![Sketch of surface plasmon](https://upload.wikimedia.org/wikipedia/commons/d/d1/Sketch_of_surface_plasmon.png)
Surface plasmons (SPs) are coherent
delocalized electron
In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref>
The term delocalization is general and can have slightly di ...
oscillations that exist at the interface between any two materials where the real part of the
dielectric function
In electromagnetism, the absolute permittivity, often simply called permittivity and denoted by the Greek letter ''ε'' (epsilon), is a measure of the electric polarizability of a dielectric. A material with high permittivity polarizes more in ...
changes sign across the interface (e.g. a metal-dielectric interface, such as a metal sheet in air). SPs have lower energy than bulk (or volume)
plasmon
In physics, a plasmon is a quantum of plasma oscillation. Just as light (an optical oscillation) consists of photons, the plasma oscillation consists of plasmons. The plasmon can be considered as a quasiparticle since it arises from the quantiz ...
s which quantise the longitudinal electron oscillations about positive ion cores within the bulk of an
electron gas
An ideal Fermi gas is a state of matter which is an ensemble of many non-interacting fermions. Fermions are particles that obey Fermi–Dirac statistics, like electrons, protons, and neutrons, and, in general, particles with half-integer spin. T ...
(or plasma).
The charge motion in a surface plasmon always creates electromagnetic fields outside (as well as inside) the metal. The ''total'' excitation, including both the charge motion and associated electromagnetic field, is called either a
surface plasmon polariton at a planar interface, or a
localized surface plasmon
A localized surface plasmon (LSP) is the result of the confinement of a surface plasmon in a nanoparticle of size comparable to or smaller than the wavelength of light used to excite the plasmon. When a small spherical metallic nanoparticle is irra ...
for the closed surface of a small particle.
The existence of surface plasmons was first predicted in 1957 by Rufus Ritchie.
In the following two decades, surface plasmons were extensively studied by many scientists, the foremost of whom were T. Turbadar in the 1950s and 1960s, and
E. N. Economou,
Heinz Raether
Heinz Artur Raether (14 October 1909 — 31 December 1986) was a German physicist. He is best known for his theoretical and experimental contributions to the study of surface plasmons, as well as for Kretschmann-Raether configuration, a commonly-u ...
, E. Kretschmann, and A. Otto in the 1960s and 1970s. Information transfer in nanoscale structures, similar to
photonics
Photonics is a branch of optics that involves the application of generation, detection, and manipulation of light in form of photons through emission, transmission, modulation, signal processing, switching, amplification, and sensing. Though ...
, by means of surface plasmons, is referred to as
plasmonics
Plasmonics or nanoplasmonics refers to the generation, detection, and manipulation of signals at optical frequencies along metal-dielectric interfaces in the nanometer scale. Inspired by photonics, plasmonics follows the trend of miniaturizing opt ...
.
Surface plasmon polaritons
Excitation
Surface plasmon polaritons can be excited by electrons
or photons. In the case of photons, it cannot be done directly, but requires a prism, or a grating, or a defect on the metal surface.
Dispersion relation
![Dispersion Relationship](https://upload.wikimedia.org/wikipedia/commons/e/e8/Dispersion_Relationship.png)
At low frequency, an SPP approaches a
Sommerfeld-Zenneck wave, where the dispersion relation (relation between frequency and wavevector) is the same as in free space. At a higher frequency, the dispersion relation bends over and reaches an
asymptotic limit called the "
plasma frequency Plasma oscillations, also known as Langmuir waves (after Irving Langmuir), are rapid oscillations of the electron density in conducting media such as plasmas or metals in the ultraviolet region. The oscillations can be described as an instability i ...
"
(see figure at right). For more details see
surface plasmon polariton.
Propagation length and skin depth
As an SPP propagates along the surface, it loses energy to the metal due to absorption. It can also lose energy due to scattering into free-space or into other directions. The electric field falls off evanescently perpendicular to the metal surface. At low frequencies, the SPP penetration depth into the metal is commonly approximated using the
skin depth
Skin effect is the tendency of an alternating electric current (AC) to become distributed within a conductor such that the current density is largest near the surface of the conductor and decreases exponentially with greater depths in the co ...
formula. In the dielectric, the field will fall off far more slowly. SPPs are very sensitive to slight perturbations within the skin depth and because of this, SPPs are often used to probe inhomogeneities of a surface.
For more details, see
surface plasmon polariton.
Localized surface plasmons
Localized surface plasmons arise in small metallic objects, including nanoparticles. Since the translational invariance of the system is lost, a description in terms of
wavevector
In physics, a wave vector (or wavevector) is a vector used in describing a wave, with a typical unit being cycle per metre. It has a magnitude and direction. Its magnitude is the wavenumber of the wave (inversely proportional to the wavelength), ...
, as in SPPs, can not be made. Also unlike the continuous dispersion relation in SPPs,
electromagnetic modes of the particle are discretized.
LSPs can be excited directly through incident waves; efficient coupling to the LSP modes correspond to resonances and can be attributed to
absorption and
scattering
Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
, with increased local-field enhancements.
