If a laser-active atom or ion is in an excited state (quantum-mechanical energy level), it may after some time spontaneously decay into a lower energy level, releasing energy in the form of a photon, emitted in a random spatial direction. This process is called spontaneous emission. However, it is also possible that the photon emission is stimulated by incoming photons , if these have a suitable photon energy (or optical frequency); this is called stimulated emission. In that case, a photon is emitted into the mode of the incoming photon. In effect, the power of the incoming radiation is amplified. This is the physical basis of light amplification in laser amplifiers and laser oscillators.
The physics of stimulated emission can be described in the context of quantum optics, although there are also semiclassical descriptions (treating the interaction of an oscillating dipole with an electromagnetic field), and the original idea of stimulated emission was published by Einstein  before quantum mechanics were fully developed.
Note that the amplification effect of stimulated emission can be reduced or entirely suppressed in a medium where too many laser-active atoms are in the lower state of the laser transition, because these atoms are absorb photons and thus attenuate light. In a simple two-level system, laser amplification requires a so-called population inversion.
The rate of stimulated emission processes for an excited atom can be calculated as the product of the so-called emission cross section and the photon flux (number of photons per unit area and time). The photon flux can be calculated as the optical intensity divided by the photon energy.
In a laser operated well above threshold, stimulated emission dominates over spontaneous emission, and the power efficiency can be high. For this condition to be fulfilled, the incident optical intensity must be higher than the saturation intensity.
|||A. Einstein, “Zur Quantentheorie der Strahlung”, Physikalische Zeitschrift XVIII, 121 (1917) (first prediction of stimulated emission)|
See also: spontaneous emission, laser transitions, photons, gain, transition cross sections, population inversion, optical amplifiers, lasers, Rabi oscillations
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