filled with liquid crystal material,and the output mode can be selected within the range of 0.5572～0.5926 THz(frequency width35.4 GHz)through external magnetic field control.(2)The interface modes of terahertz heterostructure waveguides are researched,the functions of terahertz narrowband filterings are realized.The three kinds terahertz heterostructure waveguides based on interface modes are designed.They are heterostructures with different radius fluctuations,heterostructures with different duty ratios and heterostructures with different fluctuation shapes,respectively.The heterostructure waveguides are formed by connecting two waveguides with different structural parameters and overlapping frequency domain of forbidden bands.The study finds that three kinds of heterostructure waveguides can generate local resonance at the connection interfaces,resulting in the interface modes,which appears as narrow passbands in the transmission spectrums.The heterostructure waveguides are filled with liquid crystal and regulated by temperature field,and a tunable frequency width of 20.9 GHz is obtained.The maximum Q value and sensitivity are 1045 and 0.6967 GHz/°C,respectively.The temperature dependent of interface modes can be applied to terahertz temperature controlled narrowband filtering and temperature sensing.(3)The heterostructure waveguides of double interface modes are studied,and the function of terahertz flat-top filtering is realized.Three waveguides are connected to form a heterostructure.The generation frequencies of the two interface modes can be made close,by adjusting the connection phases of the two interfaces,and a passband with flat-top characteristic can be obtained in the transmission spectrum.The effects of the duty ratio combinations and the number of waveguide periods on the bandwidth are studied,and the maximum bandwidth is 52.2 GHz.The double interface modes are regulated by temperature and magnetic field.The temperature tunable frequency width is 18.75 GHz,and the magnetic field tunable frequency width is 69.75 GHz,which realize the functions of terahertz tunable bandpass filtering.(4)The local resonances in terahertz waveguides are studied,and Bragg and non-Bragg defect modes are obtained.The two defect modes realize the functions of dual-channel narrowband filtering and ultra-narrowband filtering,respectively.The effects of defect locations on the two defect modes are investigated.The effects of defect lengths and shapes are also investigated for Bragg defect modes.The tunable dual-channel filtering function is realized through magnetic field regulation,when the waveguide cavities are filled with liquid crystal material.The narrowest linewidth is only 0.24 MHz,and the maximum Q value can reach 3×10~6.In conclusion,this thesis studies the guided wave mode resonances in four terahertz waveguide structures and their modulation characteristics.The four terahertz waveguide structures realize the functions of single high-order mode output,narrow passband,flat passband and ultra-narrow passband,respectively.The tunable properties of the output spectrums of these structures are investigated based on the magnetic field and temperature effects of liquid crystal materials.By combining the four terahertz waveguide structures with liquid crystal materials,they can realize the functions of mode selection,temperature sensing,tunable flat filtering and tunable ultra-narrowband filtering,respectively.The work of this thesis opens up new ideas for the development and application of terahertz waveguide functional devices.