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Physical Review A

Effects of spontaneously generated coherence on transient process in a Λ system

Weihua Xu, Jin‐Hui Wu, Jin-Yue Gao

Published Dec 26, 2002• 73 citations
Publisher

Abstract

Due to interaction with the vacuum of the radiation field, near-degenerate lower levels in a $\ensuremath{\Lambda}$ system have an additional coherence term, the spontaneously generated coherence term. In this paper, we investigate the effects of this kind of coherence on the transient process in the presence of a weak probe and a strong coherent field, with or without an incoherent pump. We find that the transient gain (absorption) properties can be dramatically affected by the spontaneously generated coherence. The transient gain can be greatly enhanced due to the effect of spontaneously generated coherence, and the transient absorption can be completely eliminated just by choosing the proper relative phase between the two fields.

DOI
10.1103/physreva.66.063812

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Due to interaction with the vacuum of the radiation field, near-degenerate lower levels in a \ensuremathΛ\ensuremath{\Lambda}\ensuremathΛ system have an additional coherence term, the spontaneously generated coherence term. In this paper, we investigate the effects of this kind of coherence on the transient process in the presence of a weak probe and a strong coherent field, with or without an incoherent pump. We find that the transient gain (absorption) properties can be dramatically affected by the spontaneously generated coherence. The transient gain can be greatly enhanced due to the effect of spontaneously generated coherence, and the transient absorption can be completely eliminated just by choosing the proper relative phase between the two fields.
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We investigate the effects of spontaneously generated coherence (SGC) on both the steady and transient gain properties in a four-level inverted-Y-type atomic system in the presence of a weak probe, two strong coherent fields, and an incoherent pump. For the steady process, we find that the inversionless gain mainly origins from SGC. In particular, we can modulate the inversionless gain by changing the relative phase between the two fields. Moreover, the amplitude of the gain peak can be enhanced and the additional gain peak can appear by changing the detuning of the coupling field. As for the transient process, the transient gain properties can also be dramatically affected by the SGC. Compared to the case without SGC, the transient gain can be greatly enhanced with completely eliminated transient absorption by choosing the proper relative phase between the two fields. And the inverted-Y-type system with SGC can be simulated in both atomic and semiconductor quantum well systems avoiding the conditions of SGC.
Phase Dependence of the Inversionless Gain in a Λ\LambdaΛ System with Near-Degenerate Levels
Near-degenerate lower levels in a $\ensuremath{\Lambda}$ system have an additional coherence term due to interaction with the vacuum of the radiation field. We report the effects of this spontaneously generated coherence on the formation of a trapped state in the presence of two coherent fields of arbitrary intensity. We show that such coherence preserves both electromagnetically induced transparency and coherent population trapping (CPT) phenomena. However, it changes the time scales associated with the formation of the CPT state, and brings about quantitative changes in the line profiles. We present a clear analytical explanation for our numerical results. We also report the dependence of line shapes on the relative phase between the two applied fields.
The effects of incoherent pumping on the phase control of amplification without inversion (AWI) are investigated in a three-level Λ system with a weak probe field. It is shown that, similar to spontaneously generated coherence, incoherent pumping is necessary to make the system become sensitive to the relative phase. We find that the region in which AWI can be exhibited is widened with the enhancement of incoherent pumping, and that the amplitude of the probe gain can be also changed where there exists an extremum of the amplitude for a given probe detuning.