In this work, spoof surface plasmon polaritons (SSPPs) supported by microstrip lines with T-shaped complementary grooves are proposed. Compared with the traditional SSPP structure based on single-conductor transmission lines, a broadband high-order mode of SSPPs can be predicted from the dispersion diagrams. Besides, a decrease in transverse size of 75% can be realized under the same asymptotic frequency. Then, a smooth and simple transition, which is composed of gradient complementary grooves, is designed to realize high-efficiency excitation and support practical applications of the high-order mode of SSPPs. As a result, a seamless connection with microstrip lines is easily achieved without using flaring ground, which simplifies the design procedure. Based on the proposed structure, a prototype of a SSPP waveguide is designed and fabricated. Both numerical and experimental results validate efficient excitation and broadband propagation (up to 12 GHz) of the high-order mode of SSPPs. This work will greatly accelerate the development of advanced plasmonic integrated circuits at microwave frequencies.

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