LSP resonances largely depend on the shape of the particle; spherical particles can be studied analytically by
Mie theory.
Experimental applications
The excitation of surface plasmons is frequently used in an experimental technique known as
surface plasmon resonance
Surface plasmon resonance (SPR) is the resonant oscillation of conduction electrons at the interface between negative and positive permittivity material in a particle stimulated by incident light. SPR is the basis of many standard tools for measu ...
(SPR). In SPR, the maximum excitation of surface plasmons are detected by monitoring the reflected power from a prism coupler as a function of incident angle or
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
. This technique can be used to observe
nanometer
330px, Different lengths as in respect to the molecular scale.
The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...
changes in thickness, density fluctuations, or molecular absorption. Recent works have also shown that SPR can be used to measure the optical indexes of multi-layered systems, where
ellipsometry
Ellipsometry is an optical technique for investigating the dielectric properties (complex refractive index or dielectric function) of thin films. Ellipsometry measures the change of polarization upon reflection or transmission and compares it t ...
failed to give a result.
Surface plasmon-based circuits have been proposed as a means of overcoming the size limitations of photonic circuits for use in high performance data processing nano devices.
The ability to dynamically control the plasmonic properties of materials in these nano-devices is key to their development. A new approach that uses plasmon-plasmon interactions has been demonstrated recently. Here the bulk plasmon resonance is induced or suppressed to manipulate the propagation of light.
This approach has been shown to have a high potential for nanoscale light manipulation and the development of a fully
CMOS
Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFE ...
-compatible electro-optical plasmonic modulator, said to be a future key component in chip-scale photonic circuits.
Some other surface effects such as
surface-enhanced Raman scattering and
surface-enhanced fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
are induced by surface plasmon of
noble metal
A noble metal is ordinarily regarded as a metallic chemical element that is generally resistant to corrosion and is usually found in nature in its raw form. Gold, platinum, and the other platinum group metals (ruthenium, rhodium, palladium, o ...
s, therefore sensors based on surface plasmons were developed.
In
surface second harmonic generation, the second harmonic signal is proportional to the square of the electric field. The electric field is stronger at the interface because of the surface plasmon resulting in a
non-linear optical effect. This larger signal is often exploited to produce a stronger second harmonic signal.
The wavelength and intensity of the plasmon-related absorption and emission peaks are affected by molecular adsorption that can be used in molecular sensors. For example, a fully operational prototype device detecting
casein
Casein ( , from Latin ''caseus'' "cheese") is a family of related phosphoproteins (CSN1S1, αS1, aS2, CSN2, β, K-casein, κ) that are commonly found in mammalian milk, comprising about 80% of the proteins in cow's milk and between 20% and 60% of ...
in milk has been fabricated. The device is based on monitoring changes in plasmon-related absorption of light by a gold layer.
See also
*
Biosensor
A biosensor is an analytical device, used for the detection of a chemical substance, that combines a biological component with a physicochemical detector.
The ''sensitive biological element'', e.g. tissue, microorganisms, organelles, cell rece ...
*
Dual-polarization interferometry
Dual-polarization interferometry (DPI) is an analytical technique that probes molecular layers adsorbed to the surface of a waveguide using the evanescent wave of a laser beam. It is used to measure the conformational change in proteins, or othe ...
*
Extraordinary optical transmission
*
Free electron model
In solid-state physics, the free electron model is a quantum mechanical model for the behaviour of charge carriers in a metallic solid. It was developed in 1927, principally by Arnold Sommerfeld, who combined the classical Drude model with quantu ...
*
Gap surface plasmon
*
Heat-assisted magnetic recording
Heat-assisted magnetic recording (HAMR) (pronounced "''hammer")'' is a magnetic storage technology for greatly increasing the amount of data that can be stored on a magnetic device such as a hard disk drive by temporarily heating the disk materia ...
*
List of plasma (physics) articles
This is a list of plasma physics topics.
A
* Ablation
* Abradable coating
* Abraham–Lorentz force
* Absorption band
* Accretion disk
* Active galactic nucleus
* Adiabatic invariant
* ADITYA (tokamak)
* Aeronomy
* Afterglow plasma
* Airg ...
*
Multi-parametric surface plasmon resonance
*
Plasma oscillation Plasma oscillations, also known as Langmuir waves (after Irving Langmuir), are rapid oscillations of the electron density in conducting media such as plasmas or metals in the ultraviolet region. The oscillations can be described as an instability ...
*
Plasmonic lens
*
Plasmonics (journal)
*
Spinplasmonics
*
Surface plasmon resonance microscopy
Surface plasmon resonance microscopy (SPRM), also called surface plasmon resonance imaging (SPRI), is a label free analytical tool that combines the surface plasmon resonance of metallic surfaces with imaging of the metallic surface.
The heterogen ...
*
Waves in plasmas In plasma physics, waves in plasmas are an interconnected set of particles and fields which propagate in a periodically repeating fashion. A plasma is a quasineutral, electrically conductive fluid. In the simplest case, it is composed of electron ...
Notes
References
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Quasiparticles
Plasmonics