population inversion

noun

, Physics.

1. a condition of matter in which more electrons are in a high energy state than in a lower energy state, as is required for the operation of a laser.

population inversion

1. The condition of having enough excited or high-energy states distributed throughout a substance to sustain a chain-reaction of stimulated emission. Lasers, for example, need a constant power source that maintains population inversion in order to generate radiation continuously, since each stimulated emission reduces the population of high-energy states.
2. See also stimulated emission

Word History and Origins

Origin of population inversion¹

First recorded in 1960–65