Transmission And Coupling Of Terahertz Metal Waveguide

Terahertz（THz）science and technology have great applications in wireless communications;safety monitoring;non-destructive image;sensor and so on.These wide applications rely on various THz devices.As one of them,THz waveguide is the most important basic device.Just as its name implies,the THz waveguide is used to guide the transmission of the THz wave,which is transmitted from one place to another one where it needs to arrive.A lot of waveguide structures have been investigated by researcher.At the same time,functional devices based on the THz waveguides have also been developed.According to the material of waveguide,there are three kinds of waveguide in THz band:metal waveguide;metal-dielectric waveguide;and pure dielectric waveguide.The transmission loss of waveguide is an important index to evaluate its performance.Besides,coupling is also a research hotspot.This dissertation is based on the research of the transmission and coupling of THz wave in waveguide.Then we propose the evancesent resonant mode of the T-shape cavity based on a parallel-plate metal waveguide（PPMWG）,theoretically analyse the resonant phenomenon,and explain the essence of this phenomenon.In addition,we suggest a new waveguide structure based on the dielectric-metal waveguide and analyse its propagation loss and the coupling efficiency of the cos-Gaussian mode with Gaussian beam.Last,we design an elliptical-hyperbolic connecter,which can improve the coupling efficiency between the Gaussian beam and hyperbolic metal waveguide.This doctoral dissertation mainly works on the following three points:1.The evanescent resonant mode is proposed for the T-shape cavity based on a THz PPMWG to understand the essence of the resonant phenomenon.For the T-shape cavity based on a THz PPMWG,there is a narrow resonant dip in the transmission spectrum and the power transmitivity is almost zero at the resonant point.This feature makes the T-shape cavity an important application in sensor and filter.A mode-matching method was used by Mittleman et.al.to analyze the resonant phenomenon,and this resonant mode is called the fundamental cavity mode by Chen et.al.However,there is still short of a reasonable explanation for the resonant phenomenon.The proposed evanescent resonant mode consists of three evanescent fields outside the cross region and one propagation field inside the cross region.Based on the orthogonality between modes,the resonant frequency the input TE₁ mode can not couple the THz wave into the T-shape cavity.This is a reasonable understanding of the resonance phenomenon.In addition,the resonant frequency depends closely on the geometrical parameters of T-shape cavity.The modal field and the resonant frequency by model show good agreement with those by simulation.This work has the following innovations:（a）The evanescent resonant mode can well explain the resonant phenomenon.When the TE₂-like evanescent resonant mode is excited,the input TE₁ mode can not be coupled into the T-shape cavity due to the orthogonality between modes.Most of the incident wave energy is reflected,and a small portion is localized in the T-shaped cavity,with little energy passing through.（b）The resonant frequency shows a complicated relation with the geometrical parameters of the T-shape cavity.There is strong effect of the separation of PPMWG on the resonant frequency,slight effect of the width of the groove on the resonant frequency,and little effect of the height of the groove on the resonant frequency.In order to avoid the non-evanescent electric field in the groove,the separation of PPMWG must be twice larger than the width of the groove.（c）We have not only successfully investigated the TE₂-like evanescent resonant mode,but also predicted the existence of the TE₁-like and TE₃-like evanescent resonant modes in the T-shape cavity.Among those modes,the TE₁-like evanescent resonant mode is localized in the T-shape cavity.2.Cos-Gaussian mode of a rectangular metal waveguide（RMWG）filled with multiple slices dielectric is proposed.In THz region,the TE₀₁ mode of a RMWG has lower propagation loss,but its modal field is almost invariable along one direction,which does not match the Gaussian beam.The fundamental mode of a graded-index fiber is Gaussian,but it suffers the dielectric material loss.Based on the TE₀₁ mode of a RMWG and the Gaussian mode of a fiber,we propose the cos-Gaussian mode of a RMWG filled with dielectric.First,an ideal graded-index distribution along one direction dielectric is used,and its propagation characteristics is discussed.Furthermore,a RMWG filled with multiple slices of dielectric is suggested to fulfill graded-index distribution according to the effective medium theory.The cos-Gaussian mode has higher coupling efficiency with Gaussian beam,and at the same time has a lower loss.This work has the following innovations:（a）According to the Gaussian approximation used in analyzing the fiber and the RMWG theory,the eigen equation of RMWG filled with graded-index dielectric is established.The modal field and effective index are solved numerically.（b）The shapes and polarization of the cos-Gaussian mode and Gaussian beam are very similar,which leads to the larger mode overlap and higher coupling efficiency.（c）Because of the Gaussian field distribution,the field value is relatively small at the metal boundary,which decreases the ohmic loss.The cos-Gaussian mode has a lower loss.（d）Graded-index distribution is achieved with multiple slices dielectric.Due to the introduction of air,the equivalent loss is reduced.3.Tapered elliptical-hyperbolic metal waveguide（EHMWG）is designed to efficiently couple the THz Gaussian beam into the hyperbolic metal waveguide（HMWG）.HMWG has the ability of focusing and enhancing the THz wave.However,the modal field of HMWG is elliptically polarized,which leads the lower coupling efficiency with the linearly polarized Gaussian beam.EHMWG has the ability of transforming the TE₀₁ mode of RMWG into the elliptically polarized mode of HMWG adiabatically.According to WKB approximation,the input THz wave can propagate without the reflection and scattering.Compared with the elliptically polarized mode,the TE₀₁ mode is linearly polarized and has a high coupling efficiency with a Gaussian beam.This work has the following innovations:（a）The TE₀₁ mode of RMWG is gradually transformed into the elliptically polarized mode of HMWG.（b）Through this EHMWG connector,the coupling efficiency of the Gaussian beam with the HMWG is greatly improved.This dissertation includes our studies on metal waveguide.We have laid emphasis on the evanescent resonant mode for T-shape cavity based on a PPMWG,the properties of cos-Gaussian mode of RMWG filled with multiple slices dielectric,the coupling between Gaussian beam and HMWG through the EHMWG connector.These results may have a certain applying prospect in filter,sensor,frequency-multiplexing and so on.