Abstract
Although there have been many discussions on negative group velocity (NGV), disagreements over its counter-intuitive explanations are still on the way. In this paper, it is proposed that the negative group velocity is just a component of the actual wave group, which has to be accompanied by a positive group velocity (PGV) component. A composite wave with the NGV component propagates forward in the form of a tumbling vortex as an attenuated or evanescent wave. Negative group velocity exists only in the anomalous dispersion band with resonance absorption. This wave group model is confirmed by the NGV characteristics of spoof surface plasmon polaritons (SSPPs) of spiral-shaped periodic units. In this case, the PGV and NGV waves are formed by the resonance coupling of periodic cells, and the composite SSPP modes have both resonance absorption attenuation and vortex cancellation attenuation.
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