Manipulating multipole resonances in spoof localized surface plasmons for wideband filtering

The spoof localized surface plasmon (LSP) has been widely investigated but mostly with fixed multipole resonances. This Letter proposes a method to generate multipole resonances by adding a slit on the metallic ring of a complementary LSP. This slit theoretically introduces two new boundary conditions and new modes. To validate this approach, complementary LSPs with and without slits at three different angular positions are theoretically analyzed and numerically simulated. To validate and demonstrate the potential application of the proposed LSP structure, a bandpass filter (BPF) in a single-layer substrate is designed and measured by exciting the LSP with a slit on the metallic ring. The measured results show that by simply adding a slit, the BPF achieves a fractional bandwidth of 42.7% (2.5 GHz), for both |S₁₁|<-10dB and |S₂₁| within 1 dB variation. In the passband, a flat group delay between 0.57 ns and 0.75 ns is obtained. Moreover, the proposed structure features a low profile and a compact radius of 0.136 wavelength. By dynamically controlling slit/slits with varactors or diodes, the proposed structure is theoretically promising to be reconfigurable at microwave and even terahertz frequencies.