Dispersion and transitions of dipolar plasmon modes in graded plasmonic waveguides

Coupled plasmon modes are studied in graded plasmonic waveguides, which are periodic chains of metallic nanoparticles embedded in a host with gradually varying refractive indices. The authors identify three types of localized modes called “light,” “heavy,” and “light-heavy” plasmonic gradons outside the passband, according to various localizations. The authors also demonstrate different transitions among extended and localized modes when the interparticle separation d is smaller than a critical dc, whereas the three types of localized modes occur for d>dc, with no extended modes. The transitions can be explained with phase diagrams constructed for the lossless metallic systems.