Population inversion

In science, specifically statistical mechanics, a population inversion occurs while a system (such as a group of atoms or molecules) exists in a state in which more members of the system are in higher, excited states than in lower, unexcited energy states. It is called an "inversion" because in many familiar and commonly encountered physical systems, this is not possible. The concept is of fundamental importance in laser science because the production of a population inversion is a necessary step in the workings of a standard laser.

To understand the concept of a population inversion, it is necessary to understand some thermodynamics and the way that light interacts with matter. To do so, it is useful to consider a very simple assembly of atoms forming a laser medium.

Assume there are a group of N atoms, each of which is capable of being in one of two energy states: either

The number of these atoms which are in the ground state is given by N₁, and the number in the excited state N₂. Since there are N atoms in total,

The energy difference between the two states, given by

determines the characteristic frequency ${\textstyle \nu _{12}}$ of light which will interact with the atoms; This is given by the relation

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