stimulatedemission

Stimulated emission is a quantum mechanical process in which an incoming photon causes an excited atom or molecule to emit a second photon of the same energy, direction, phase, and polarization, effectively duplicating the stimulating photon. The two photons are identical in frequency and phase; this is essential for coherent light.

The process occurs when a system with more atoms in an excited state than in lower levels is illuminated by radiation at the transition frequency. The rate of stimulated emission is proportional to the population of the upper level and the density (or flux) of stimulating photons. In Einstein’s formalism, the rate is described by the B21 coefficient times the spectral energy density; spontaneous emission is described by A21.

In contrast to spontaneous emission, which is random, stimulated emission is deterministic with respect to the incident field. It also underpins optical amplification: a photon arriving triggers emission of another photon in phase, increasing light intensity with each interaction. The concept is central to lasers, where a population inversion and a resonator convert stimulated emission into a coherent beam.

Historically, Einstein introduced A and B coefficients in 1917 to explain black-body radiation; stimulated emission became a practical foundation for laser technology in the 1960s. Today it is observed in atomic, ionic, molecular transitions and in semiconductor and solid-state gain media, providing the mechanism for light amplification and coherent light generation.