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Inphysics, aplasmon is aquantum ofplasma oscillation. Just aslight (an optical oscillation) consists ofphotons, the plasma oscillation consists of plasmons. The plasmon can be considered as aquasiparticle since it arises from the quantization of plasma oscillations, just likephonons are quantizations of mechanical vibrations. Thus, plasmons are collective (a discrete number) oscillations of thefree electron gas density. For example, at optical frequencies, plasmons cancouple with aphoton to create another quasiparticle called a plasmonpolariton.
The field of study and manipulation of plasmons is calledplasmonics.
The plasmon was initially proposed in 1952 byDavid Pines andDavid Bohm[1] and was shown to arise from aHamiltonian for the long-range electron-electron correlations.[2]
Since plasmons are the quantization of classical plasma oscillations, most of their properties can be derived directly fromMaxwell's equations.[3]
Plasmons can be described in the classical picture as anoscillation of electron density with respect to the fixed positiveions in ametal. To visualize a plasma oscillation, imagine a cube of metal placed in an externalelectric field pointing to the right.Electrons will move to the left side (uncovering positive ions on the right 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 theplasma frequency until theenergy is lost in some kind ofresistance ordamping. Plasmons are aquantization of this kind of oscillation.
Plasmons play a huge role in theoptical properties ofmetals and semiconductors. Frequencies oflight below theplasma frequency arereflected by a material because the electrons in the materialscreen theelectric field of the light. Light of frequencies above the plasma frequency is transmitted by a material because the electrons in the material cannot respond fast enough to screen it. In most metals, the plasma frequency is in theultraviolet, making them shiny (reflective) in the visible range. Some metals, such ascopper[4] andgold,[5] have electronic interband transitions in the visible range, whereby specific light energies (colors) are absorbed, yielding their distinct color. Insemiconductors, thevalence electron plasmon frequency is usually in the deep ultraviolet, while their electronic interband transitions are in the visible range, whereby specific light energies (colors) are absorbed, yielding their distinct color[6][7] which is why they are reflective. It has been shown that the plasmon frequency may occur in the mid-infrared and near-infrared region when semiconductors are in the form ofnanoparticles with heavy doping.[8][9]
The plasmon energy can often be estimated in thefree electron model as
where is theconduction electron density, is theelementary charge, is theelectron mass, thepermittivity of free space, thereduced Planck constant and theplasmon frequency.
Surface plasmons are those plasmons that are confined to surfaces and that interact strongly with light resulting in apolariton.[10] They occur at the interface of a material exhibiting positive real part of their relative permittivity, i.e.dielectric constant, (e.g. vacuum, air, glass and other dielectrics) and a material whose real part of permittivity is negative at the given frequency of light, typically a metal or heavily doped semiconductors. In addition to opposite sign of the real part of the permittivity, the magnitude of the real part of the permittivity in the negative permittivity region should typically be larger than the magnitude of the permittivity in the positive permittivity region, otherwise the light is not bound to the surface (i.e. the surface plasmons do not exist) as shown in the famous book byHeinz Raether.[11] At visible wavelengths of light, e.g. 632.8 nm wavelength provided by a He-Ne laser, interfaces supporting surface plasmons are often formed by metals like silver or gold (negative real part permittivity) in contact with dielectrics such as air or silicon dioxide. The particular choice of materials can have a drastic effect on the degree of light confinement and propagation distance due to losses. Surface plasmons can also exist on interfaces other than flat surfaces, such as particles, or rectangular strips, v-grooves, cylinders, and other structures. Many structures have been investigated due to the capability of surface plasmons to confine light below the diffraction limit of light. One simple structure that was investigated was a multilayer system of copper and nickel. Mladenovicet al. report the use of the multilayers as if its one plasmonic material.[12] Oxidation of the copper layers is prevented with the addition of the nickel layers. It is an easy path the integration of plasmonics to use copper as the plasmonic material because it is the most common choice for metallic plating along with nickel. The multilayers serve as a diffractive grating for the incident light. Up to 40 percent transmission can be achieved at normal incidence with the multilayer system depending on the thickness ratio of copper to nickel. Therefore, the use of already popular metals in a multilayer structure prove to be solution for plasmonic integration.
