Near-field observation of surface plasmon polariton propagation on thin metal stripes

We use a photon scanning tunneling microscope to probe the field of surface plasmon polariton modes excited on finite-width thin metal films (metal stripes). We first investigate the coupling between surface plasmons launched by a focused beam on a homogeneous thin film and the modes sustained by metal stripes of different widths. We show that. if the width of the metal stripe is about a few micrometers, a strong coupling with the stripe modes can be achieved at visible frequencies. A sharp transverse confinement of the field associated with the surface plasmon modes propagating on the metal stripe is unambiguously observed on the constant height photon scanning tunneling microscope images. The back-reflection of these modes at the end of the stripe leads to a surface-wave interference pattern from which the wavelength of the stripe surface plasmon modes is directly measured. We finally demonstrate that metal stripes could be used for optical addressing put-poses at the micrometer scale since a stripe with a triangularly shaped termination performs the focusing of the stripe surface plasmon field.