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In this paper, we theoretically investigate the guiding properties of two vertically coupled cylindrical graphene-coated nanowires (GNWs) integrated with a thin high-index dielectric substrate. We show that the plasmonic properties of the proposed structure can be effectively modulated by changing the thickness of the dielectric substrate, the nanowire radius, the gap distance between the substrate and GNWs, and the Fermi level of graphene. Through optimizing the geometric and structural parameters, a surface plasmon mode with high optical performance and low propagation loss can be achieved in the studied structure. Compared with the plasmon mode guided in a single GNW over substrate, a highly improved figure of merit with nearly two-fold electric field enhancement can be yielded in the double GNW over substrate counterpart.

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