Population inversion is a phenomenon that occurs in a system, such as a group of atoms or molecules, when more members of the system are in higher, excited states than in lower, unexcited energy states. It is a state of a system where a higher-lying energy level is more strongly populated than a lower-lying level. This cannot occur under conditions of thermal equilibrium, where the level populations are described by a Boltzmann distribution. However, a population inversion is often easily achieved by optical pumping at a suitable optical wavelength.
To understand the concept of population inversion, it is necessary to understand some thermodynamics and the way that light interacts with matter. For example, in a two-level energy system of energies E1 and E2, the population in the ground state or lower state is always greater than the population in the excited state or higher states under normal conditions. However, by pumping energy into the system, a population inversion can take place in which more atoms will exist in the higher energy states than in the lower.
Population inversion is essential for laser action, as it creates a situation where there are relatively many atoms in excited states, capable of emitting light of a particular wavelength, and few atoms in the ground state. When a population inversion exists, an upper-state system will eventually give off a photon of the proper wavelength and drop to the ground state. This photon, however, can stimulate the production of other photons of exactly the same wavelength because of stimulated emission of radiation. Thus, many photons of the same wavelength (and phase, and other similar characteristics) can be generated in a short time.
