Near-field analysis of surface waves generated by nanostructures

By Scanning Near-field Optical Microscopy (SNOM), we study the propagation of surface waves created by nanostructures on a thin gold film. The nanostructures are slits and ridges fabricated by electron or ion beam lithography techniques. We will first show that the light scattered by a slit made in a gold film illuminated in transmission is composed of two components: a diffracted field and a surface plasmon polariton that propagates on the gold surface over several tens of nanometers. When two slits are illuminated, the created waves encounter and form an interference pattern which involves both the surface plasmon polariton and the diffracted waves. The situation is more complicated when the nanostructures are illuminated in a reflection mode at oblique incidence. In that case, the created waves are superimposed to the incident and reflected fields. Despite a larger number of waves, the analysis of the interference pattern provides several informations on the nature of the scattered waves and their generation rate. In this article, we provide a qualitative analysis of the waves created by slits, and by linear and curved ridges located on a gold surface.