Abstract

A thin metal film supports two surface plasmon excitations, one associated with each of the sample’s two surfaces. For an asymmetric geometry, each metal surface borders a different dielectric medium, which leads to two distinct branches of the surface plasmon dispersion relation. It is possible to phase match and hence couple these two excitations if the metal film is corrugated to form a thin grating of period Λ. Cross coupling occurs, in first order, when the grating constant k_g = 2π/Λ. matches the difference between the propagation constants of the two surface plasmon modes for some frequency ω. We have observed this cross coupling on samples prepared by depositing thin silver films onto holographic gratings formed in layers of photoresist. Our experiments make use of the fact that photoresist exhibits a broad fluorescence spectrum under laser illumination. Surface plasmons (SP) with this same spectrum are excited at the Ag-photoresist interface by excited molecules in the photoresist. Part of this spectrum near the phase-matching frequency excites SP on the Ag-air interface, which in turn radiates via the usual grating-coupling mechanism. We report our observations of this cross-coupling effect and its dependence on the grating period and film thickness.

© 1986 Optical Society of America