Rami Arieli: "The Laser Adventure" Chapter 2.6 page 1
2.6 Population Inversion

We saw that in a thermodynamic equilibrium Bolzman equation shows us that :

N1 > N2 > N3
Thus, the population numbers of higher energy levels are smaller than the population numbers of lower ones.
This situation is called "Normal Population" (as described in Figure 2.3a). In a situation of normal population a photon impinging on the material will be absorbed, and raise an atom to a higher level.

By putting energy into a system of atoms, we can achieve a situation of "Population Inversion". In population inversion, at least one of the higher energy levels has more atoms than a lower energy level.

An example is described in Figure 2.3b. In this situation there are more atoms (N3) in an higher energy level (E3), than the number of atoms (N2) in a lower energy level (E2).

Figure 2.3: "Normal Population" compared to "Population Inversion".
You can play with an interactive demo that helps to explain what population inversion means.
Click on Population Inversion Demonstration Applet

As we shall see later , this is one of the necessary conditions for lasing.
The process of raising the number of excited atoms is called "Pumping".
If this process is done by optical excitation (electromagnetic beam), it is called "Optical Pumping".