The guiding properties of spoof surface plasmon polaritons (SPPs) in an array of hollowed-out metallic blocks standing on a metallic surface are theoretically investigated in this study. For such spoof SPPs, there exists an additional guiding mode except for the fundamental mode and the former is closely related to hollowing out metal blocks. The dispersion band of this additional mode has a negative slope (corresponding to a group velocity) and it intersects the light line and then enters the radiation region as the frequency grows, which leads to a frequency steering pencil-like radiation. The transmission and radiation properties of the additional mode of spoof SPPs are finally verified experimentally.
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