The stimulated emission alone cannot lead to the amplification of light; else, the laser would have come into existence more than one and half a century ago along with the development of fluorescent light sources based on a cylindrical geometry. Although it provides countless excited atoms, spread along the length of the tube, for their stimulation by spontaneously emitted resonant photons, amplification of light fails to happen. This is in spite of the fact that the avalanche of electrons produced in the electric discharge results in the creation of many excited atoms capable of giving out enough spontaneously emitted photons to break the darkness of an entire room! The crucial question here is: how many? Unless the number of excited atoms in the discharge tube outnumbers the unexcited atoms, a condition identified as population inversion in the common parlance, amplification of light is not possible. In addition to providing deeper insight into the concept of population inversion, this chapter has also been planned to enlighten readers on the inherent difficulties in its realization and some practical ways to overcome them. A significant part of the chapter is also devoted to identifying the medium most suitable for effecting and sustaining the condition of population inversion and, in turn, lasing.

1. 1.

   The photon here is a resonant photon as its energy matches exactly the difference of energy between the excited and ground states.

2. 2.

   A metastable state is an excited energy level of a microscopic particle like an atom/molecule that has a lifetime much longer than that of typical excited states.

Authors and Affiliations

1. Former Head, Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Mumbai, India

   Dhruba J. Biswas

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