How Laser Cooling works - Explained

[Davide86]

How does the laser cooling work? I have read that it is used to cool ions in vacuum, by absorbing their kinetic energy with a laser beam; but I don't understand why the laser cool the ions and not raise their energy by moment transfer

 

[borgwal]

In a nutshell: The atoms or ions absorb a laser photon of a certain energy, then emit a photon with a slightly larger energy, thus losing kinetic energy in the process.

There are more subtle ways of cooling, but this is by far the easiest to understand.

 

[Redbelly98]

Davide86, a good explanation requires figures, and knowledge (on your part) of the Doppler effect for light. So it would be extremely difficult to provide a full explanation here.

Instead, I will refer you to 2 Scientific American articles that have dealt with the subject.

W. D. Phillips and H. J. Metcalf, Cooling and Trapping Atoms, March 1987.
Steven Chu, Laser Trapping of Neutral Particles, February 1992.

You might find old Scientific American articles in a local university physics or chemistry department library.

 

[Enthalpy]

One intriguing thought is what happens with the second law of thermodynamics here. After all, laser light is generally considered hot, and now it cools.

To keep coherent (if I dare to say) you have to choose the appropriate definition of the temperature for a light beam. It could have been its colour, or its power density over surface-angle-frequency... But here, its interesting temperature is related to its bandwidth. As the laser light has a narrow bandwidth, we may call it cold that time, and it does cool the ions - whereas broadband light would heat them.

Just one more example where the second law is more treacherous than useful.

 

[borgwal]

One intriguing thought is what happens with the second law of thermodynamics here. After all, laser light is generally considered hot, and now it cools.

To keep coherent (if I dare to say) you have to choose the appropriate definition of the temperature for a light beam. It could have been its colour, or its power density over surface-angle-frequency... But here, its interesting temperature is related to its bandwidth. As the laser light has a narrow bandwidth, we may call it cold that time, and it does cool the ions - whereas broadband light would heat them.

Just one more example where the second law is more treacherous than useful.

The second law is only treacherous of you have no idea what you're talking about.
Laser light doesn't have a temperature, it is FAR from thermal equilibrium.
Instead, you should talk about the entropy of laser light, which is very low, the quantum state being (almost) pure. The total entropy of atoms + light increases by a lot, because the atoms, in the process of being laser cooled, spontaneously emit photons: those photons are certainly not in a pure state, and are emitted in random directions. If you calculate how much entropy is gained by photons, vs. how much the atoms' entropy decreases per photon absorption/emission event, then you'll find the gain in entropy is about 5 or 6 orders of magnitude more than the decrease.