Encyclopedia … combined with a great Buyer's Guide!

Sponsors:     and others

Doppler Cooling

Definition: a technique of laser cooling based on velocity-dependent absorption processes

German: Doppler-Kühlung

Categories: quantum optics, methods

How to cite the article; suggest additional literature

Author:

URL: https://www.rp-photonics.com/doppler_cooling.html

Doppler cooling is a technique for laser cooling of small particles, typically atoms or ions. The basic principle is that absorption and subsequent spontaneous emission of photons lead to light forces, which serve to reduce the average particle velocity if the frequency of the light is somewhat below the center frequency of an electronic transition of the particles:

  • The faster an atom (or ion or molecule) is moving towards that red-detuned light, the more will the Doppler effect bring it towards resonance of the electronic transition. That means that the rate of photon absorption events increases, and that leads to a transfer of momentum, i.e., a force, which is opposite the direction of movement and thus decelerates the atoms.
  • The contrary happens for atoms moving away from the light: the (in that case accelerating) light force is reduced by the Doppler effect.
  • There is also a random force resulting from spontaneous emission shortly after each absorption event. Due to the random direction of spontaneous emission, that force is zero on average. It is only that its fluctuations prevent one from getting to arbitrarily low temperatures of the atom cloud.

The simplest situation is that a beam of atoms in a vacuum chamber can be stopped and cooled (in one spatial dimension) with a counterpropagating single-frequency laser beam, the optical frequency of which is first chosen to be somewhat higher than the atomic resonance, so that only the fastest atoms can absorb photons. Subsequently, the laser frequency is reduced so that slower and slower atoms participate in the interaction, and finally all atoms have a greatly reduced speed and a reduced range of speeds (in the direction of the laser beam). Thats corresponds to a lower temperature, assuming that thermal equilibrium can be reestablished by collisions.

An alternative to sweeping the laser frequency is sweeping the atomic resonances via a spatially varying magnetic field (Zeeman slowing).

Doppler cooling can also be used in an arrangement called optical molasses, where cooling occurs in all three dimensions [5].

The minimum temperature achievable with Doppler cooling is the Doppler limit. In some cases, however, cooling well below the Doppler limit (down to the region of the recoil limit) has been observed and explained as Sisyphus cooling.

Questions and Comments from Users

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here; we would otherwise delete it soon. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him e.g. via e-mail.

Your question or comment:

Spam check:

  (Please enter the sum of thirteen and three in the form of digits!)

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.

Bibliography

[1]T. W. Hänsch and A. L. Schawlow, “Cooling of gases by laser radiation”, Opt. Commun. 13 (1), 68 (1975), doi:10.1016/0030-4018(75)90159-5
[2]V. S. Letokhov, V. G. Minogin and B. D. Pavlik, “Cooling and capture of atoms and molecules by a resonant light field”, JETP 45, 698 (1977)
[3]D. J. Wineland, R. E. Drullinger and F. L. Walls, “Radiation-pressure cooling of bound resonant absorbers”, Phys. Rev. Lett. 40 (25), 1639 (1978), doi:10.1103/PhysRevLett.40.1639
[4]S. Chu et al., “Three-dimensional viscous confinement and cooling of atoms by resonance radiation pressure”, Phys. Rev. Lett. 55 (1), 48 (1985), doi:10.1103/PhysRevLett.55.48
[5]P. D. Lett et al., “Optical molasses”, J. Opt. Soc. Am. B 6 (11), 2084 (1989), doi:10.1364/JOSAB.6.002084
[6]D. J. Wineland and W. M. Itano, “Laser cooling of atoms”, Phys. Rev. A 20 (4), 1521 (1979), doi:10.1103/PhysRevA.20.1521
[7]C. Savage, “Introduction to light forces, atom cooling, and atom trapping” (1995), https://arxiv.org/abs/atom-ph/9510004
[8]R. Chang et al., “Three-dimensional laser cooling at the Doppler limit”, Phys. Rev. A 90 (6), 063407 (2014), doi:10.1103/PhysRevA.90.063407

(Suggest additional literature!)

See also: Doppler broadening, optical molasses, laser cooling, Sisyphus cooling
and other articles in the categories quantum optics, methods

preview

If you like this page, please share the link with your friends and colleagues, e.g. via social media:

These sharing buttons are implemented in a privacy-friendly way!