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Physical Review A

Absorption mechanism in a four-level system

Weihua Xu, Jin-Yue Gao

Published Mar 28, 2003• 18 citations
Publisher

Abstract

We investigate the absorption mechanism in a four-level system proposed by Harris and Yamamoto [Phys. Rev. Lett. 81, 3611 (1998)] by using the quantum-jump approach. We show that the one-photon and three-photon absorption processes are both responsible for the whole absorption at the line center of the probe field. In particular, we show the important role of the one-photon absorption process.

DOI
10.1103/physreva.67.033816

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We investigate the absorption mechanism in a four-level system proposed by Harris and Yamamoto [Phys. Rev. Lett. 81, 3611 (1998)] by using the quantum-jump approach. We show that the one-photon and three-photon absorption processes are both responsible for the whole absorption at the line center of the probe field. In particular, we show the important role of the one-photon absorption process.
We investigate the amplification mechanism in a four-level system by both the density-matrix and quantum-jump approaches. We show that the asymmetry between three-photon stimulated emission and one-photon absorption process is responsible for the inversionless amplification of the probe field. We also investigate the effects of spontaneously generated coherence (SGC) on the probe gain and find that due to the SGC, the probe gain can be greatly enhanced and can be modulated by changing the relative phase between the applied fields.
We report an experimental demonstration of photon switching by quantum interference in a four-level atomic system proposed by Harris and Yamamoto [Phys. Rev. Lett. 81, 3611 (1998)]. Quantum interference inhibits single-photon absorption but enhances third-order, two-photon-type absorption in the four-level system. We have observed greatly enhanced nonlinear absorption in the four-level system realized with cold 87Rb{}^{87}\mathrm{Rb}87Rb atoms and demonstrated fast switching of the nonlinear absorption with a pulsed pump laser.
Pump–probe spectroscopy of a four-level system (FLS) consisting of a Λ system with an additional close lying excited level is investigated. The level configuration gives rise to two simultaneously excited Λ resonances driven by the same pump and probe laser beams. Two such four-level configurations, 5s1/2F=1,2→5p1/2F′=1,2 (FLS1) and 5s1/2F=1,2→5p3/2F′=1,2 (FLS2) respectively in the D1 and D2 transitions of 87Rb, are used as model systems to study theoretically the probe absorption spectrum. It is observed that the signature of the quantum interference is dependent on the nature of the FLS; FLS1 exhibits probe amplification as against probe absorption in FLS2. This contrasting behaviour is further corroborated by analytical results, which highlight the importance of transition dipole moments, atom–field interaction parameters and incoherent decays associated with the FLS. The discussion is further augmented using quantum jump formalism, which provides useful insight into the underlying mechanism responsible for probe amplification versus absorption.
Summary form only given. It has been shown that two-photon absorption can be controlled by quantum interference in the multiple excitation paths created by a coupling field. Recently, Harris and Yamamoto (1998) described a four-level system that utilizes electromagnetically induced transparency (EIT) to switch on and off the two-photon excitation. Because the cancellation of the one-photon excitation, the enhanced two-photon excitation may be observable under very low light intensities. We have carried out an experimental study of two-photon excitation via quantum interference in multi-level Rb atomic systems based on the D/sub 1/ and D/sub 2/ transitions. The Rb systems can be viewed as quasi-four level systems. The experiments were done in an /sup 87/Rb isotope vapor cell.
In this paper, we analyze and discuss the absorption properties of the probe beam in a four-level atomic system with a nearly hyper-fine doublet structure of two higher-lying excited levels based on electromagnetically induced transparency (EIT) for the two cases of transient process and steady-state process. The main gain of this work is to investigate theoretically the influence of the nearly hyper-fine levels on the probe absorption. For the transient process,using the numerical calculations by a simple Mathematica code we find that the magnitude of the probe absorption at the line center is small compared to the typical three-level atomic system in the context of electromagnetically induced transparency. For the case of the steady state, our results show that the probe absorption can be completely eliminated at the line center of the probe transition just as the usual three-level EIT scheme.
In this paper, we analyze and discuss the absorption properties of the probe beam in a four-level atomic system with a nearly hyper-fine doublet structure of two higher-lying excited levels based on electromagnetically induced transparency (EIT) for the two cases of transient process and steady-state process. The main gain of this work is to investigate theoretically the influence of the nearly hyper-fine levels on the probe absorption. For the transient process, using the numerical calculations by a simple Mathematica code we find that the magnitude of the probe absorption at the line center is small compared to the typical three-level atomic system in the context of electromagnetically induced transparency. For the case of the steady state, our results show that the probe absorption can be completely eliminated at the line center of the probe transition just as the usual three-level EIT scheme.