Optical near-field distributions of surface plasmon waveguide modes

Thin gold stripes, featuring various widths in the micrometer range, were microfabricated to obtain surface-plasmon guides on a glass substrate. Each metal stripe (MS) was excited by an incident surface-plasmon polariton which was itself launched on an extended thin gold film by the total internal reflection of a focused beam coming through the substrate. The optical near-field distributions of the surface-plasmon (sp) modes sustained by the stripes were then recorded using a photon scanning tunneling microscope (PSTM). For a fixed frequency of the incident light, these field distributions are found to depend on the widths of the stripes. We first provide an experimental study of the various order modes which arise as a function of the decreasing width. Specifically, we show that the lateral confinement of a MS surface-plasmon mode is not related to a reflection of the SP on the edges of the stripe. On the basis of PSTM images recorded over a gold thin-film step discontinuity, we show that the metal stripe plasmon modes are hybrid modes created by the coupling of interface and boundary modes. Using MSs of various thicknesses, we finally demonstrate that, similarly to the symmetric mode of an extended metal thin film bounded by different dielectric media, the field of the MS modes is mostly localized at the interface between the metal and the dielectric medium with the lowest refractive index.