| In common media,the transmission of electromagnetic waves is reciprocal.However,in gyromagnetic media and magneto-optical media,the transmission of electromagnetic waves can be non-reciprocal.Non-reciprocal transmission refers to the different characteristics of transmission of electromagnetic wave in two opposite directions.Non-reciprocal transmission is the basis of some important functional devices in radar and communication system,and has always been one of the research hotspots.In recent years,with the continuous development of microwave integrated circuit,the performance of non-reciprocal devices has become more demanding.In the microwave frequency band,the ultra-thin and flexible metal strip spoof surface plasmon polaritons(SSPPs)waveguide structure can support the transmission of electromagnetic wave well because of its characteristics of high field enhancement and confinement.At the same time,the structure has the unique advantages of miniaturization and conformal as well.Therefore,non-reciprocal devices designed with this structure usually have the characteristics of small size,light weight,easy preparation and high transmission efficiency,and have important application value in the field of radar and communication.In this thesis,we use the way of ultra-thin spoof surface plasmon polaritons waveguide combined with gyromagnetic ferrite to control the non-reciprocal transmission of electromagnetic waves and propose a variety of plasmonic circulator design methods.The main research contents are summarized as follows:(1)A four-port plasmonic circulator is designed and its transmission characteristics are analyzed and studied.The scheme is based on the interaction between the resonance mode of gyromagnetic ferrites and the spoof surface plasmon polaritons mode to control the directional transmission of waves.The circulator consists of spoof surface plasmon polaritons waveguides and ferrite disks on which a magnetic field is applied to realize non-reciprocal performance.In order to improve efficiency,the waveguide structure adopts matching transition.The simulation result shows that in the frequency range of 6.0-8.5 GHz,the return loss is less than-15 d B,the isolation degree reaches-10 d B,and the insertion loss is more than-0.8 d B.The simulation results are basically consistent with the calculation results of equivalent circuit model and the experimental results,which verifies the good performance of the proposed four-port circulator.(2)A design method of reconfigurable multi-port plasmonic circulators is proposed and the transmission performance of the circulators is studied.Firstly,we designed a T-shaped three-port circulator which is realized the low loss and wide band non-reciprocal transmission performance through simulation and optimization.The simulation result shows that in the 5.8-9.5 GHz frequency range,the return loss is less than-10 d B,and insertion loss is more than-1 d B.In order to manipulate the one-way transmission of electromagnetic wave to a greater extent,we transition the T-shaped three-port circulator to get the double T-shaped four-port circulator through series and H-shaped four-port circulator through parallel.And two sets of ferrite disks are introduced into the structure.The surface wave mode supported by ferrite can interact with the mode of SSPPs to realize non-reciprocal wave transmission.The surface wave pattern varies with the direction of the external biased magnetic field.We study the transmission of electromagnetic wave incident from different ports under the same magnetic field condition.And the transmission of electromagnetic wave incident from the same port under different magnetic field directions.The results show that high performance unidirectional transmission can be achieved in a certain frequency range.The design method of electromagnetic wave transmission along different channels by controlling the direction of the magnetic field increases the freedom degree of manipulating one-way transmission of electromagnetic waves and provides a reference for more flexible non-reciprocal transmission of control waves as well. |
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