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

Superfluorescence without inversion

Viktor A. Malyshev, Igor Viktorovich Ryzhov, Evgeni D. Trifonov, А. И. Зайцев

Published Sep 26, 1997• 8 citations
Publisher

Abstract

In three-level systems one can realize amplification of radiation without inversion by creating a coherent superposition of lower states. This concept applied to the superradiant emission may allow us to get inversionless superfluorescence. Such a possibility is demonstrated for (Lambda) -scheme of the operating transitions. We discuss the problem how to create the coherence of lower states and the effect of the lower level splitting on superfluorescence.

DOI
10.1117/12.287668

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In three-level systems one can realize amplification of radiation without inversion by creating a coherent superposition of lower states. This concept applied to the superradiant emission may allow us to get inversionless superfluorescence. Such a possibility is demonstrated for (Lambda) -scheme of the operating transitions. We discuss the problem how to create the coherence of lower states and the effect of the lower level splitting on superfluorescence.
We study the propagation of a bichromatic field in three-level and four-level media. In the case of a three-level medium, we consider the \ensuremath{\Lambda} configuration and demonstrate that if the upper level is not empty there is a critical value for the low-frequency coherence such that below this threshold and for intense fields, coherent bleaching occurs. This implies a sharp reduction of the field absorption and the fluorescence emission whereas above this threshold a parametric instability takes place. It corresponds to an amplification of the field that does not need a population inversion between the optical transitions. Finally we prove that the field alone cannot create a low-frequency coherence exceeding this threshold. Therefore we analyze the double-\ensuremath{\Lambda} configuration involving a four-level medium in which the upper and lower pairs of atomic levels have a low-frequency separation while optical transitions take place between the two pairs of states. An intense bichromatic field that is resonant with one upper state and the lower levels and a weak bichromatic field resonant with the other upper state and the same two lower states are sent into the medium. We analyze the limit in which the low-frequency coherence created between the lower pair of states has a relaxation time which is large. Then there will be population trapping in the lower states leading to coherent bleaching if there is no inversion between the upper levels but there will be amplification if there is inversion between the upper levels. In this way amplification at the optical transitions without population inversion between these states is realized.
A superfluorescence effect in a V-type medium with initial Raman inversion, driven by a strong coherent field on one of the transitions, is studied analytically and numerically. It is shown that the collective spontaneous emission evolves in the presence of a minor population of an upper state of a superfluorescent transition. In the limit of the Rabi frequency of a driving field large compared to that of a superfluorescent pulse, the system of two equations for the fields and density-matrix equations for three states of the medium is reduced to Maxwell-Bloch equations for an effective two-level system. The shape of a superfluorescent pulse is perfectly described in terms of a slowly varying envelope function (which is a solution of the Maxwell-Bloch equations), filled with fast oscillations associated with Rabi floppings on the driving transition. In the average, i.e., after integrating over a period of Rabi oscillations, the upper superfluorescent state is populated less than other states, such that the phenomenon may be called ``superfluorescence without inversion'' by analogy with lasing without inversion.