Transverse Anderson localization of surface plasmon polaritons

We investigate the effect of disorder on the propagation of surface plasmon polaritons in arrays of evanescently coupled dielectric loaded surface plasmon polariton waveguides. Diagonal disorder is implemented by randomly varying the heights of the waveguides. Real-space as well as momentum-space images of the surface plasmon polariton intensity distribution in the waveguide arrays are recorded by leakage radiation microscopy. In real space, increasing disorder results in a transverse localization of surface plasmon polaritons. In the momentum distribution, we observe for the first time a disorder-induced transition from a continuous band to a set of discrete modes.