Surface plasmon polaritons are collective longitudinal oscillations of electrons near a material surface, strongly coupled to an electromagnetic wave. The existence of coherent electron oscillations bound to the surface of a conductor were first predicted by Ritchie in 1957 and demonstrated by Powell and Swan[2,3] in 1959, but have recently experienced an explosion of interest due to their ability to efficiently manipulate light on a deep sub-wavelength scale. Because the electromagnetic wave is coupled to the motion of conduction band electrons bound to the material surface, the fields are strongly localized to the surface, opening up possibilities of sub-wavelength optical detectors and waveguides; compact, sensitive chemical detectors; and enhanced light-matter interaction.
|Fig. 1: Dispersion relation for bulk plasmons, free-space electromagnetic waves, and surface plasmons.|
Both bulk and surface plasmons have associated electromagnetic waves, and can consequently be described by Maxwell's equations. The coherent oscillations of electron motion can be encapsulated in the dielectric constant of the material. The derivation of the electromagnetic fields which characterize bulk and surface plasmons is done below, and those interested are encouraged to work out the results. To keep things brief however, the basic form of the bulk and surface plasmon solutions are shown below, along with a plot of the dispersion relation for these modes.
The dispersion diagram relates the time-variation of the wave (given by its frequency &omega) to the spatial variation of the wave (given by its wave-vector kx). Bulk plasmons are associated with purely transverse electromagnetic waves, with the electric and magnetic fields perpendicular to the direction of propagation, and can only exist for &omega > &omegap (the plasma frequency). &omegap is the resonant frequency of free electrons in response to a perturbation (such as an electromagnetic field). For &omega < &omegap, the wave-vector for bulk plasmons becomes imaginary, giving 