Plasmons are the Quanta associated with longitudinal waves propagating in matter through the collective motion of large numbers of electrons. Surface plasmons are a subset of these 'eigen-modes' of the electrons, which are bound to regions in the material where the optical properties reverse, i.e. the interface between a dielectric and conducting medium. However in the low k-vector limit these surface modes couple with the free EM field to yield a polariton type excitation. The principal consequence is that the quantum nature of the excitation is relaxed and it exists over the entire frequency range from zero to an asymptotic value determined by the classical surface plasmon energy. This means that surface plasmon polaritons can be accessed optically over a large frequency range and are typically evidenced by a resonant dip in the reflectance of the material under the correct coupling conditions. We currently use this resonant effect as a means of probing the optical properties of materials (high temperature superconductors, niobium and platinum silicide are on the agenda at present) and of enhancing the response of photodetectors and the effects of laser ablation.
Surface Plasmons cannot couple directly to free-space electromagnetic radiation of the same energy because they travel too slowly, their associated wavevector being too large to satisify conservation of energy and momentum. Experimentally two methods have been developed to bypass this problem, Attenuated total reflection and grating coupling.
Attenuated total reflection The incident electromagnetic radiation
(in red- diagram below) is passed through an optically dense medium which
increases its wavevector. This beam then reflects off the boundary between
the optically dense medium and either a less dense dielectric (Otto configuration)
or metallic layer (Kretchmann-Raether configuration). An evenescent field
extends from this interface to 'drive' the electrons on the dielectric
metal interface producing a Surface Plasmon
Last revised 17/5/96