Plasmon

In physics, the plasmon is the quasiparticle resulting from the quantization of plasma oscillations just as photons and phonons are quantizations of light and sound waves, respectively. Thus, plasmons are collective oscillations of the free electron gas density, often at optical frequencies. They can also couple with a photon to create a third quasiparticle called a plasma polariton.

Since plasmons are the quantization of classical plasma oscillations, most of their properties can be derived directly from Maxwell's Equations.

Additional recommended knowledge

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Explanation

Plasmons are explained in the classical picture using the Drude model of metals. The metal is treated as a three dimensional crystal of positively charged ions, and a delocalized electron gas is moving in a periodic potential of this ion grid.

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. Some metals, such as copper and gold, have electronic interband transitions in the visible range, whereby specific light energies (colors) are absorbed, yielding their distinct color. In doped semiconductors, the plasma frequency is usually in the deep ultraviolet^[1]. That is why they are reflective, too.

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

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 vacuum or 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 material's 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. These effects are illustrated by the historic stained glass which adorn medieval cathedrals. In this case, the color is given by metal nanoparticles of a fixed size which interacts with the optical field to give the glass its vibrant color. In modern science, these effects have been engineered for both visible light and 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.

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). Plasmons involve rapid motion of electrons through the solid, but the ohmic losses vanish. 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 led 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 makeup by L’Oréal among others.^[2]

See also

• Surface plasmon resonance
• Plasmonic cover
• Spinplasmonics

References

• Stefan Maier (2007). Plasmonics: Fundamentals and Applications. Springer. ISBN 978-0387331508. 

• Heinz Raether (1980). Excitation of plasmons and interband transitions by electrons. Springer-Verlag. ISBN 0-387-09677-9. 

• A.V. Zayats, I.I. Smolyaninov, A.A. Maradudin, "Nano-optics of surface plasmon polaritons," Phys. Rep., vol. 408, pp. 131-314 (2005).

• Harry A. Atwater (2007). The Promise of Plasmonics. In Scientific American, April 2007 v.296 n.4, pg.56-63

1. ^ Kittel, C.: "Introduction to Solid State Physics", 8th edition, Wiley 2005, Table 2 on p. 403