Large positive and negative lateral optical beam displacements due to surface plasmon resonance

Abstract

We report abnormally large positive and negative lateral optical beam shifts at a metal-air interface when the surface plasmon resonance of the metal is excited. The optimal thickness for minimal resonant reflection is identified as the critical thickness above which a negative beam displacement is observed. Experimental results show good agreement with theoretical predictions and the large observed bidirectional beam displacements also indicate the existence of forward and backward surface propagating waves at the surface plasmon resonance of the metal. © 2004 American Institute of Physics. (DOI: 10.1063/1.1775294) As described by Newton,1 when a finite size light beam is total internally reflected by a dielectric interface, the elec- tromagnetic field partially penetrates into the rear medium and builds up an evanescent wave field whose amplitude decreases exponentially with the distance from the surface. The static Poynting vector of the evanescent wave is directed along the interface with a complex wave vector. Hence, after re-emerging into the former medium, the actual reflected en- ergy flux is laterally displaced with respect to the geometri- cal optics beam. This effect, now known as the Goos- Hänchen(GH) effect, was experimentally demonstrated by Goos and Hänchen in l947. 2 Most investigations have exam- ined this phenomenon by using a beam that is reflected from the interface of two dielectric media with the incidence angle sufficiently close to the total internal reflection (TIR) angle. Normally, the lateral displacement is proportional to the pen-