Fundamentals of laser action

As the acronym laser Qight amplification by stimulated emission of radiation) suggests, laser radiation involves an amplification process for photons, i.e. if a photon beam passes through a medium, with a reso- 

When such a particle system is exposed to photons that are in resonance with a transition between two energy levels, the three processes introduced by Einstein, i.e. absorption, stimulated emission and spontaneous emission, need to be considered. Overall, the interaction can be described by the so-called rate 

Laser Chemistry Spectroscopy, Dynamics and Applications Helmut H. Telle, Angel Gonzalez Urena Robert J. Donovan 2007 John Wiley Sons, Ltd ISBN 978-0-471-48570-4 (HB) ISBN 978-0-471-48571-1 (PB) 

Note that stimulated emission, ultimately responsible for laser amplification, does indeed take place in the two-level photon-matter interaction addressed here however, stimulated emission is less important than the other processes, under such conditions. 

It should be noted that the simple principles of the absorption and emission processes discussed above for the two-, three- and fom-level systems are based on optical photon transitions. However, non-radiative processes need to be considered as well. They can even be the dominant ones, as would be the case for electron impact excitation, pumping in a discharge, or for coUisional energy transfer between levels (e.g. as encountered in the HeNe gas laser or the Nd YAG solid-state laser). Regardless of the way the pumping and relaxation steps are implemented, the overall formalism remains more or less unchanged only the transition probability expressions have to be adapted. 

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