physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...

the Exciton–polariton is a type of

polariton In physics, polaritons are quasiparticles resulting from strong coupling of electromagnetic waves with an electric or magnetic dipole-carrying excitation. They are an expression of the common quantum phenomenon known as level repulsion, also ...

; a hybrid

light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...

and

matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic partic ...

quasiparticle In physics, quasiparticles and collective excitations are closely related emergent phenomena arising when a microscopically complicated system such as a solid behaves as if it contained different weakly interacting particles in vacuum. For exam ...

arising from the strong coupling of the electromagnetic dipolar oscillations of

exciton An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and some liquids. The ...

s (either in bulk or

quantum well A quantum well is a potential well with only discrete energy values. The classic model used to demonstrate a quantum well is to confine particles, which were initially free to move in three dimensions, to two dimensions, by forcing them to occupy ...

s) and

photons A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alway ...

. Because light excitations are observed classically as

, which are massless particles, they do not therefore have

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementar ...

, like a physical particle. This property makes them a quasiparticle.

Theory

The coupling of the two oscillators,

modes in the semiconductor

optical microcavity An optical microcavity or microresonator is a structure formed by reflecting faces on the two sides of a spacer layer or optical medium, or by wrapping a waveguide in a circular fashion to form a ring. The former type is a standing wave cavity, and ...

and

s of the

s, results in the energy anticrossing of the bare oscillators, giving rise to the two new

normal mode A normal mode of a dynamical system is a pattern of motion in which all parts of the system move sinusoidally with the same frequency and with a fixed phase relation. The free motion described by the normal modes takes place at fixed frequencies. ...

s for the system, known as the upper and lower polariton resonances (or branches). The energy shift is proportional to the coupling strength (dependent, e.g., on the field and polarization overlaps). The higher energy or upper mode (UPB, upper polariton branch) is characterized by the photonic and exciton fields oscillating in-phase, while the LPB (lower polariton branch) mode is characterized by them oscillating with phase-opposition. Microcavity exciton–polaritons inherit some properties from both of their roots, such as a light effective mass (from the photons) and a capacity to interact with each other (from the strong exciton nonlinearities) and with the environment (including the internal

phonons In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, specifically in solids and some liquids. A type of quasiparticle, a phonon is an excited state in the quantum mechanic ...

, which provide thermalization, and the outcoupling by radiative losses). In most cases the interactions are repulsive, at least between polariton quasi-particles of the same spin type (intra-spin interactions) and the nonlinearity term is positive (increase of total energy, or blueshift, upon increasing density). Recently, researchers measured the long-range transport in organic materials coupled to optical micro-cavities and showed that exciton-polaritons propagate over several microns.

Other features

Polaritons are also characterized by non-parabolic

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...

–

momentum In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...

dispersion relations In the physical sciences and electrical engineering, dispersion relations describe the effect of dispersion on the properties of waves in a medium. A dispersion relation relates the wavelength or wavenumber of a wave to its frequency. Given the d ...

, which limit the validity of the parabolic effective-mass approximation to a small range of momenta . They also have a

spin Spin or spinning most often refers to: * Spinning (textiles), the creation of yarn or thread by twisting fibers together, traditionally by hand spinning * Spin, the rotation of an object around a central axis * Spin (propaganda), an intentionally b ...

degree-of-freedom, making them spinorial fluids able to sustain different polarization textures. Exciton-polaritons are composite

bosons In particle physics, a boson ( ) is a subatomic particle whose spin quantum number has an integer value (0,1,2 ...). Bosons form one of the two fundamental classes of subatomic particle, the other being fermions, which have odd half-integer ...

which can be observed to form Bose-Einstein condensates, and sustain

polariton superfluid Polariton superfluid is predicted to be a state of the exciton-polaritons system that combines the characteristics of lasers with those of excellent electrical conductors. Researchers look for this state in a solid state optical microcavity coupled ...

ity and

quantum vortices In physics, a quantum vortex represents a quantized flux circulation of some physical quantity. In most cases, quantum vortices are a type of topological defect exhibited in superfluids and superconductors. The existence of quantum vortices was f ...

and are prospected for emerging technological applications. Many experimental works currently focus on

polariton laser A polariton laser is a novel type of laser source that exploits the coherent nature of Bose condensates of exciton-polaritons in semiconductors to achieve ultra-low threshold lasing. In 1996, Imamoglu ''et al.'' proposed such a novel type of coher ...

s, optically addressed transistors, nonlinear states such as solitons and shock waves, long-range coherence properties and phase transitions, quantum vortices and spinorial patterns. Modelization of exciton-polariton fluids mainly rely on the use of GPE (

Gross–Pitaevskii equation The Gross–Pitaevskii equation (GPE, named after Eugene P. Gross and Lev Petrovich Pitaevskii) describes the ground state of a quantum system of identical bosons using the Hartree–Fock approximation and the pseudopotential interaction model. A ...

s) which are in the form of

nonlinear Schrödinger equations In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many othe ...

References

{{Reflist

External links

YouTube animation explaining what a polariton is in a semiconductor micro-resonator.

Description of experimental research on polariton fluids at the Institute of Nanotechnologies within the Italian CNR.
Quasiparticles

Theory

Other features

See also

References

External links