Exciton-polariton

In physics, the exciton–polariton is a type of polariton; a hybrid light and matter quasiparticle arising from the strong coupling of the electromagnetic dipolar oscillations of excitons (either in bulk or quantum wells) and photons.^[1] Because light excitations are observed classically as photons, which are massless particles, they do not therefore have mass, like a physical particle. This property makes them a quasiparticle.

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Polaritons are on the forefront of research in solid state physics. They exhibit many intriguing phenomena such as superfluidity and condensation. Polaritons offer unique possibilities to study quantum physics. In this video we will discuss what polaritons are and how they acquire their remarkable properties. Think about a guitar. This is a simple mechanical resonator. When the string is hit... ...it starts to oscillate and produces sound waves with a characteristic frequency, or pitch. These resonant frequencies, as they are called,... ...are the ones for which exactly a whole number of oscillations fit between the clamped points. For waves with other frequencies the string tries to move at the points where it is clamped down... ...and the waves quickly become very weak. Only waves at the resonant frequencies can survive on the string. Just like the sound waves on the guitar string,... ...light can also be thought of as a wave. Each colour of light has a different frequency.... ...and every frequency corresponds to a different wavelength,... ...which is the distance between the neighbouring peaks of the wave. Visible light has wavelengths ranging from 450 to 700 nanometres –... ...a size smaller than one thousandth of a millimetre. Now imagine two parallel mirrors facing each other. This is an optical resonator. It confines light inside by making it bounce between the mirrors. The light waves are trapped between the mirrors... ...in a similar way to the sound waves of the guitar string being trapped between the two clamps. As a result only some frequencies will be resonant... ...and able to survive between the two mirrors. The smallest possible resonator... ...is the one where only half of the wave fits between the mirrors. The wavelength of light is less than a micrometre... ...so an optical resonator that confines less than several wavelengths of visible light... ...is therefore called a microresonator. To make a polariton we need to add something to the light. In between the mirrors we place a quantum well,... ...which is only a few tens of atoms thick. It is called a well... ...because it traps electrons in its plane... ...so that they cannot escape to the surrounding regions. If a light wave with the right frequency passes through a quantum well... ...it can give up its energy to release an electron... ...from the atom it is bound to,... ...creating a vacancy. Relative to the surroundings,... ...this vacancy is positively charged... ...and is called a hole. Since the electron and hole have opposite charges... ...they attract and for a while orbit around each other. This behaviour forms a new particle,... which we call an exciton. Eventually the exciton recombines, when the electron falls back into the hole. The energy is released as a light wave with the same frequency... ...as the one which originally created the exciton. The quantum well is then placed in the plane... ...plane that intersects the peak of the light wave inside the microresonator. So, when the microresonator is calibrated... ...so the resonant light wave and the exciton have the same energy,... ...a single exciton can be converted into a light wave -... ...which then converts into an exciton, and so on. The resonant optical cavity can trap the light wave emitted by the exciton... ...so that it returns and creates another exciton which also decays. A repeating cycle between light wave and exciton is made possible. Since the exciton and light are constantly interchanging... ...they behave overall like a new particle called a polariton,... ...which has some of the properties of each.

Theory

The coupling of the two oscillators, photons modes in the semiconductor optical microcavity and excitons of the quantum wells, results in the energy anticrossing of the bare oscillators, giving rise to the two new normal modes 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, 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).^[2]

Researchers also studied the long-range transport in organic materials linked to optical microcavities and demonstrated that exciton-polaritons propagate over several microns.^[3] This was done in order to prove that exciton-polaritons propagate over several microns and that the interplay between the molecular disorder and long-range correlations induced by coherent mixing with light leads to a mobility transition between diffusive and ballistic transport.^[4]

Other features

Polaritons are also characterized by non-parabolic energy–momentum dispersion relations, which limit the validity of the parabolic effective-mass approximation to a small range of momenta.^[5] They also have a spin degree-of-freedom, making them spinorial fluids able to sustain different polarization textures. Exciton-polaritons are composite bosons which can be observed to form Bose–Einstein condensates,^[6]^[7]^[8]^[9] and sustain polariton superfluidity and quantum vortices^[10] and are prospected for emerging technological applications.^[11] Many experimental works currently focus on polariton lasers,^[12] optically addressed transistors,^[13] 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 equations) which are in the form of nonlinear Schrödinger equations.^[14]

