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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.
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