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| Lasers have become powerful technological tools today to do precision science and engineering. From our point of view, the interest lies in understanding them precisely through “molecular science”, i.e., energy levels, transitions between them, populating the higher levels in a non-equilibrium distribution and finally getting “laser action” by stimulating the excitations “stored” in higher occupied levels to fall down to lower levels, with the accompanying laser light. The acronym laser expands as light amplification by stimulated emission of radiation. The principles involved in the action of a laser are outlined below. The first difference between normal emission and laser is that in a Laser, there is a stimulated emission. Most excited states are unstable and have a short life span. The excited states, on their own (spontaneously), decay to the ground or lower states. It is these spontaneous processes which are responsible for “equilibrium” or the Boltzmann distribution of energies. However, some excited states are metastable, i.e., they can exist for a sufficiently long time so that they can be populated a lot more than what is permitted by thermal equilibrium. This corresponds to a "population inversion". |
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If the equilibrium population of states 3, 2 and 1 are, say 103, 107 and 1015 a non–equilibrium or inverted population of these states is like 103 1014 and 0.9*1014. In this example, the second state is very highly populated. In the three level laser depicted below the ground state G is excited to the state E by pumping the energy (absorbing radiation) into the system. If there is a metastable state M very near the level E, then considerable energy from E is transferred to M and M gets populated. M is said to be a laser state, because by stimulating M with a photon of frequency  , laser action can be accomplished. |
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