Tunable Rejections of Metamaterial Filter Based on Spoof Surface Plasmon Polaritons

According to the dispersion property, spoof surface plasmon polaritons (SPPs) can be supported within a low-pass propagating band. When the operating frequency rises to approach the asymptotic limit, which is also known as the surface plasma frequency, the spoof SPP modes vanish and the propagation cuts off. In this paper, we propose a method to achieve tunable rejections of spoof SPPs by producing independent absorptions. A spoof SPP waveguide consisting of corrugated microstrip line and matching convertors is introduced and electrically resonant metamaterials, the electric-field-coupled-LCs (ELCs), are employed and arranged along both sides of the waveguide. Since the electric field around the SPP waveguide excites strong and tight coupling to ELCs at resonant frequencies and their relative impedance keeps matching within a broadband, the propagating SPP modes are absorbed by such metamaterial particles and SPP filters with broad propagating band and significant rejection are achieved. Furthermore, the transmissions of the SPP filter with a series of ELCs scaled by different factors are investigated to clarify that the resonant frequency is directly controlled by the dimension of the metamaterial particles and that the coupling effect between different ELCs are weak enough to be ignored. As a result, it is convenient to control the absorptions by simply modulating the scaling factors of ELCs to construct a multiple rejection filter.Simulation results prove that the filter exhibits several significant rejections of -15 dB within a broad high-efficient propagating band. The proposed highly-efficient ultrathin planar filter is able to play an important role of filtering SPP waves in plasmonic circuits and systems.