In physics, a plasmon is a quantum of plasma
oscillation. The plasmon can be considered
a quasiparticle since it arises from the quantization
of plasma oscillations, just like phonons
are quantizations of mechanical vibrations.
Thus, plasmons are collective oscillations
of the free electron gas density, for example,
at optical frequencies. Plasmons can couple
with a photon to create another 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.
Explanation
Plasmons can be described in the classical
picture as an oscillation of free electron
density with respect to the fixed positive
ions in a metal. To visualize a plasma oscillation,
imagine a cube of metal placed in an external
electric field pointing to the right. Electrons
will move to the left side until they cancel
the field inside the metal. If the electric
field is removed, the electrons move to the
right, repelled by each other and attracted
to the positive ions left bare on the right
side. They oscillate back and forth at the
plasma frequency until the energy is lost
in some kind of resistance or damping. Plasmons
are a quantization of this kind of oscillation.
Role of plasmons
Plasmons play a large role in the optical
properties of metals. Light of frequencies
below the plasma frequency is reflected, because
the electrons in the metal screen the electric
field of the light. Light of frequencies 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. Some metals, such as copper
and gold, have electronic interband transitions
in the visible range, whereby specific light
energies are absorbed, yielding their distinct
color. In semiconductors, the valence electron
plasma frequency is usually in the deep ultraviolet,
which is why they are reflective.
The plasmon energy can often be estimated
in the free electron model as
where is the conduction electron density,
is the elementary charge, is the electron
mass, the permittivity of free space, the
reduced Planck constant and the plasmon frequency.
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 and material
with a small positive imaginary and large
negative real dielectric constant. They play
a role in Surface Enhanced Raman Spectroscopy
and in explaining anomalies in diffraction
from metal gratings, among other things. Surface
plasmon resonance is used by biochemists to
study the mechanisms and kinetics of ligands
binding to receptors.
Surface Plasmon may also be observed in the
X-ray emission spectra of metals. A dispersion
relation of surface plasmon in the X-ray emission
spectra of metals has been derived
More recently surface plasmons have been used
to control colors of materials. This is possible
since controlling the particle's shape and
size determines 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 interact 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.
Recently, graphene has also shown to accommodate
surface plasmons, observed via near field
infrared optical microscopy techniques and
infrared spectroscopy. Potential applications
of graphene plasmonics mainly addressed the
terahertz to midinfrared frequencies, such
as optical modulators, photodetectors, biosensors.
Possible applications
Position and intensity of plasmon absorption
and emission peaks are affected by molecular
adsorption, which can be used in molecular
sensors. For example, a fully operational
prototype device detecting casein in milk
has been fabricated. The device is based on
detecting a change in absorption of a gold
layer. Localized surface plasmons of metal
nanoparticles can be used for sensing different
types molecules, proteins, etc.
Plasmons are being considered as a means of
transmitting information on computer chips,
since plasmons can support much higher frequencies.
However, for plasmon-based electronics to
be useful, the analog to the transistor, called
a plasmonster, first needs to be created.
Plasmons 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.
In 2009, a Korean research team found a way
to greatly improve organic light-emitting
diode efficiency with the use of plasmons.
A group of European researchers led by IMEC
has begun work to improve solar cell efficiencies
and costs through incorporation of metallic
nanostructures that can enhance absorption
of light into different types of solar cells:
crystalline silicon, high-performance III-V,
organic, and dye-sensitized solar cells.
Full color holograms using plasmonics have
been demonstrated.
See also
References
Stefan Maier. Plasmonics: Fundamentals and
Applications. Springer. ISBN 978-0-387-33150-8. 
Michael G. Cottam and David R. Tilley. Introduction
to Surface and Superlattice Excitations. Cambridge
University Press. ISBN 0-521-32154-9. 
Heinz Raether. Excitation of plasmons and
interband transitions by electrons. Springer-Verlag.
ISBN 0-387-09677-9. 
Barnes, W. L.; Dereux, A.; Ebbesen T.W.. "Surface
plasmon subwavelength optics". Nature 424:
824–830. Bibcode:2003Natur.424..824B. doi:10.1038/nature01937.
PMID 12917696. 
Zayats, A. V.; Smolyaninov, I. I.; Maradudin,
A. A.. "Nano-optics of surface plasmon polaritons".
Physics Reports 408: 131–314. Bibcode:2005PhR...408..131Z.
doi:10.1016/j.physrep.2004.11.001. 
Atwater, Harry A.. "The Promise of Plasmonics".
Scientific 
American 296: 56–63. doi:10.1038/scientificamerican0407-56.
PMID 17479631. 
Ozbay, Ekmel. "Plasmonics: Merging Photonics
and Electronics at Nanoscale Dimensions".
Science 311: 189–193. Bibcode:2006Sci...311..189O.
doi:10.1126/science.1114849. PMID 16410515. 
Schuller, Jon; Barnard, Edward; Cai, Wenshan;
Jun, Young Chul; White, Justin; Brongersma,
Mark L.. "Plasmonics for Extreme Light Concentration
and Manipulation". Nature Materials 9: 193–204.
Bibcode:2010NatMa...9..193S. doi:10.1038/nmat2630.
PMID 20168343. 
Brongersma, Mark; Shalaev, Vladimir. "The
case for plasmonics". Science 328: 440–441.
Bibcode:2010Sci...328..440B. doi:10.1126/science.1186905. 
External links
A selection of free-download papers on Plasmonics
in New Journal of Physics
http:www.plasmonicfocus.com
http:www.sprpages.nl
http:www.qub.ac.ukconsp1.html
http:www.nano-optics.org.uk
Plasmonic computer chips move closer
Progress at Stanford for use in computers
Slashdot: A Plasmonic Revolution for Computer
Chips?
A Microscope from Flatland Physical Review
Focus, January 24, 2005
http:en.wikinews.orgInvisibility_shield_gets_blueprint
http:www.plasmonanodevices.org
http:www.eu-pleas.org
http:www.plasmocom.org
Test the limits of plasmonic technology
http:www.activeplasmonics.org
http:www.plaisir-project.eu
