A plasmon is a quasiparticle resulting from the quantization of plasma oscillations. Plasmons are collective excitations of the electron gas that exist in metals and semiconductors at subwavelength length scales, where free electrons strongly interact with each other via Coulombic forces. The name “plasmon” was first introduced by Gustav Mie in 1908 when he studied electromagnetic waves in metal nanoparticles.
Mie theory shows that when light shines on a metal nanoparticle, the free electrons in the metal are set into oscillation. These oscillating electrons create an electromagnetic field that extends beyond the particle itself. The coupled system of free electrons and their generated electromagnetic field is known as a plasmon. Because plasmons are created by free electrons, they exist only in metals and semiconductors; insulators do not support free electron motion and therefore cannot support plasmons. In addition, because plasmons are essentially an interaction between free electrons, they only exist on length scales smaller than the mean free path of those electrons; at larger length scales, individualfree electrons have lost memory of their neighbors and no longer interact strongly enough to form a collective excitation (i.e., a plasmon). For this reason, plasmons typically exist on length scales smaller than 100 nm.
The strength of the coupling between an individual electron and the surrounding electromagnetic field depends on both the wavelength of light shining on the particle (which sets how fast the electron must oscillate) and on the size ofthe particle (which sets how many other electrons there are for that one electron to interact with). For small particles or long wavelengths (low frequencies), individualelectrons couple very weakly to their surroundings—so weakly, in fact, that we do not observe any effects due to this coupling in everyday life. However, as either or bothof these parameters increase—particle size gets smaller or wavelength decreases (frequency increases)—the coupling becomes stronger until it eventually dominates over all other interactions present in nature at those length/time scales: This is when we say that “plasma effects” begin to be important!
In general terms then, plasma effects become important when two conditions are met: 1) there must be enough particles so that each one can significantly influence its immediate surroundings via Coulombic interactions; 2) those interactions must happen quickly enough so that changes happening at one location can propagate throughout the entire system before anything else changes significantly elsewhere (this timescale is typically set by light speed divided by some characteristic distance scale). When both these criteriaare met—a large number of charges interacting rapidly over short distances—we saythat we have a “dense plasma”