May 29 2014
A population inversion is a condition of the laser medium, where a system consisting of a group of molecules or atoms exists in a higher excited state, with more quantum systems than that of the lower energy state of a laser transition.
In general, an optical gain in the laser transition occurs due to the stimulated emission between the two energy levels – the upper and lower laser level of the gain medium.
The cross sections of a laser transition for stimulated emission and absorption will be the same at any wavelength, while a positive net gain is obtained when the population, i.e. laser-active atoms/ions of the upper energy state, is greater than that of the lower state.
Population inversion is not applicable in the processes of Raman scattering and lasing without inversion. Pi-pulse inversion and three-level inversion are two ways of creating population inversion in a system.
In pi-pulse inversion, intense bombardment of coherent laser radiation triggers the atomic dipoles in phase, using an electromagnetic field when the population is in the ground state. As a result, the population oscillates between the excited and ground state. If the propagation of laser pulse is terminated after the population completes half of a Bloch oscillation, a population inversion will be produced.
Three-level inversion is the process in which the presence of a third metastable state below the upper level creates the population inversion between the third and ground state.
Population inversion can be maintained in the system by providing a certain minimum power as a pump, to deviate the state of laser-active molecules from thermodynamic equilibrium. This is due to the fact that, in thermal equilibrium, positive net gain can never be achieved, and the population of a lower energy level is always higher.
Deviation from thermal equilibrium can be further accomplished by modulating the relaxation rates of different quantum processes occurring at various quantum levels. Selective populating of one energy level with respect to another level can be performed in different ways, such as rapid three body recombination, photon bombardment, or electron collision.
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