References

^ S.I. Pekar (1958). "Theory of electromagnetic waves in a crystal with excitons". Journal of Physics and Chemistry of Solids. 5 (1–2): 11–22. Bibcode:1958JPCS....5...11P. doi:10.1016/0022-3697(58)90127-6.
^ Vladimirova, M; et al. (2010). "Polariton-polariton interaction constants in microcavities". Physical Review B. 82 (7): 075301. Bibcode:2010PhRvB..82g5301V. doi:10.1103/PhysRevB.82.075301.
^ Georgi Gary Rozenman; Katherine Akulov; Adina Golombek; Tal Schwartz (2018). "Long-Range Transport of Organic Exciton-Polaritons Revealed by Ultrafast Microscopy". ACS Photonics. 5 (1): 105–110. doi:10.1021/acsphotonics.7b01332.
^ Balasubrahmaniyam; Arie Simkhovich; Adina Golombek; Gal Sandik; Guy Ankonina; Tal Schwartz (2023). "From enhanced diffusion to ultrafast ballistic motion of hybrid light–matter excitations". Nature Materials. 22 (3): 338. arXiv:2205.06683. doi:10.1038/s41563-022-01463-3.
^ Pinsker, F.; Ruan, X.; Alexander, T. (2017). "Effects of the non-parabolic kinetic energy on non-equilibrium polariton condensates". Scientific Reports. 7 (1891): 1891. arXiv:1606.02130. Bibcode:2017NatSR...7.1891P. doi:10.1038/s41598-017-01113-8. PMC 5432531. PMID 28507290.
^ Deng, H (2002). "Condensation of semiconductor microcavity exciton polaritons". Science. 298 (5591): 199–202. Bibcode:2002Sci...298..199D. doi:10.1126/science.1074464. PMID 12364801. S2CID 21366048.
^ Kasprzak, J (2006). "Bose–Einstein condensation of exciton polaritons". Nature. 443 (7110): 409–14. Bibcode:2006Natur.443..409K. doi:10.1038/nature05131. PMID 17006506.
^ Deng, H (2010). "Exciton-polariton Bose–Einstein condensation". Reviews of Modern Physics. 82 (2): 1489–1537. Bibcode:2010RvMP...82.1489D. doi:10.1103/RevModPhys.82.1489. S2CID 122733835.
^ Byrnes, T.; Kim, N. Y.; Yamamoto, Y. (2014). "Exciton–polariton condensates". Nature Physics. 10 (11): 803. arXiv:1411.6822. Bibcode:2014NatPh..10..803B. doi:10.1038/nphys3143.
^ Dominici, L; Dagvadorj, G; Fellows, JM; et al. (2015). "Vortex and half-vortex dynamics in a nonlinear spinor quantum fluid". Science Advances. 1 (11): e1500807. arXiv:1403.0487. Bibcode:2015SciA....1E0807D. doi:10.1126/sciadv.1500807. PMC 4672757. PMID 26665174.
^ Sanvitto, D.; Kéna-Cohen, S. (2016). "The road towards polaritonic devices". Nature Materials. 15 (10): 1061–73. Bibcode:2016NatMa..15.1061S. doi:10.1038/nmat4668. PMID 27429208.
^ Schneider, C.; Rahimi-Iman, A.; Kim, N. Y.; et al. (2013). "An electrically pumped polariton laser". Nature. 497 (7449): 348–352. Bibcode:2013Natur.497..348S. doi:10.1038/nature12036. PMID 23676752.
^ Ballarini, D.; De Giorgi, M.; Cancellieri, E.; et al. (2013). "All-optical polariton transistor". Nature Communications. 4 (2013): 1778. arXiv:1201.4071. Bibcode:2013NatCo...4E1778B. doi:10.1038/ncomms2734. PMID 23653190.
^ Moxley, Frederick Ira; Byrnes, Tim; Ma, Baoling; Yan, Yun; Dai, Weizhong (2015). "A G-FDTD scheme for solving multi-dimensional open dissipative Gross–Pitaevskii equations". Journal of Computational Physics. 282: 303–316. Bibcode:2015JCoPh.282..303M. doi:10.1016/j.jcp.2014.11.021. ISSN 0021-9991.

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This page was last edited on 27 January 2024, at 03:14

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