Surface Plasmons : Physical Principles and Applications

In metals, there are special waves called plasma waves, which correspond to an oscillation in charge density. These waves have a longitudinal structure, i.e. the wave vector associated with them is parallel to the electric field. They cannot therefore be generated optically, given the transverse structure of the electromagnetic light wave. However, it is possible to overcome this constraint at the interface between a metal and a dielectric, provided that an evanescent wave with a longitudinal component is generated at this interface. The mixed mode of light and plasma oscillation thus generated constitutes the plasmon. In practice, the coupling between the plasma wave and the light is only possible if the phase velocities of the two waves are matched. This is achieved when the wave vectors are identical along the interface. A simple and effective way of generating an evanescent wave conducive to coupling with the plasmon is to work in total internal reflection in a prism, one side of which is covered with a nanometric metallic layer. When the incident light is coupled with the plasmonic wave, there is no longer any reflected light, since the light energy transferred to the plasmon is dissipated in the metal. This dissipation is linked to the imaginary part of the metal's dielectric constant and results in a certain resonance width. As surface plasmon waves are highly sensitive to changes in the refractive index of the external dielectric medium, they are naturally exploited for fine refractometry. Main applications include the measurement of dielectric constants in metals, the creation of (bio)chemical sensors, spectroscopy...

This physical principle, first observed over 100 years ago, has been extensively studied and documented. Numerous books detail the operating principle of surface plasmon waves and their use. The aim of this dossier is to introduce the reader to the physical principle underlying the generation of surface plasmons in optical fibers. It then looks at the main configurations used for surface plasmon excitation. Finally, concrete examples are discussed.