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Physical Review A

Inversionless amplification in a three-level medium

Olga A. Kocharovskaya, Paul Mandel, Yevgeny V. Radeonychev

Published Feb 1, 1992• 115 citations
Publisher

Abstract

We study all possible resonant three-level configurations where a low-frequency coherent pump can be converted into a high-frequency field in the absence of population inversion at both atomic high-frequency transitions. We identify two different mechanisms that provide such inversionless amplification.
DOI
10.1103/physreva.45.1997

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We consider the problem of amplification without inversion in a three-level atomic system. When atoms are driven by a field reservoir and a low-frequency coherent field, amplification of a high-frequency probe field may occur even when there is no population inversion on any atomic transition. We prove that, depending on the ratio of the probe and coherent drive field frequencies, which we term the frequency up-conversion factor, this process may or may not require hidden inversion in the field reservoir. In the special case of a frequency up-conversion factor less than 2, we demonstrate that amplification is possible even if the field reservoir is in complete thermal equilibrium: this represents a new kind of coherent radiation source, distinct from both conventional lasers and lasers without inversion.
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.
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.
We consider a three-level medium with low-frequency separation between the two upper levels. The medium interacts with an external field that induces a coherence between the two upper levels. We analyze the amplification condition for the probe monochromatic optical field in a general nonresonant case. We show that amplification without population inversion arises either at line center or at the sidebands, depending on the sign of the population difference between the upper sublevels. This amplification occurs in a limited range of the coherent pump intensity.