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arXiv:quant-ph/9908055 · Physical Review A

Gain components in the Autler-Townes doublet from quantum interferences in decay channels

Sunish Menon, G. S. Agarwal

Published Dec 13, 1999• 132 citations
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Abstract

We consider nondegenerate pump-probe spectroscopy of $V$ systems under conditions such that interference among decay channels is important. We demonstrate how this interference can result in new gain features instead of the usual absorption features. We relate this gain to the existence of a new vacuum induced quasi-trapped-state. We further show how this also results in a large refractive index with low absorption.

DOI
10.1103/physreva.61.013807

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Physical Review A•Feb 3, 2004•14 citations
We consider nondegenerate pump-probe spectroscopy of VVV systems under conditions such that interference among decay channels is important. We demonstrate how this interference can result in new gain features instead of the usual absorption features. We relate this gain to the existence of a new vacuum induced quasi-trapped-state. We further show how this also results in a large refractive index with low absorption.
In a couple of papers [Phys. Rev. A, 63, 033818 (2001); 63, 023810 (2001)], P. Zhou, S. Swain, and L. You studied the cavity-induced modifications to the Autler-Townes spectrum of three-level atoms due to the phenomenon of quantum interference. They demonstrated that probe gain can occur in either of the Autler-Townes doublet due to the cavity-induced interference among spontaneous decay channels. In this Comment, we show that this conclusion is incorrect and it contradicts the thermodynamic equilibrium conditions of the atomic system in steady state.
We have analyzed the properties of weak probe absorption in a closed four-level Y -type system driven by a coherent driving field. The system with two nearly degenerate uppermost levels interacting with the same modes of the vacuum radiation field exhibits the interference effect between the spontaneous decay channels. The phenomenon of this quantum interference can significantly change the absorptive response of the weak probe field. On varying the amplitude of the resonant driving field, in the absence of incoherent pumping in the upper transitions the system can display probe transparency as well as enhanced absorption peak under the condition of resonant detuning of the probe field. With the application of incoherent pumping to the system the phenomenon of symmetric light amplification with two distinct gain peaks one each at the positions of Autler–Townes resonances can be achieved in the presence of the decay-interference effect. We have also shown that the unidirectional incoherent pumping mode is necessary for the evolution of probe gain.
We examine the propagation of a weak probe light through a coherently driven $\mathsf{Y}\mathsf{\text{\ensuremath{-}}}\mathsf{type}$ system. Under the condition that the excited atomic levels decay via the same vacuum modes, the effects of quantum interference in decay channels are considered. It is found that the interference in decay channels results in a lossless anomalous dispersion between two gain peaks. We demonstrate that the probe pulse propagation can, in principle, be switched from subluminal to superluminal due to the decay-induced interference. We also show that the system exhibits a high index of refraction with negligible absorption for the driving fields. A dressed-state picture of the atom-light interaction is described to explain the numerical results.
We study the Autler-Townes spectrum of a V-type atom coupled to a single-mode, frequency-tunable cavity field at finite temperature, with a pre-selected polarization in the bad cavity limit, and show that, when the mean number of thermal photons $N\ensuremath{\gg}1$ and the excited sublevel splitting is very large (of the same order as the cavity linewidth), probe gain may occur at either sideband of the doublet, depending on the cavity frequency, due to cavity-induced quantum interference. It is also possible to choose the cavity detuning so that one of the Autler-Townes sidebands vanishes.
We discuss the effects of the incoherent pumping in both probe and coupling channels on the absorption and dispersion of the probe laser with quantum interference. The incoherent population pumping in the coupling channel decreases the probe transparency. When decay rate in the coupling channel is greater than that in the probe channel, the incoherent population pumping in the probe channel can create the gain without inversion. The probe absorption pattern is upsidedown if the interference is present. When the decay rates in the probe and channels are equal, the incoherent pumping in the probe channel doesn't create the gain until the existence of the interference.
In this paper we analyze the steady-state populations and gain lineshape of a V-type three-level atom with a closely spaced excited doublet. The atom is driven by a strong coherent field, a weak probe, and a single broadband squeezed vacuum. We focus our attention in the interplay between the quantum interference and the squeezed field on the probe gain. It is shown that the relative phases between the two coherent fields and the squeezed field play an important role in the optical properties of the atom. Specifically, we find that the probe can experience gain without population inversion for proper values of the parameters characterizing the squeezed field and in the absence of incoherent pumping. The system can be tailored to exhibit multiple dispersion regimes accompanied by negligible gain or absorption over a large bandwidth, a desirable feature for obtaining propagation of pulses with negligible distortion.