Surface plasmons can play a role insurface-enhanced Raman spectroscopy and in explaining anomalies in diffraction from metalgratings (Wood's anomaly), among other things.Surface plasmon resonance is used bybiochemists to study the mechanisms and kinetics of ligands binding to receptors (i.e. a substrate binding to anenzyme).Multi-parametric surface plasmon resonance can be used not only to measure molecular interactions but also nanolayer properties or structural changes in the adsorbed molecules, polymer layers or graphene, for instance.
Surface plasmons may also be observed in the X-ray emission spectra of metals. A dispersion relation for surface plasmons in the X-ray emission spectra of metals has been derived (Harsh and Agarwal).[13]
More recently surface plasmons have been used to control colors of materials.[14] This is possible since controlling the particle's shape and size determines the types of surface plasmons that can be coupled into and propagate across it. This, in turn, controls the interaction of light with the surface. These effects are illustrated by the historicstained glass which adorn medieval cathedrals. Some stained glass colors are produced by metal nanoparticles of a fixed size which interact with the optical field to give glass a vibrant red color. In modern science, these effects have been engineered for both visible light andmicrowave radiation. Much research goes on first in the microwave range because at this wavelength, material surfaces and samples can be produced mechanically because the patterns tend to be on the order of a few centimeters. The production of optical range surface plasmon effects involves making 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 been shown to accommodate surface plasmons, observed via near field infrared optical microscopy techniques[15][16] and infrared spectroscopy.[17] Potential applications of graphene plasmonics mainly addressed the terahertz to midinfrared frequencies, such as optical modulators, photodetectors, biosensors.[18]
The position and intensity of plasmon absorption and emission peaks are affected by molecularadsorption, which can be used inmolecular sensors. For example, a fully operational device detectingcasein in milk has been prototyped, based on detecting a change inabsorption of a gold layer.[19] Localized surface plasmons of metal nanoparticles can be used for sensing different types of molecules, proteins, etc.
Plasmons are being considered as a means of transmitting information oncomputer chips, since plasmons can support much higher frequencies (into the 100 THz range, whereas conventional wires become very lossy in the tens ofGHz). However, for plasmon-based electronics to be practical, a plasmon-based amplifier analogous to thetransistor, called aplasmonstor, needs to be created.[20]
Plasmons have also beenproposed as a means of high-resolutionlithography 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 oncolloid films, such as screening and quantifyingprotein binding events. Companies such asBiacore have commercialized instruments that operate on these principles. Optical surface plasmons are being investigated with a view to improve makeup byL'Oréal and others.[21]
In 2009, a Korean research team found a way to greatly improveorganic light-emitting diode efficiency with the use of plasmons.[22]
A group of European researchers led byIMEC began work to improvesolar cell efficiencies and costs through incorporation of metallic nanostructures (using plasmonic effects) that can enhance absorption of light into different types of solar cells: crystalline silicon (c-Si), high-performance III-V, organic, and dye-sensitized.[23] However, for plasmonicphotovoltaic devices to function optimally, ultra-thintransparent conducting oxides are necessary.[24]Full colorholograms usingplasmonics[25] have been demonstrated.
Plasmon-soliton mathematically refers to the hybrid solution of nonlinear amplitude equation e.g. for a metal-nonlinear media considering both the plasmon mode and solitary solution. A soliplasmon resonance is on the other hand considered as a quasiparticle combining thesurface plasmon mode with spatial soliton as aresult of a resonant interaction.[26][27][28][29] To achieve one dimensional solitary propagation in aplasmonic waveguide while thesurface plasmons should be localized at the interface, the lateral distribution of the field envelope should also be unchanged.
Agraphene-based waveguide is a suitable platform for supporting hybrid plasmon-solitons due to the large effective area and huge nonlinearity.[30] For example, the propagation of solitary waves in a graphene-dielectric heterostructure may appear as in the form of higher order solitons or discrete solitons resulting from the competition betweendiffraction and nonlinearity.[31][32]
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