Backward and forward modes guided by metal-dielectric-metal plasmonic waveguides

We revisited the problem of the existence of plasmonic modes guided by metal-dielectric-metal slot waveguides. For the case of lossless slot waveguides, we classify the guided modes in the structure with the metal dispersion and found that, in a certain parameter range, three different guided modes coexist at a fixed frequency, two (symmetric and antisymmetric) forward propagating modes and the third, backward propagating antisymmetric mode. We study the properties of the forward and backward plasmonic guided modes in the presence of realistic losses, and discuss the importance of evanescent modes in lossy structures.