When an atom (or a laser ion in a gain medium) is excited into a higher-lying energy level, e.g. by absorption of a photon, it may after some time spontaneously return to its ground state, or to some intermediate energy level, by releasing the energy in the form of a photon, which carries the energy in some random direction. (More precisely, the photon can correspond to any propagation mode of the medium surrounding the atom or ion.) This process is called spontaneous emission.
Spontaneous emission is a quantum effect, which in a semiclassical picture can be described as an emission which is stimulated by vacuum noise, i.e. by the zero point fluctuations of the optical field. This view is supported by the fact that spontaneous emission can be suppressed or modified by placing an atom or ion in a microcavity structure, which modifies the mode structure of the optical field.
The rate of spontaneous emission (and thus the lifetime of the excited level) is determined both by the properties of the atom and by the mode structure of the surrounding medium. For an atom in free space (or in a homogeneous medium), transition cross sections can be used for calculating the rate of spontaneous emission events (→ radiative lifetime). Typical upper-state lifetimes of atoms are a few nanoseconds if there are allowed transitions to lower levels; much longer values can occur for forbidden transitions.
Light produced by spontaneous emission is called luminescence. The luminescence emitted by a laser gain medium typically carries a total power which equals around one-tenths to one-third of the laser's output power. The optical spectrum of the luminescent light is essentially governed by transition cross sections.
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See also: stimulated emission, quantum noise, radiative lifetime, upper-state lifetime, transition cross sections, fluorescence, laser threshold, parametric fluorescence
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