Research On Terahertz Functional Devices Based On Spoof Surface-Plasmon-Polaritons

Surface plasmon polaritons(SPPs)are electromagnetic waves that propagate along the metal-dielectric interface.Due to the subwavelength confinement and fieldenhancing properties,SPPs have broad application prospects in many fields.However,in the low frequencies(microwave and terahertz frequencies),metals have poor confinement to the SPPs,making it difficult to control the propagation of SPPs.Corrugated metal surfaces support bound SPP modes at low frequencies,and the physical characteristics of this electromagnetic mode displayed similarly to those in optical frequencies,which is called spoof SPPs.In this paper,surface gradient index(GRIN)lenses,multimode interferometers and logic gates are designed by using metal microstructures,which are realized the transmission control of spoof SPPs in terahertz and microwave frequencies.The main contents are listed below:Firstly,three kinds of gradient index lenses based on spoof SPPs were designed.The lenses with variable indices are constructed by fine-tuning the size of the pillars.Among them,the surface self-focusing lens can output a mirrored spoof SPPs with offaxis incidence at 0.61 THz.The surface Luneburg lens and surface fisheye lens can achieve sub-wavelength focusing of parallel incident spoof SPPs at 0.65 THz,with a half-maximum width of the focusing spot of about 1.08λ and 0.702λ,respectively,and efficiencies of about 6.7% and 16.1%,respectively.The measurement results obtained from scanning near-field terahertz microscopy show good consistency with the CST simulation results.Secondly,a multimode interference coupler based on spoof SPPs was designed.By designing multi-row metal pillars on the metal substrate,the metathesis function of THz spoof SPPs was realized.Through CST simulation optimization,under the symmetric interference condition,beam splitting and self-imaging output of the input light field can be achieved at 0.591 THz and 0.575 THz.Under the general interference condition,mirror imaging and self-imaging output of the input light field can be achieved at 0.586 THz and 0.57 THz,with good spectral splitting ratios in both interference conditions.Finally,two kinds of active control logic gates based on conformal surface plasmons(CSPs)were designed.The spoof SPPs mode dispersion relation supported on the structure is changed by rotating the unit in the waveguide.Based on the MachZehnder interferometer,the proposed structure is applied to the dual-port and threeport input logic gates.By changing the rotation state of different regions,the dual-port logic gate can realize the mutual conversion between NOR and OR,and the three-port logic gate can realize the mutual conversion between NAND,NOR,AND and OR.