Abstract
We theoretically demonstrate that a metallic film covered by a dielectric grating of graded thickness can strongly slow light as the propagation velocities of surface plasmon polaritons (SPPs) are reduced over a large frequency bandwidth at visible frequencies. Since the dispersive relation of SPPs is dependent on the dielectric grating thickness, the guided SPPs at different frequencies can be localized at different spatial positions of the plasmonic grating. We numerically demonstrate that a true rainbow from violet to red colors can be separately localized, resulting in the spatial separation of the visible spectrum on a chip.