Plasmon

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In physics, the plasmon is the quasiparticle resulting from the quantization of plasma oscillations. They are a hybrid of the electron plasma (in a metal or semiconductor) and the photon. Thus, plasmons are collective oscillations of the free electron gas at optical frequencies.

A plasmon is basically just an oscillation of the conduction electrons in a metal.

This definition suggests that plasmons are strictly quantum mechanical entities, but many of their important properties can be derived directly from Maxwell's Equations.

It is the classical electrodynamical picture that is implied in most of the modern literature on plasmons. And for simulations of plasmons in complex geometry no free electrons are simulated, but simply the dielectric constant for a given frequency is used (or the local impulse response if you use time instead of frequency). No nonlocal interaction (like the one which is needed for high-precision UV lithography lens simulation) is needed.

1 Explanation
2 Surface plasmons
3 Possible applications
4 External links
5 See also
6 References

[edit] 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 (reflective) 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:

$E_{p} = \hbar \sqrt{\frac{n e^{2}}{m\epsilon_0}}$

where $n$ is the valence electron density, $e$ is the elementary charge, $m$ is the electron mass and $ε 0$ the permittivity of free space.

[edit] Surface plasmons

Surface plasmons are those plasmons that are confined to surfaces and that 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 Spectroscopy and in explaining anomalies in diffraction from metal gratings (Wood's anomaly), among other things. Surface plasmon resonance is used by biochemists to study the mechanisms and kinetics of ligands binding to receptors (i.e. a substrate binding to an enzyme).

More recently surface plasmons have been used to control colours of materials^[1]. This is possible since controlling the materials surface shape controls the types of surface plasmons that can couple to it and propagate across it. This in turn controls the interaction of light with the surface. This has been done both for visible light and for microwave radiation. Much research goes on first in the microwave range because at this wavelength material surfaces can be produced mechanically as the patterns tend to be of the order a few centimeters. To produce optical range surface plasmon effects involves producing surfaces which have features <400 nm. This is much more difficult and has only recently become possible to do in any reliable or available way.

The study of butterflies and beetles has revealed that many of the optical effects (from simple colour to iridescence) are actually produced by natural nanometre-length structures which occur in nature.^[2]

[edit] Possible applications

Plasmons have been considered as a means of transmitting information on computer chips, since plasmons can support much higher frequencies (into the 100 THz range, while conventional wires become very lossy in the tens of GHz). They have also been proposed as a means of high-resolution lithography and microscopy due to their extremely small wavelengths. Both of these applications have seen successful demonstrations in the lab environment. Finally, surface plasmons have the unique capacity to confine light to very small dimensions which could enable many new applications.

Surface plasmons are very sensitive to the properties of the materials on which they propagate. This has lead to their use to measure the thickness of monolayers on colloid films, such as screening and quantifying protein binding events. Companies such as Biacore have commercialized instruments which operate on these principles. Optical surface plasmons are being investigated with a view to improve make up by L’Oréal among others.^[3]

[edit] External links

[edit] See also

[edit] References

Heinz Raether (1980). Excitation of plasmons and interband transitions by electrons. Springer-Verlag. ISBN 0-387-09677-9.
Harry A. Atwater (2007). The Promise of Plasmonics. In Scientific American, April 2007 v.296 n.4, pg.56-63

Particles in physics - quasiparticles

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Categories: Plasma physics | Condensed matter physics