10 - What in the world are surface plasmons?

Summary

Despite its scary name, a surface plasmon is simply an inhomogeneous plane-wave solution to Maxwell's equations. Typically, a medium with a large but negative dielectric constant ε is a good host for surface plasmons. Because in an isotropic medium having refractive index n and absorption coefficient κ we have ε = (n + iκ), whenever κ ≫ n the above criterion, large but negative ε, is approximately satisfied; as a result, most common metals such as aluminum, gold, and silver can exhibit resonant absorption by surface plasmon excitation. In order to excite, within a metal, a plane wave that has a large enough amplitude to carry away a significant fraction of the incident optical energy, one must create a situation whereby the metal is “forced” to accept such a wave; otherwise, as normally occurs, the wave within the metal ends up having a small amplitude, causing nearly all of the incident energy to be reflected, diffracted, or scattered from the metallic surface, depending upon the condition of that surface.

In this chapter several practical situations in which surface plasmons play a role will be presented. We begin by describing the results of an experiment that can be readily set up in any optics laboratory, and we give an explanation of the observed phenomenon by scrutinizing the well-known Fresnel's reflection formula at a metal-to-air interface.