Spoof Surface Plasmon Polaritons On Ultrathin Film Structures And Their Applications

Surface plasmon polaritons(SPPs),which are surface electromagnetic(EM)waves propagating along the metal/dielectric interface,have attracted increasing attentions owing to their remarkable capability of guiding and localizing EM wave into sub-wavelength scales and applications in miniaturized optical integrated circuits and devices with scales smaller than the light wavelength.Unlike in the optical frequency band,at terahertz or microwave band,metals resemble a perfect electric conductor(PEC)since their plasmon frequencies are usually lie in the ultraviolet region,thus do not support the SPP waves.Recently,plasmonic metamaterials have been proposed by J.B.Pendry to support highly confined surface EM waves at lower frequencies,which are also called as the spoof SPPs.However,most of the existing plasmonic metamaterials rely on three-dimensional(3D)structures of sub-wavelength scaled geometry on metal surfaces,making them not convenient to be fabricated and integrated with other existing terahertz or microwave circuitry.More recently,T.J.Cui et al.proposed an ultrathin plasmonic metamaterial to support spoof SPPs,which are named as conformal surface plasmon(CSP)due to its ultra-thin and flexible nature,paving the way of developing versatile surface wave integrated devices or circuits at lower frequencies bands,especially at terahertz region.In the view of above development trend,this dissertation focuses on the studies of CSP structures and its application.We analyzed different types of modes of spoof SPPs on the CSP structures in detail.According on the property of spoof SPPs,we designed a series of prototype devices,including splitter,coupler,filter,and resonator.Furthermore,we proposed and demonstrated for the first time a specific type of CSP structure which can support the backward wave propagation of spoof SPPs.The main contributions of this dissertation are listed below:(1)We theoretical analyzed the dispersion and transmission characteristic of the fundamental and high-order modes of spoof SPPs on the asymmetric CSP structure and established the condition of occurrence for the high-order modes.We also verified the theoretical findings by measuring the transmission spectrum on fabricated samples.Based on the analysis,we verified the high-order mode spoof SPPs propagation with relatively low bending loss on curved structure by simulation and experiment.The research findings provide the important reference for us at designing the spoof SPPs prototype devices with high-order modes.(2)The fundamental mode of spoof SPPs on the symmetric CSP structure are theoretical analyzed,which includes odd and even modes.A way to excite the odd mode of spoof SPPs is proposed and verified by the simulated and measured transmission spectrum of the symmetric CSP structure.Furthermore,we proposed a new symmetric CSP structure,which presents strong confinement and lower-loss of spoof SPPs at given frequency bands.The proposed structure is very promising in applications to achieve the miniaturization of integrated circuits and devices.(3)We carried out researches on how to design CSP prototype devices.Firstly,according to the coupled mode theory,we designed frequency splitter and directional coupler at terahertz frequency bands by utilizing the even fundamental mode of symmetric CSP structure.To verify the performance of the proposed waveguide devices,we scaled down to microwave frequency by simply scaling up the geometry of the devices and made direct measurement of the transmission spectra on the fabricated prototype samples at microwave frequency band,which validate the theoretical analysis and the functionality of the frequency splitter and the directional coupler.Secondly,according to the property of single-mode propagation of the spoof SPPs,we designed a wide bandpass filter witih wide upper stopband by introducing a transmission zero at lower frequency band.The proposed filter may overcome the narrow upper stop band in ultra-wideband microstrip line filters.Thirdly,an asymmetric CSP structure with ground plane is proposed,which support the mode of spoof SPPs with lower radiative loss.We achieved the bandpass filters by utilizing the first high-order mode,of which the central frequency and the bandwidth may be easily tuned by cascading different CSP structures.Finally,we designed a straight-line resonator by utilizing the symmetric CSP structure and proposed a way to increase the Q factor by analyzing the Q factor of the resonator.Furthermore,we can indirectly measure the dispersion curve and attenuation coefficient of spoof SPPs by measuring the Q factor of the proposed resonator.(4)A novel symmetric CSP structure is proposed,which can support backward spoof SPPs wave.We designed a contra-directional coupler at microwave frequency bands by utilizing the property of backward wave,and verified the property of coupler by measuring the transmission spectra on the fabricated prototype samples.The proposed CSP structure is very promising in developing novel circuits and devices.