A Flexible Amplitude Equalizing Filter Based on Spoof Surface Plasmon Polaritons

In this article, an amplitude equalizing spoof surface plasmon polaritons (AESSPPs) filter deposited on a flexible substrate is proposed for the first time. This new hybrid device can realize both the characteristics of amplitude equalization and interference filtering when embedding into a wideband communication system, which brings the compensation process of gain fluctuation back to the microwave front end instead of traditional complex algorithm in digital chip. It is mainly composed of equalizing thin layers and spoof surface plasmon polaritons (SSPPs) waveguide. The equalizing thin layer is realized by indium tin oxide (ITO) and periodically sputtered along the propagation path of SSPPs waveguide to dissipate EM energy at different frequencies. In order to better reveal the natural property of equalization and filtering, an equivalent circuit of AESSPPs unit cell is proposed and analyzed theoretically. The equalizing value and bandwidth can be regulated independently by surface impedance of ITO thin layer and the structural parameter of SSPPs, respectively. Finally, we fabricated and measured three AESSPPs filter prototypes with different critical parameters. All the theory, simulation, and measurement results are proved to have achieved the artificially dominated equalizing curve with different equalizing values and bandwidths, which validate that the proposed filter demonstrates the desired flexibility, equalization, and filtering capabilities simultaneously.