Observation of the Wannier-Stark ladder in plasmonic waveguide arrays.

Evanescently coupled waveguides are a powerful platform to study and visualize the wave dynamics in tight-binding systems. Here, we investigate the propagation of surface plasmon polaritons in arrays of dielectric loaded surface plasmon polariton waveguides with a propagation constant gradient acting as an effective external potential. Using leakage radiation microscopy, we observe in real-space for single site excitation a periodic breathing of the wave packet and an oscillatory motion in the case of Gaussian excitation of multiple waveguides. The corresponding momentum-resolved spectra are composed of sets of equally spaced modes. We interpret these observations as the plasmonic analogs of Bloch oscillations and the Wannier-Stark ladder, respectively.