Research On The Theory And Application Of Metal-and Dielectric-type Spoof Surface Plasmon Polaritons

Surface plasmon polaritons?SPPs?are surface electromagnetic?EM?waves that propagate along the interface between two materials with opposite permittivity?i.e.,metal-dielectric in optical frequencies?,whose EM fields decay exponentially in the transverse direction and can be applied to ultra-sensitive,ultra-resolution sensing,imaging and other fields.However,in the microwave and terahertz?THz?frequencies,metals behave no longer like a plasma but resembles a perfect electric conductor?PEC?whose plasma frequencies are often in the ultraviolet part of the EM spectrum,and consequently the natural SPPs can not be excited at the metal-dielectric interface.To solve this problem,a new concept of corrugated plasmonic metamaterial,namely Spoof?or designer?SPPs which are SPP-like bound surface modes,has been proposed by Pendry et al..The Spoof SPPs inherit most of the exotic features of natural SPPs and the main advantage is that their physical characteristics can be engineered at will by changing the geometrical parameters,which provides an effective way to flexibly guide and modulate EM waves at microwave and THz frequencies.The main contents and contributions of this work include the following aspects:1.Several different structures by decorating the three-dimensional?3D?metal surfaces with subwavelength holes or grooves have been proposed to efficiently excite different types of Spoof SPPs,which provide effective ways to extend the operating band of the Spoof SPPs and flexibly guide and manipulate EM waves,including?1?An optimized bridge with two tapered rectangular waveguides connected by a gradient domino array with subwavelength gradient heights and lateral widths has been proposed to realize high efficiency mode conversion between the TE₁₀0 guided wave supported by a conventional rectangular waveguide and the domino-type Spoof SPPs supported by a domino-type hybrid plasmonic waveguide?HPW?in broadband.?2?An optimized bridge with two flaring coaxial waveguides connected with a metal cylindrical wire corrugated with subwavelength gradient radial grooves has been proposed to realize high efficiency mode conversion between the TEM guided wave supported by a conventional coaxial waveguide and the cylindrical-type Spoof SPPs supported by a three-dimensional?3D?cylindrical-type HPW in broadband.Based on the above design,deep-subwavelength guiding and superfocusing of Spoof SPPs realized on a helically grooved metal wire at microwave frequencies are proposed.Two smooth bridges with gradient helical grooves decorated on the cylindrical and conicalmetal wire are designed at microwave frequencies to realize high efficiency and broadband mode conversion from the TEM waves in a conventional coaxial waveguide to the helical-type Spoof SPPs and superfocusing,respectively.Numerical simulations quantitatively show that the magnitudes of electric field at the tip of the conical wire with gradient helical grooves can be magnified 90 times more than that of the input signal in broadband.2.Several different structures by decorating the ultrathin two-dimensional?2D?planar metal surfaces with subwavelength holes or grooves have been proposed to efficiently excite different types of Spoof SPPs,including?1?an optimized bridge with two subwavelength gradient double-layered symmetrical metal gratings of fishbone shape has been proposed to realize high efficiency mode conversion between the quasi-TEM wave supported by a conventional microstrip line and the fishbone-type Spoof SPPs supported by a fishbone-type HPW in broadband.?2?A tapered microstrip line with periodic gradient holes has been built for efficient mode conversion between the quasi-TEM waves in a common microstrip line and the hole-type Spoof SPPs supported by a hole-type HPW,which can realize dual-band trapping of Spoof SPPs in broadband.It is interesting to note that negative group velocity can be clearly observed on the first high-order mode of the hole-type Spoof SPPs due to the strong coupling between the corrugated microstrip line and ground.Moreover,a curved hole-type HPW of the same form is also investigated for testing its field confinement and circuitry function.Measurement results agree quite well with the simulation ones,which indicate this hole-type HPW can find potential applications in plasmonic integrated circuits at microwave and THz frequencies.3.It is demonstrated that composite Spoof SPPs,which are composed of two different Spoof SPP modes propagating along a periodically corrugated metallic thin film simultaneously,can be excited by coplanar waveguide.These two Spoof SPPs corresponding to the dominant modes of one-dimensional?1D?periodical arrays consist of rectangular holes and symmetric grooves respectively.We have designed a ultrathin planar composite plasmonic waveguide at microwave frequencies,which shows that different Spoof SPP modes can achieve non-interference,multi-channel and independent signal transmission with good performance.4.A new type surface mode supported by periodic subwavelength dielectric high contrast gratings?HCGs?on a perfect conductor plane has been proposed,called as HCGs-based Spoof SPPs.The dispersion relation of the new Spoof SPPs is derived analytically by combining multimode network theory with rigorous mode matching method and effective medium approximation respectively,which has nearly the same form with and can be degenerated into that of conventional Spoof SPPs arising from metallic gratings.Moreover,we experimentally realized a wide-band,ultra-low loss,high-confinement plasmonic waveguide constructed by a high refractive-index dielectric array with deep-subwavelength periodicity on a metal substrate.Simulation and measurement results on the near field distributions and S-parameters at microwave frequencies provide explicit evidences of strong field localization and show that subwavelength dielectric HCGs offer a route to effectively suppress the losses and hence dramatically increase the propagation length of surface EM waves.This work sets up a unified theoretical framework for Spoof SPPs and opens up new vistas in surface plasmon optics.