Spoof surface plasmon polariton waveguide with spiral structure units Qian Qiao, Yong Xu, Liangcai Zhang, Weiwen Li and Zhiyuan Shi Eur. Phys. J. Plus, 136 8 (2021) 874 Published online: 24 August 2021 DOI: 10.1140/epjp/s13360-021-01880-4