Laser Cooling and Trapping: Research
If an atom is moving towards or away from a laser beam, the atom sees a Doppler-shifted frequency. If the atom moves towards the beam, as shown in the drawing, the atom sees a slight increase in the laser frequency.
An atom can make a transition between two energy levels by absorbing or emitting a photon-a light quantum- provided the frequency f of the photon corresponds to the energy difference between two atomic levels. (The energy of the photon is directly proportional to the photon's frequency.) The drawing shows an atom with energies E1 and E2. Actually, each level has a small width in energy, so an atom can absorb energy over a small range of photon frequencies. The graph shows this range and displays the probability of photon absorption as a function of frequency for an atomic transition.
In laser cooling and trapping, the laser frequency is tuned below the center frequency of the center of this curve, as shown in the graph. If the atom moves towards the laser beam, the Doppler shift increases the light frequency in the atom's frame of reference. Thus the atom moving towards the beam has a higher probability of absorbing the photon and receiving a kick of momentum that slows the atom. However, if the atom is moving away from the beam, the Doppler shift reduces the light frequency and moves it further from the center frequency, thus further reducing the probability of photon absorption.
Within about 10-8 seconds of absorbing a photon, the atom emits another photon and returns to its original state. The emission of this second photon gives the atom a kick in a random direction. The net effect of many such events is a slowing of the atom.
Laser Cooling and Trapping Laboratory at NIST (Gaithersburg, MD). Photo by Paul Lett.
