A Hybrid Circuit for Spoof Surface Plasmons and Spatial Waveguide Modes to Reach Controllable Band-Pass Filters

Abstract We propose a hybrid circuit for spoof surface plasmon polaritons (SPPs) and spatial waveguide modes to develop new microwave devices. The hybrid circuit includes a spoof SPP waveguide made of two anti-symmetric corrugated metallic strips and a traditional substrate integrated waveguide (SIW). From dispersion relations, we show that the electromagnetic waves only can propagate through the hybrid circuit when the operating frequency is less than the cut-off frequency of the SPP waveguide and greater than the cut-off frequency of SIW, generating efficient band-pass filters. We demonstrate that the pass band is controllable in a large range by designing the geometrical parameters of SPP waveguide and SIW. Full-wave simulations are provided to show the large adjustability of filters, including ultra wideband and narrowband filters. We fabricate a sample of the new hybrid device in the microwave frequencies, and measurement results have excellent agreements to numerical simulations, demonstrating excellent filtering characteristics such as low loss, high efficiency, and good square ratio. The proposed hybrid circuit gives important potential to accelerate the development of plasmonic integrated functional devices and circuits in both microwave and terahertz frequencies. Introduction Surface plasmon polaritons (SPPs) are a kind of propagating surface waves bounded on the interface of metal and dielectric at the optical frequency1. In virtue of strong local-field enhancement and the diffraction limit broken, SPPs have attracted many scientists to make investigations2. The plasmonic waves propagate in directions parallel to the surface of metal and decay exponentially normal to the surface3,4. Different from metal with the characteristic of negative permittivity at the optical frequency, the metal behaves like a perfectly electric conductor in the terahertz and microwave frequencies where we cannot find SPPs tightly confined as a result5. Therefore, the spoof SPP metamaterials6,7 have been proposed to obtain the plasmon propagation at such frequencies. The metamaterials of corrugated metal structures7,8,9,10,11,12,13 such as one dimensional or two dimensional periodic arrays enchased with slits, holes or blocks have been employed to support the spoof SPP modes, which have the same dispersion relations and spatial confinements with SPPs in the optical region. For facilitating the application of spoof SPPs, broadband and high-efficiency conversion and transition14,15 between traditional transmission line and conformal SPPs with double grating and single grating have been reported, which allow for successful combination between the spatial guided waves and SPPs. At the same time, some devices based on spoof SPPs have been developed, including ultra-wideband surface plasmonic filter16, efficient converter17,18 between SPP modes and spatial radiated modes, and controlling rejecter of SPPs19 and so on20,21,22,23,24. All of the reported papers attest that the spoof SPPs can provide the advantage of miniaturization and localized electromagnetic (EM) waves in subwavelength scales. The substrate integrated waveguide (SIW) and similar schemes25,26,27,28,29 have been put forward in recent years as new spatial guided-wave structures, which can be integrated in the dielectric substrate with the characteristics of low insertion loss and low radiation. Compared with the traditional expensive and cumbersome waveguide, SIWs have similar performance but more compact dimensions, which makes it possible to downsize the microwave system into a small package. In this article, we propose a hybrid circuit of the spoof SPP waveguide which supports SPP modes and SIW that is a barrier of spatial guided modes. The spoof SPP waveguide takes the advantage of the anti-symmetrical corrugated metallic strips22 which bring further tighter EM field confinement and smaller propagating wavelength for decreasing the interference and miniaturizing the system at the same frequency. Linear tapered microstrip lines27,30 serve as a bridge to connect the SPP waveguide, SIW and microstrip line, attaining the conversions among the transverse magnetic (TM) mode, transverse electric (TE) mode, and quasi-transverse-electromagnetic (TEM) mode. Numerical and experimental resulted are present to validate the new features of the hybrid circuit. Results Hybrid circuit of spoof SPP waveguide and SIW We propose a hybrid circuit that contains a spoof SPP waveguide, SIW, and tapered microstrip lines, as illustrated in Fig. 1. The hybrid circuit is divided into two main sections of SPP waveguide and SIW. The commercial printed circuit board, F4B31, is used as the dielectric substrate with thickness of t = 0.5 mm and relative permittivity of 2.65. We choose annealed copper (electric conductivity σ = 5.8e + 007 S/m) as metal layers whose thickness is 0.018 mm. The first main section of the hybrid circuit is the spoof SPP waveguide, which is composed of two (...truncated)