| Surface plasmon polaritons(SPPs)exist naturally in the optical regime,which essentially are the interaction between the incident electromagnetic waves and the free electrons at the metal-dielectric interface.Due to the resonance effect between the surface charge oscillation and the electromagnetic field of the light,this special surface wave has extraordinary field confinements and enhancements.When the frequency scales down to the far infrared,terahertz,and microwave bands,spoof surface plasmon polaritons(SSPPs)on an ultrathin and flexible film printing serrated metal strip also has the advantages of lightweight,conformal,low profile and easy to combine with traditional circuits.In recent years,microwave devices and systems based on the ultra-thin SSPP structure have become one of the important means of using artificial microstructures to control electromagnetic waves.In this paper,the physical mechanism and engineering application of SSPPs are systematically studied.Through the special design of sub-wavelength artificial microstructure,the fine control of surface plasmon is realized and the unique advantages of the application of artificial surface plasmon in microwave system are demonstrated from two aspects of transmission and radiation respectively.The main research contents and contributions are summarized as follows:1.The general theory that the electromagnetic characteristics of the SSPP waveguides vary with the modulation period size is studied.The results show that as the periodic structure increases from the depth sub-wavelength scale to the wavelength scale,SSPPs will undergo the process from high-efficiency transmission to Bragg reflection,and finally form directional leaky wave and multi-beam leaky wave radiation.The above conclusion is also consistent with the grating theory.2.Two novel SSPP filters are designed.Firstly,a miniaturized SSPP unit is proposed which can significantly increase the slow wave effect and field confinement of SSPPs.A bandpass filter with this new structure has shown the advantages of ultrawideband and miniaturization.Then,a design method of multi-mode SSPP strucrure loaded with split-ring resonator is proposed.The bandgap between multi-modes can form transmission stopband.By loading varactor diode,the stopband is reconfigurated to act as the radio frequenccy(RF)switch,and finally RF logic gates can be realized by cascading two units with different geometric sizes.3.Three SSPP leaky wave antennas(LWAs)are studied.Firstly,the spatial multi-polarization electromagnetic radiation is designed by using the annular omnidirectional radiation characteristics of single conductor SSPP LWA;secondly,the polarization customization of leaky wave radiation is realized by using the combination of orthogonal components,which simplifies the design process of electromagnetic wave polarization.The above work provides a new idea for flexible control of leaky wave polarizations.In addition,based on the theory of coding metamaterials,a hybrid digital coding metasurface is proposed to deal with both space wave and SSPPs with orthogonal polarizations at the same time.Using the hybrid digital coding metasurface,one can control the leaky wave direction of SSPPs and the reflection direction of plane wave simultaneously,which improves the utilization efficiency of the antenna and is conducive to the integrated design of the system.4.Two SSPP LWAs with fixed-frequency beam-scanning function are proposed.By loading varactor diodes in the modulation periodic structure,the surface impedance of the SSPP waveguide can be reconstructed in real time by using external bias voltage,so as to control the leaky wave direction.In addition,for the SSPP unit with dual-mode characteristics,the interaction between multi-modes supports dual-frequency periodic modulation,then a dual-band beam-reconfigurable LWA can be constructed.Above reconfigurable LWAs greatly reduce the cost of beam scanning and is expected to be widely used in modern radar and communication systems. |
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