The below figure shows schematically how a population inversion can be achieved so that laser action-or lasing us it is called can occur. Atoms from the ground state E1 are "pumped" up to an excited state E3, for example by the absorption of light energy from an intense continuous spectrum source that surrounds the lasing material.
The basic three - level scheme for laser operation.
Metastable state E2 has a greater population than the ground state E1
From E3, the atoms decay rapidly to a state of energy E2. For lasing to occur, this state must be metastable; that is, it must have a relatively long mean life against decay by spontaneous emission. If conditions are right, state E2 can then become more heavily populated than state E1, thus providing the needed population inversion. A stray photon of the right energy can then trigger an avalanche of stimulate emission events, resulting in the production of laser light. A number of lasers using crystalline solids (such as ruby) as a lasing material operate in this three-level mode.
