Plasmon: Encyclopedia II - Plasmon - Explanation

Plasmon - Explanation

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. On the other hand, some metals, such as copper, have a plasmon frequency in the visible range, yielding their distinct color. For other metals, such as gold, the plasma frequency lies deeply in the ultraviolet, but geometric factors come into play which reduce the plasmon frequency to the visible. In doped semiconductors, the plasma frequency is usually in the infrared.

The plasmon energy can often be estimated in the free electron model as:

where n is the valence electron density, e is the elementary charge, m is the electron mass and ε₀ the permittivity of free space.

Surface plasmons (those plasmons confined to surfaces) interact strongly with light, resulting in a polariton. They occur at the interface of a material with a positive dielectric constant with that of a negative dielectric constant (usually a metal or doped dielectric). 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.

Adapted from the Wikipedia article "Explanation", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki