Superfluorescence is a phenomenon of quantum optics. It is the collective emission of fluorescent light by an ensemble of excited atoms or ions . Initially, the atoms (or ions) are incoherently excited (e.g. by optical pumping), so that there is no macroscopic dipole moment. The process starts slowly with spontaneous emission. The radiation field then couples the radiation phases of the different atoms, so that they become synchronized, and a collective emission releases the stored energy as a superfluorescent pulse. The pulse maximum occurs after some delay, and the pulse duration can be short compared with the upper-state lifetime of a single atom. The maximum intensity of the emitted light scales with the square of the number of atoms.
Superfluorescence is similar to superradiance, but in the latter case there is a macroscopic dipole moment from the beginning, created by the excitation process: the excitation is created with a light pulse such that all involved atoms have about the same location on the Bloch sphere. (The intensity must be sufficiently uniform, and the involved atoms should behave all in the same way, e.g. not influenced by locally varying conditions.) Unfortunately, however, many authors do not distinguish between superfluorescence and superradiance.
The phenomenon of superfluorescence, as defined above, is also substantially different from superluminescence or amplified spontaneous emission (ASE). The latter process does not require close proximity and coherent excitation, and is based only on fluorescence and stimulated emission. Superfluorescence and superluminescence are processes which can compete in certain situations, depending on, e.g., the dephasing rate of the atomic system .
Unfortunately, there appears to be no authoritative definition of the term superfluorescence, and the literature contains many scientific papers where the term has been used with different meanings. For example, the term superfluorescence source (or superfluorescent source) is in most cases used such that it means the same as the actually more suitable term superluminescent source.
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