  We report on the observation of a topologically protected edge state at the
interface between two topologically distinct domains of the
Su-Schrieffer-Heeger model, which we implement in arrays of evanescently
coupled dielectric-loaded surface plasmon polariton waveguides. Direct evidence
of the topological character of the edge state is obtained through several
independent experiments: Its spatial localization at the interface as well as
the restriction to one sublattice is confirmed by real-space leakage radiation
microscopy. The corresponding momentum-resolved spectrum obtained by Fourier
imaging reveals the midgap position of the edge state as predicted by theory.
