Compact wideband plasmonic filter with flat-top transmission response based on corrugated metal-insulator-metal ring resonator.

We demonstrate a novel route to control the filtering of spoof localized surface plasmons (LSPs) on the corrugated metal-insulator-metal (MIM) ring resonator. The spoof LSPs resonance modes can be effectively tuned to achieve broad passband (covering the quadrupole mode and the hexapole mode) by selecting proper lengths in the input and output coupling area. The mutual coupling between the input and output lines produces the flat-top transmission response and sharp out-of-band rejection. Compared with the wideband bandpass filters based on spoof plasmonic waveguides, the proposed filter is ultra-compact and only 0.35λ*0.35λ. In order to further improve the property of the bandpass plasmonic filter, all the out-of-band frequencies (the dipole mode and the octopole mode) have been rejected by introducing a shunt stepped-impedance resonator and double C-shaped rings on the back of the substrate of the filter. Simulated results are confirmed via experiment, showing good rejection and wideband filtering performance with low insertion loss, flat-top transmission response and sharp out-of-band suppression. The proposed filter can find more applications in the highly integrated plasmonic circuits and systems in both terahertz and microwave regimes.