Spontaneous emission luminescent. And there are different forms of luminescence depending on how excited atoms are produced (electroluminescence, chemiluminescence etc.). If the excitation is affected by the absorption of radiation the spontaneous emission is called fluorescence. Sometimes molecules have a metastable level and continue to fluoresce long after the exciting radiation is turned off; this is called phosphorescence. Figurines that glow in the dark are phosphorescent. Lasers start via spontaneous emission, then during continuous operation work by stimulated emission. Spontaneous emission cannot be explained by classical electromagnetic theory and is fundamentally a quantum process. The first person to derive the rate of
Sound amplification by stimulated emission of radiation emission takes place in the structure under electrical pumping. A bias voltage is applied to a weakly coupled n-doped GaAs/AlAs superlattice and increases the amplitude of the coherent hypersound oscillations generated by a femtosecond optical pulse. An evidence of hypersound amplification by stimulated emission of phonons emerges, in a system where the inversion of the electron populations for phonon-assisted transitions exists. This evidence is provided by the bias-induced amplitude increase and experimentally observer spectral narrowing of the superlattice phonon mode with a frequency of 441 GHz. The main target of this type of experiments is to highlight the realization probability