Non-inversion Laser

For decades, lasers have been explained as the result of an inverted population of states with stimulated emission of photons in a high-Q cavity. However, lasers can be made without an inverted population. Can you explain how this type of stimulated emission process works?

Answer

Yes, lasing without inversion (LWI) can occur whenever absorption cancellation is established. Light amplification is then possible even when the upper-level population is less than the lower-level population. This cancellation can be set up in a three-level system in an atom in which the two absorption transitions to the same final state interfere and cancel, making the absorption probability zero. In the diagram, upper-level state | a > is connected to lower levels | b > and | c >. Use incident photons of the

appropriate energies E1 and E2, which correspond to transitions | a > to | b > and | a > to | c >, respectively. The uncertainty in these atomic transitions leads to interference, since the transitions end in the same final state. There is no way to determine which absorption transition to the final state actually occurred, so like the Young double-slit experiment, one must have the interference. There is no interference between the emission paths, since they have different final states. By arranging the phases of the two incoming light rays properly, one can make the interference completely destructive for absorption. Then stimulated emission is the only process left. For details of the probability calculations, see the references below.