Population Inversion
A non-equilibrium state of a quantum system where more atoms reside in a higher excited energy state than in lower energy states.
Governing formula
N₂ > N₁ (Excited state density > Ground state density)
SI unit
Dimensionless Condition
In depth
Thermal equilibrium dictates that lower energy states are more populated according to Boltzmann statistics. Optical or electrical pumping achieves population inversion, enabling stimulated emission to overcome absorption to produce laser gain.
Examples in the real world
Optical pumping in Ruby lasers or gas discharge pumping in Helium-Neon (He-Ne) lasers.