Near-field characterization of guided polariton propagation and cutoff in surface plasmon waveguides

We present a series of near-field experiments characterizing cutoff for the three lowest order leaky surface plasmon modes supported by Au stripes on glass substrates. These studies demonstrate that the propagation of light along surface plasmon waveguides is mediated by a discrete number of guided polariton modes as well as a continuum of radiation modes. To distinguish the contribution of the guided modes from that of the radiation continuum, a parametric study of propagation length as a function of varying stripe width is performed. Discontinuities consistent with the loss of a guided mode are observed near cutoff widths predicted by numerical simulations, and a severe decrease in propagation length is observed below cutoff for the fundamental surface plasmon mode. Contrary to previous interpretations, experimental and numerical investigations confirm that the finite propagation lengths observed along the narrowest stripes are in good agreement with an intuitive model for the radiation continuum. Furthermore, multimode interference studies provide direct evidence for multiple guided modes and support previous findings with regard to modal cutoff.