I recently came upon this fantastic technique through reading, but I found it difficult to comprehend how it operates. Would the ions not initially heat up if photons were pumped in? In what way does the energy loss exceed the input? Many thanks ahead of time.
In: Physics
Atoms (and ions, which are just atoms with the wrong number of electrons) can’t just absorb any frequency of photons that hit them. In the frequency range that is used for laser cooling, they can only absorb a photon if that photon’s energy corresponds to an electron transition in that atom. For example, a hydrogen atom in its lowest energy state can only absorb a photon if its energy is equal to the difference between the atom in its ground state and the atom in an excited state (such as the electron being promoted to the n=2 orbital).
The energy of a photon is equal to its frequency times Planck’s constant. So if an atom can only absorb certain energies of photons, then it can also only absorb certain frequencies of photons. This is where the Doppler effect comes in. If you expose some atoms to photons that are slightly too low in energy, then the atoms won’t absorb them… unless the atom is moving *towards* the source of the photons. In that case, the atom will see the photon as having slightly higher frequency and energy, and thus it can absorb it. Atoms that aren’t moving towards the laser, or that are moving away from it, will see a lower energy and thus won’t absorb the photons.
So if you put these lasers all around your cloud of atoms, you have a situation where if an atom is standing still, it ignores the photons, but if it moves towards one of the lasers, it absorbs a photon. And when an atom is moving towards the laser and absorbs a photon, it will slow down, because it’s a head-on collision. So atoms that are standing still will keep standing still, but atoms that move towards one of the lasers will slow down. This is sometimes called “optical molasses”, because it applies a kind of drag or friction on the atoms that move, without speeding up the ones that are already standing still. The end result is that the atoms all slow down and collectively have a very low temperature.
The atoms do need to release the energy they absorb, of course, so there is a limit to how much you can cool the atoms with these photons. The atoms will end up with random motion equivalent to the momentum of one photon. But that’s still a very good cooling effect.
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