Plasmonic Rainbow Trapping by a Silica–Graphene–Silica on a Sloping Silicon Substrate

We give a proposal for plasmonic rainbow trapping based on a novel structure comprised of a silica-graphene-silica on a sloping silicon substrate, which, importantly, overcomes the intrinsic constraints that are required by metal/dielectric interface. As compared with previous plasmonic grating structures for rainbow trapping, the adiabatic control of the dispersion curve for the present one is achieved by gradually changing the equivalent permittivity of the graphene monolayer via the gap separation between the graphene monolayer and the silicon substrate. We attribute the rainbow trapping effect to the correlative dispersive relation between the slow plasmonic mode and the gap separation between the graphene monolayer and silicon substrate, which leads to the localization of light waves of different frequencies at different positions on the graphene surface. The group velocity can be reduced to be 1000 times smaller than light velocity in air, which is 1-2 smaller than that was previously reported in dielectric gratings-based plasmonic structures.