Abstract
This chapter discusses the coherent emission in rare-earth materials. The three types of coherent emissions connected with spontaneous emission, which differ from laser emission by the common feature that they do not require a closed optical cavity are—Dieke superradiance (SR), superfluorescence (SF), and spontaneous emission amplified by stimulated emission (ASE). As for SF, atoms being prepared incoherently in their excited state emit a cooperative spontaneous emission by a self-organization stemming from their common emitted field. Though sometimes called “superradiance,” ASE would happen in a long laser medium without a mirror. Propagation plays an important role which is not necessary for pure SR and SF cases. Typical of the coherent spontaneous emissions is the reduced fluorescent lifetime, which can become much shorter than the non-radiative decay. Then the radiative process may even overtake the non-radiative one. Spontaneous emission is characterized by its independence of the initial photon number, that is, n =0.
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