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We summarize our study of the ultrawideband slow-light system based on plasmonic-graded metallic gratings for “rainbow” trapping at terahertz (THz) frequencies. The dispersion relations for a perfect conducting plane hosting an array of 1-D grooves having a constant depth were derived. Since the dispersion relations for the fundamental surface mode resemble those for the surface plasmon polaritons (SPPs) at optical frequencies, such a uniform grating can be used to effectively guide THz waves. The dispersion relations for these spoof SPPs can be tailored by choosing the geometric parameters of the surface structures. We show that a graded metallic surface grating can be used to trap THz waves at different locations along the surface grating corresponding to the different wavelengths. Theoretical and experimental studies of rainbow trapping at shorter wavelengths, e.g., in visible and near-infrared domains, are also briefly summarized.