A non-equilibrium condition where more quantum systems are in the higher excited state than the lower state of a particular transition. A quantitative measurement of population inversion applies only to the two levels which are directly involved in the transition. This unusual distribution among the excited state is unstable and will tend to decay back to thermodynamic equilibrium which consists of more electrons in the lower states.

This inverted distribution among the excited states is is required for most lasers, stimulated raman scattering and Lasing Without Inversion (LWI) are two special cases in which a population inversion is not required.

To create or maintain a population inversion requires that a certain minimum power be provided as a pump in order to push the quantum system away from thermodynamic equilibrium. This can be accomplished by taking advantage of the varying relaxation rates of different microscopic quantum processes among the various quantum levels. Selective populating of one level over another can be achieved in many ways such as electron collision, photon bombardement or rapid three body recombination.


If the population is initially in the ground state, intense bombardment by coherent laser radiation can drive the small atomic dipoles in phase with the incoming electromagnetic field. This results in a macroscopic electric polarization vector which causes the populations to oscillate between the ground and excited state with a frequency which is proportional to the electric field intensity of the the pump. If the pulse is turned off after the population has performed one half of a Bloch oscillation then they would all be left in the excited state, hence a population inversion has been created. The practical disadvantage of this scheme is that it requires a very precisely timed powerful coherent laser pulse.


Of course this simplified treatment of power saturation neglects the other quantum levels that are likely to occur in a real system. For example: if there was a third metastable quantum level just below the upper level to which the excited states could decay then a population inversion could build up between this third state and the ground state. The ruby laser functions in this way.

Home Page.