Population inversion

A state in which more particles occupy an excited energy level than a lower one. It enables stimulated emission to exceed absorption, allowing optical amplification.

Stimulated emission: An excited particle emits a photon when triggered by an incoming photon, producing a matching photon. Inversion lets this process outnumber absorption, producing net gain.
Energy level: A discrete allowed value of the energy of an atom, molecule, or other quantum system. Inversion compares how many particles occupy two such levels.
Laser: A device that produces light through optical amplification by stimulated emission. Its active medium uses population inversion to amplify selected modes of light.
Thermal equilibrium: A state in which a system's macroscopic properties remain stable and its components share a common temperature. A system with ordinary positive temperature has fewer particles in higher energy levels.
Absorption (electromagnetic radiation): The transfer of electromagnetic energy to matter when a particle takes up a photon. Below inversion, absorption exceeds stimulated emission and light is attenuated.
Boltzmann distribution: The equilibrium distribution of states across energies at a specified temperature. At thermal equilibrium, this distribution favors lower energy rather than inversion.
Optical pumping: The use of light to redistribute particles among quantum states. It can create an inverted population by selectively exciting atoms or molecules.
Negative temperature: A thermodynamic condition in a bounded energy system where higher-energy states are more populated than lower ones. It is a formal thermodynamic description possible for certain inverted populations, not simply a temperature below zero kelvin.
Three-level laser: A laser energy-level scheme that pumps particles from the ground state to a higher state before they accumulate in a metastable state. Its inversion threshold is high because more than half the particles must leave the ground state.
Einstein coefficients: Coefficients describing the rates of absorption, spontaneous emission, and stimulated emission between two energy levels. Their relations show why equal-level populations do not generally produce optical gain.

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