I am a high school student trying to understand how laser cooling works.

What I understand so far is that scientist take a bunch of atoms in vacuum and point lasers at them from all directions. The atoms will absorb the photons but only if they have a specific frequency that matches the one of the atom since an atom will only absorb a particle that has enough energy to transfer it to a higher energy level. I also understand that if an atom absorbs a photon moving in the same direction of it the atom will increase in velocity (meaning an increase in temperature which is opposite the desired effect) so to ensure that only atoms moving in the opposite direction of the photons absorbs them scientists take advantage of the doppler effect, so they tune the laser beam at a frequency slightly lower than the one the atom desires so that the atoms moving towards the photons see them as perfect frequency and absorbs them and the ones moving away see tham at lower frequency and therefore do not absorb them. All of this makes sense to me, however I do not understand what happens after that. I know that the atoms will return to their ground levels after reaching an excited state and will release a photon (in a random direction) of the same energy as the one it absorbed but what happens then?


1 Answer 1


You have a very good understanding of the principle so far. But there is one misunderstanding: the photon released from the system has more energy than the photon that was initially absorbed.

In order to make up the energy difference between the photon energy and the atom's "desired" energy, the atom has to use up some of its own kinetic energy. When a photon is later emitted, the kinetic energy it used up gets lost as well as the initial energy.

The net result is that the atom gets slower. If you carry out this cooling process again and again, the atoms keep on getting slower and slower. Their speed is linked to their temperature, so the atoms are being cooled down by this process.

  • $\begingroup$ But why is that? Does it take more energy to to go from a higher energy level to a lower one than it takes to go from a lower to a higher? Does the atom need to get rid of more energy to go back to its ground state and therefore need to release more enegy (in form of a photon) than it absorbed? Thank you! $\endgroup$
    – Manar
    Commented Sep 7, 2022 at 19:03
  • 1
    $\begingroup$ A good way to think about it, is to consider the Doppler shift. Suppose an atom has a blue (higher energy) transition, and a scientist puts in red (lower energy) light. If the atom was stationary, it would not absorb the red light. When it moves, Doppler shift might be enough so that the red light "looks" blue to the atom. Then the atom can absorb the light. When it emits, it emits light, which the scientist measures to be blue. On average then, the atom is converting its kinetic energy into light energy which it emits. $\endgroup$
    – David
    Commented Sep 7, 2022 at 19:08
  • $\begingroup$ So the atom absorbs light that is red but "appears" blue and then emitts blue light which has more energy compared to what it technically absorbed (red light)? $\endgroup$
    – Manar
    Commented Sep 7, 2022 at 19:18
  • $\begingroup$ That sounds about right. $\endgroup$
    – David
    Commented Sep 7, 2022 at 19:22

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