In physics, the plasmon is the quasiparticle resulting from the quantization of plasma oscillations, which are density waves of the charge carriers in a conducting medium such as a metal, semiconductor, or plasma. Plasmons are longitudinal excitations .
Plasmons play a large role in the optical properties of metals. Light of frequency below the plasma frequency is reflected, because the electrons in the metal screen the electric field of the light. Light of frequency above the plasma frequency is transmitted, because the electrons cannot respond fast enough to screen it. In most metals, the plasma frequency is in the ultraviolet, making them shiny in the visible range, though some, like Gold and Copper, have plasma frequencies in the visual range, yielding their distinct colour. In doped semiconductors, the plasma frequency is usually in the infrared.
Surface plasmons (those plasmons near surfaces) interact strongly with light, resulting in a polariton. They play a role in surface-enhanced Raman scattering and in explaining anomalies in diffraction from metal gratings, among other things. Surface plasmon resonance is used by biochemists to detect the presence of a molecule on a surface.
Plasmons have been considered as a means of transmitting information on computer chips, since they propagate considerably faster than electrons but can be channeled with conductive paths similar to those on existing chips.
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