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). That 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 .
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.
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