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Photons

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Definition: quanta of light energy

German: Photonen

Categories: general optics, physical foundations, quantum optics

How to cite the article; suggest additional literature

When a weak light beam hits a sensitive photodetector, energy is found to be delivered in the form of small bunches, rather than continuously. This can be interpreted so that the light beam consists of small bunches of energy, called photons or light quanta (German 'Lichtquanten' = portions of light). The photon energy is h ν = h c / λ, i.e. the product of Planck's constant h and the optical frequency ν, and is also related to the vacuum wavelength λ. The idea that light consists of such energy bunches had already been used early in the 20th century by Max Planck in the context of thermal radiation, and by Albert Einstein when investigating the photoelectric effect. The term photon, however, was coined only in 1926 by the physical chemist Gilbert N. Lewis [1].

Although a 'naïve' interpretation of photons as particles of light gives a useful picture for the intuitive understanding of many quantum phenomena, it can be seriously misleading to apply it without understanding its limitations. A consistent and very powerful, but certainly not simple description of the nature of light is achieved by modern quantum optics. Here, photons are seen as the elementary excitations of the electromagnetic quantum field. This theory attributes fairly strange properties to photons, which cannot be reconciled either with a simple particle picture or with a pure wave picture.

Some Key Properties of Photons

Of course, quantum theory can be applied to any kind of electromagnetic wave phenomena, not only to visible light. However, quantum effects are not as important e.g. in the field of radio technology, as in optics and laser technology. This is because the photon energy of radio waves is very tiny compared with the thermal energy kBT at room temperature, whereas the opposite is true for optical phenomena.

In laser physics, a frequently considered case is that of photons propagating in media, e.g. in transparent crystals or glasses, including laser gain media. Strictly, the term photon is then no longer appropriate, as electromagnetic waves interact with such media, and what propagates are quasi-particles, sometimes called polaritons, which resemble coupled excitations of the electromagnetic field and a polarizable medium.

Bibliography

[1]G. N. Lewis, “The conservation of photons”, Nature 118, 874 (1926)
[2]R. A. Beth, “Mechanical detection and measurement of the angular momentum of light”, Phys. Rev. 50 (2), 115 (1936)
[3]R. Hanbury Brown and R. Q. Twiss, “Correlation between photons in two coherent beams of light”, Nature 177, 27 (1956)
[4]G. Molina-Terriza, J. P. Torres and L. Torner, “Twisted photons”, Nat. Phys. 3, 3050 (2007)
[5]K. E. Ballantine, J. F. Donegan and P. R. Eastham, “There are many ways to spin a photon: Half-quantization of a total optical angular momentum”, Science Advances 2 (4) (2016)
[6]C. Cohen-Tannoudji, J. Dupont-Roc and G. Grynberg, Photons and Atoms: Introduction to Quantum Electrodynamics, Wiley, New York (1997)
[7]C. Roychoudhuri, A. F. Kracklauer, and K. Creath (eds.), The Nature of Light. What is a Photon?, CRC Press, Boca Raton, FL (2008)

(Suggest additional literature!)

See also: single photon counting, quantum optics, spontaneous emission, stimulated emission, nonclassical light, phonons, Spotlight article 2007-03-23, Spotlight article 2008-05-05
and other articles in the categories general optics, physical foundations, quantum optics

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