Optimal design of composite nanowires for extended reach of surface plasmon-polaritons

We theoretically investigate composite cylindrical nanowires for the waveguiding of the lowest-order surface plasmon-polariton (SPP) mode. We find that the confinement of the SPP fields in a metallic nanowire can be significantly improved by a dielectric cladding and show that by adjusting the thickness of the optically-pumped cladding, the gain required to compensate for the losses can be minimized. If this structure is coated with an additional metal layer to form a metal-dielectric-metal (MDM) nanowire, we show that the field can be predominantly confined within the dielectric layer, to have amplitudes of three orders of magnitude higher than those in the metallic regions. We also show that the propagation lengths of SPPs can be maximized by the proper selection of the geometrical parameters. We further demonstrate that the mode is strongly confined in subwavelength scale, e.g., ∼ λ ₀² /1220 for a 60-nm-thick nanowire, where λ₀ is the wavelength in vacuum. We also find that regardless of the size of nanowire, it is possible to carry over 98.5% of the mode energy within the nanowire. In addition, we demonstrate that by appropriate choice of the material thicknesses, the losses of an MDM nanowire can be compensated by a considerably low level of optical gain in the dielectric region. For example, the losses of a 260-nm-thick Ag-ZnO-Ag nanowire can be entirely compensated by a gain of ∼ 400 cm^-1. Our results will be useful for the optimum design of nanowires as interconnects for high-density nanophotonic circuit integration.