Laser action relies on a non-Boltzmann population inversion formed by the absorption of radiation and vibrational deactivation that forms a long lived excited electronic state.
An excited state molecule can move to a lower energy state or return to the ground state by the two different types of radioactive process.
Spontaneous emission produces the fluorescence and phosphorescence treated in preceding sections. Now let us consider the important induced emission device known as the laser amplification by stimulated emission of radiation. The equilibrium population of the higher energy state m is always less than that of the lower energy state l, except in the limit of infinite temperature, where the populations become equal. Equalization of the populations can also be brought about by high radiation densities. Then the spontaneous emission term in equation is neglible and the equilibrium established is described by:
Nl Blm p (Vlm) = Nm Bml p (Vlm)
Since Blm = Bml, the equilibrium produced when induced transitions overwhelm other effects is such that:
Nl = Nm
In such a system a great deal of energy might be stored in the high energy m-state molecules. The radiation that establishes this population is continually inducing m-state molecules to emit radiation and return to the lower energy l state, for this equilibrium system we obtain no release of radiant energy and move molecules from state l to state m.
Laser action depends on a population of the higher energy state that is greater than that of the lower energy state. Under such circumstances, instead, or stimulated, emission can release more radiant energy than is stored by the concurrent included absorption process. Thus a population inversion the opposite of that for an equilibrium system at infinite temperatures or radiation densities, must be produced if laser action is to occur. Let us observe how this can be done.
Three types of energy can be delivered to suitable systems so that a population inversion is produced, namely, radiation energy, collision energy, and chemical energy.
The preceding section on phosphorescence suggests how radiation might establish a population inversion. An indirect approach must circumvent the equality of the coefficients for induced absorption and emission. Two general arrangements of energy levels, known as a three level laser system, are illustrated. In the former, the lower energy state is the ground state, and a large amount of optical pumping is necessary to produce a population inversion between the two excited states that can produce laser action is more easily attained. The first practical laser device, the ruby laser, corresponds, however, to the two level systems.
The second excitation procedure, which applies primarily to gas lasers, depends on collisions. The primary energetic particles are usually electrons produced by an electric discharge. They collide with the molecules which will produce laser action or intermediary.
In some cases the excited state products o not themselves undergo laser action but excite a species that does. Thus the population inversion necessary for CO2 laser action can be produced by allowing the products of chemical reactions to interact with the CO2 molecules and excite them.
Thus, by various means, population inversions can be produced. Suppose that in such a system a photon enters along a laser tube. Emission in excess of absorption will be stimulated. This additional emission, moreover, will be in phase and in the same direction as the light that stimulated the emission. The augmented light beam will induce additional emissions, all with the same phase and in the same direction as the original beam. It follows that a beam of radiation described as coherent will be produced as transitions from that a beam of radiation described as coherent will be produced as transitions from the highly populated high energy state to a lower state are induced. Numerous optical devices, e.g. reflecting mirrors and pulse arrangements, can be used to enhance the intensity of the laser beam without affecting the chief characteristics of the beam, its coherence and its directionality.