Research Of Spoof Surface Plasmon Polaritons And Subwavelength Waveguides

Surface plasmon polaritons(SPPs)are a kind of surface electromagnetic waves that can propagate along a metal/insulator interface with high confinement.They appear in ultraviolet,visible and near-infrared frequency bands.SPPs have attracted much attention since they are promising in breaking diffraction limits and have potentials in miniaturization and integration of photonic devices.Spoof SPPs(SSPPs)are important developments of SPPs in terahertz and microwave bands.It is actually a kind of propagating surface electromagnetic waves bounded by periodic geometry,such as grooves,holes,metal particles and so on,at a metal surface.Their dispersion relations are mainly controlled by structure parameters.Thus they are more flexible in practical applications.In recent years,researches have shown that SSPPs have contacts with many fields,including terahertz technology,microwave technology,metamaterials and photonic crystals.Therefore,they have a broad application prospect.In this thesis,SSPPs and their subwavelength waveguides are analyzed theoretically.The ultrathin subwavelength SSPPs structures with special functions and features are designed.The main works are listed as follows:An SSPPs waveguide drilled with periodic L-shaped grooves is proposed and investigated.Dispersion formulas are first deduced based on the modal expansion method and continuity of electromagnetic field and a verification is given by simulation.Then the sensibility of dispersion relations of SSPPs to the transversal parts of L-shaped grooves are discussed analytically.Based on the results,the application of the SSPPs waveguides with L-shaped grooves in rainbow-trapping effect are investigated.In the end,SSPPs waveguides with multi-transversal-grooves are proposed and discussed,which solves the problem that the transversal groove of an L-shaped groove is limited by its period.Our investigation shows that an L-shaped groove can be equivalent to a deeper rectangular groove.Adding one or more transversal grooves to a rectangular groove is similar to increasing of its depth.A calculation method of dispersion relations for SSPPs waveguides drilled with periodic non-rectangular grooves is proposed.At first,dispersion formulas for waveguides with T-shaped grooves are derived with the help of modal expansion method and continuity of electromagnetic field.The influence of structure parameters on SSPPs is discussed analytically.Then theoretical formulas for waveguides with multi-layers composite grooves are obtained using analogical method based on the dispersion formulas for waveguides with rectangular and T-shaped grooves.The theoretical formulas can also be used to deal with SSPPs waveguides with grooves whose widths change continuously since those grooves can be regarded as composition grooves.As examples,we present analytical processes for the waveguides with trapezoidal and slanted rectangular grooves.It indicates that a multi-layers composite groove can be equivalent to a rectangular groove.The equivalent width and depth are controlled by the width and depth of each layer.A calculation method of dispersion relations for dual-grating waveguides with nonrectangular grooves is proposed.A dual-grating waveguide is formed by two SSPPs waveguides located face to face.A symmetric dual-grating waveguide supports two coupled modes,a symmetric one and an anti-symmetric one.The shapes of grooves can influence the coupled modes.Using modal expansion method and continuity of electromagnetic field,dispersion formulas for the symmetric and anti-symmetric modes supported by the dual-grating waveguides with rectangular grooves are respectively deduced.Based on the equivalent relations between multi-layers composite grooves and rectangular grooves,theoretical formulas for the dual-grating waveguides with multi-layers composite grooves are achieved and a verification is presented by simulation.The theoretical formulas are applicable to the dual-grating waveguides with grooves whose widths change continuously.The dual-grating waveguides with trapezoidal grooves are analyzed analytically using the formulas.The analytical researches show that characteristics of the dual-grating waveguides are controlled by two factors.One is the SSPPs waveguides forming them.The other is their gap widths.A dual-grating waveguide composed of two ultrathin SSPPs waveguides with rectangular grooves is investigated and an excitation system is proposed to generate coupled modes.Considering a thought of indirect excitation,we take use of the coupling between two symmetric or anti-symmetric SSPPs and design an excitation system,which includes a coplanar waveguide,a convertor,a coupler and a dual-grating waveguide.The S parameters and electromagnetic fields are simulated to demonstrate that the system can excite symmetric and anti-symmetric modes effectively.A dual-grating waveguide consisting of two SSPPs waveguides with periodic rhomboidal grooves is investigated.Two SSPPs waveguides with periodic rhomboidal grooves can form a symmetric or an asymmetric dual-grating waveguides.Both of the two dualgrating waveguides are discussed.It indicates that the asymmetric one is more sensitive to the tilt of grooves.At some tilts,the group velocities of the coupled modes become negative.It means that the asymmetric dual-grating waveguides can support backward modes.At a special tilt,there appear accidental modes degeneracy at the edge of the first Brillouin zone.The influence of waveguide parameters on the special tilt are shown.The excitation and propagation of coupled modes sustained by the asymmetric dual-grating waveguides are demonstrated with the help of transition between a slot and a microstrip.