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
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
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.
THREE LEVEL INVERSION
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.