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 momentumresolved 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. (C) 2022 Optica Publishing Group