domingo, 27 de junio de 2010

Laser Cooling

Laser cooling is usually not meant to be the cooling of lasers, but rather the use of dissipative light forces for reducing the random motion and thus the temperature of small particles, typically atoms or ions. Depending on the mechanism used, the temperature achieved can be in the regime of millikelvins, microkelvins, or even nanokelvins. A totally different kind of laser cooling, where macroscopic samples are cooled, is treated in a separate article on optical refrigeration.

Methods of Laser Cooling

A simple scheme for laser cooling is Doppler cooling, where light forces are exerted by absorption and subsequent spontaneous emission of photons and the rate of these processes depends on the velocity of an atom or ion due to the Doppler shift. For example, a beam of atoms in a vacuum chamber can be stopped and cooled 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 (at least in one dimension). This corresponds to a lower temperature, assuming that thermal equilibrium can be re-established.

Doppler cooling can also be used in an optical molasses for damping the atomic motion in one to three spatial dimensions.

The method of Doppler cooling is limited in terms of the reachable temperature (→ Doppler limit). There are other methods, most notably Sisyphus cooling, which allow one to get substantially below the Doppler limit, down to the much lower recoil limit associated with the recoil momentum related to the absorption or emission of a single photon. Even the recoil limit is not the final one: specifically the method of velocity-selective coherent population trapping allows sub-recoil temperatures in the nanokelvin regime.

Another technique is evaporative cooling, where the capturing potential in an atom or ion trap is gradually reduced so that the fastest particles can escape, and the average energy of the remaining particles is reduced. Subsequent collisions can re-establish a thermal equilibrium, corresponding to a reduced temperature.


Some examples of applications of laser cooling are:

high-resolution spectroscopic measurements (e.g. for frequency standards in optical clocks) by the elimination of Doppler broadening studying the behavior of ultracold gases, which can exhibit interesting phenomena such as Bose–Einstein condensation (BEC), for example ultraprecise measurement of gravitational fields (used e.g. for gravitational physics or for oil field exploration), based on the Doppler shift of free-falling cooled atoms, on Bloch oscillations lithography with cold atomic beams to form very accurately controlled structures.

In 1997, the Nobel Prize in Physics was awarded to Steven Chu, Claude Cohen-Tannoudji and William D. Phillips, for the development of methods to cool and trap atoms with laser light. Important early contributions to this field were also brought by Theodor W. Hänsch [1], Nobel Prize winner in 2005 (for other achievements).
Jesús Ramirez
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