The quanta of waves produced by collective effects of large numbers of electrons in matter when the electrons are disturbed from equilibrium. Metals provide the best evidence of plasmons, because they have a high density of electrons free to move. The results of plasmon stimulation are seen in the illustration. The graph shows the probability of energy losses by fast electrons transmitted through a thin aluminum foil. The number of detected electrons in a beam is plotted against their energy loss during transit through the foil. Each energy-loss peak corresponds to excitation of one or more plasmons. Within experimental error, the peaks occur at integral multiples of a fundamental loss. Further evidence is the fact that the areas under the peaks (a measure of the energy-loss probability) follow a Poisson distribution.
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The name plasmon derives from the physical plasma as a state of matter in which the atoms are ionized. At the lowest densities this means an ionized gas, or classical plasma; but densities are much higher in a metal, or quantum plasma, the atoms of a solid metal being in the form of ions. In both types of physical plasma, the frequency of plasma-wave oscillation is determined by the electronic density. In a quantum plasma the energy of the plasmon is its frequency multiplied by Planck's constant, a basic relationship of quantum mechanics. See Free-electron theory of metals, Quantum mechanics
The plasmon energy for most metals corresponds to that of an ultraviolet photon. However, for silver, gold, the alkali metals, and a few other materials, the plasmon energy is sufficiently low to correspond to that of a visible or near-ultraviolet photon. This means there is a possibility of exciting plasmons by light. If plasmons are confined upon a surface, optical effects can be easily observed. In this case, the quanta are called surface plasmons, and they have the bulk plasmon energy as an upper energy limit.
Surface plasmons were first proposed to explain energy losses by electrons reflected from metal surfaces. Since then, numerous experiments have involved coupling photons to surface plasmons. Potential applications extend to new light sources, solar cells, holography, Raman spectroscopy, and microscopy.
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