Two types of surface plasmon polariton waveguides consisting of square metal nanowires or nanotubes covered by low-high index dielectric layers are presented and their guiding characteristics are investigated numerically at the telecom wavelength of 1550 nm. Numerical analysis reveals that depending on the sizes of the covered low-high index dielectric layers, the nanowire based waveguides can be tuned to provide subwavelength confinement of the plasmonic modes with low propagation loss. Even enhanced optical confinement could be achieved by further increasing the nanowire size or replacing the metal nanowires by nanotubes. Consideration of directional coupling between two identical such plasmonic waveguides reveal ultra-low-crosstalk can be realized with relatively small separation distances. These waveguiding structures, compatible with modern fabrication methods, potentially enable the realization of numerous ultra-compact integrated photonic components.
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