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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Amplification without population inversion

Olga A. Kocharovskaya, Paul Mandel, Yakov I. Khanin

Published Dec 16, 1992• 2 citations
Publisher

Abstract

Three- and four-level schemes of inversionless amplification with different types of coherent pumping is reviewed. Amplification without inversion between the 'bare' states is a result of population inversion between eigenstates of the density matrix. In particular, it can be achieved due to the coherent population trapping, i.e., effective removal from the interaction with the field a fraction of atoms at lower levels.

DOI
10.1117/12.131820

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Three- and four-level schemes of inversionless amplification with different types of coherent pumping is reviewed. Amplification without inversion between the 'bare' states is a result of population inversion between eigenstates of the density matrix. In particular, it can be achieved due to the coherent population trapping, i.e., effective removal from the interaction with the field a fraction of atoms at lower levels.
Two essentially different mechanisms are known for inversionless amplification. The first is based on coherent population trapping, while the second is connected with the existence of quantum interference effects among dressed states of the coupled atom-field system. Previous investigations of four-level atoms with Raman pumping have emphasized the existence of noninversion lasing by way of the first mechanisms and have interpreted the gain as a result of a population inversion between dressed states. In this case the quantum interference generally hinders the amplification process. By extending this analysis to a broader range of parameters we find that the second mechanism can also become effective. The purpose of this presentation is to show that four-level atoms with a Raman pump can generally support both amplification mechanisms and to define the conditions under which either process may dominate.
When an atomic system interacts with light, it can amplify the electromagnetic radiation when gain exceeds loss. This requires that transitions from the upper level to the lower level prevail over the transitions from the lower level to the upper level. If one deals with a two-level system, the only way to obtain amplification is to create a state of population inversion, i.e. the upper level must be more densely populated that the lower level. "Lasing without inversion" may be possible when part of the atoms are trapped, which removes them from the subspace of states with which the upper level can have transitions. This chapter determines to what extent population inversion is necessary to achieve amplification. Independently, amplification without inversion and lasing without inversion were predicted by Harris using a different type of atomic interference. The amplification threshold has a number of features that are reminiscent of the laser threshold.
We analyze the physical origin of the inversionless amplification in four-level atoms driven by a Raman field and identify the existence of two different mechanisms for the predicted gain. The first mechanism is connected with the appearance of a population inversion between the atomic states dressed by the coherent pump, while the second is linked to the buildup of coherence among these states. Proper selection of the relaxation parameters can make either the first or the second mechanism dominant over the other although usually both coexist.
In a two-level medium, the condition for amplification is population inversion. The simplest scheme in which inversionless amplification occurs is a three-level medium. It was shown recently that a trichromatic field can be amplified by a three-level medium without population inversion if the low-frequency (LF) coherence exceeds some critical value.1 The question that we address here is how to generalize the two-level amplification condition when one or both levels are split into a number of sublevels.