Surface Plasmon Resonance (SPR) is an optical surface sensing technique that utilizes a thin film of metal (called a "Surface Plasmon" or SP) on the surface of a waveguide. "The SP can be described as an oscillation of electrons on the surface of a solid," typically a conductor such as Au, Ag, Cu, Al, Pt, Ni, Co, Cr, V, W or even a semiconductor." [1]
An apparatus known as a Kretschmann prism is often used for SPR sensors. It uses a prism to couple some light to the SP film and reflect some to an optical photodetector. A biological sample is placed on the metal film surface. As this layer changes, the refractive index of the metal film/biosample pair changes, causing less (or more) light to strike the photodetector.
Optical fiber or planar waveguides can be used instead of the Kretschmann prism. But all three use the evanscent wave, which is created by total internal reflection, to excite the SP. It is the excited SP that can affect the index of refraction of the waveguide.
Evanescent waves can be best understood using the particle-in-a-box model from quantum physics. Imagine a box with a particle in it. The particle has zero potential energy inside the box, and infinite potential energy outside the box. Its probability of being inside the box, then, is one. If you were to draw the probability density function (pdf), it would be zero outside the confines of the box.
If the potential energy outside the box is a FINITE value (like in real life), quantum mechanics tells us that the pdf will extend outside the walls of the box. Consequently, there is some (finite) probability that the particle will exist outside of the box. This is called the tunneling effect.
If we compare these particle-in-a-box potential energy diagrams with the field diagrams of waveguides, they are very similar. Low-order modes exist inside the waveguide core, while higher-order modes actually extend outside of the waveguide boundary. This is the evanescent field. Its depth depends on wavelength, photon intensity, and the ratio of waveguide refractive index (n) to that of the surroundings.