A metal waveguide of finite width and thickness surrounded by a dielectric provides increased propagation length of the surface plasmon polariton and is therefore called long-range surface plasmon polaritons (LR-SPP). In this work, a new structure is proposed by modifying the refractive index of the dielectric surrounding the metal waveguide, leading to further improvement of the propagation length. It is shown that, when the single dielectric surrounding is replaced by a multilayered surrounding where each layer has different thicknesses and refractive index, the propagation loss gets reduced, leading to increased propagation length of the LR-SPP. The propagation loss is calculated semianalytically from the FWHM of the Lorentzian peak obtained in the plot of excitation efficiency of such waveguide for different values of the propagation constant. Before doing this calculation, the 2D variation of refractive index is first converted into a 1D effective refractive index. All the steps of analysis are discussed in detail, and, wherever necessary, the calculated results are matched with similar results of other researchers. (C) 2016 Optical Society of America
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