Theory of lasers without inversion

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Abstract
We here present the theory of lasing without inversion for systems with a split upper laser level. We show that the effect of amplification without inversion may be realized for at least two different situations. In the first situation, the effect is due to the asymmetry of radiative characteristics of quasidegenerate transitions. In the second situation, the effect can be provided without any asymmetry and is achievable for the definite phase of the transfer of coherence between the quasidegenerate levels (quasiparametric amplification). We also show that for laser amplification without inversion, the rate (frequency) of mixing of the quasidegenerate levels should be significantly higher than the linewidths of these levels. This mixing of quasidegenerate levels can be realized by low-frequency electromagnetic radiation or by interaction with an auxiliary third level. In addition, we derive the conditions for providing laser action for such systems rigorously, and we give some specific examples for the realization of this effect in gaseous and solid-state media. One of the most promising configurations may be realized in solid-state media when two narrow quasidegenerate levels are embedded in quasifree states (low-density state conduction band). We also discuss the use of a parametric mechanism for the creation of spatial gratings and for the construction of new types of electro-optical devices.

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