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Journal of Optics B Quantum and Semiclassical Optics

Enhanced atomic Kerr nonlinearity inbrightcoherent states

A. M. Akulshin, A I Sidorov, Russell McLean, Peter Hannaford

Published Oct 16, 2004• 26 citations
Publisher

Abstract

We show that long-lived bright Zeeman coherence that is responsible for absorption enhancement and steep anomalous dispersion in an atomic medium gives rise to a giant nonlinearity of the atomic susceptibility, in a similar manner to the dark coherence responsible for the well-known electromagnetically induced transparency. The intensity dependent refractive index of Cs vapour with a ground state Zeeman coherence in the vicinity of the D2 resonance line has been measured at low light intensity. The Kerr nonlinearity in the bright coherent state (n2 approx 1.0 × 10-6 cm2 mW-1) is higher than that in a dark state under the same experimental conditions. Higher order nonlinear components of the refractive index have also been estimated for both coherent states.

DOI
10.1088/1464-4266/6/12/002

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Mar 2, 2007•1 citation
We show that long-lived bright Zeeman coherence that is responsible for absorption enhancement and steep anomalous dispersion in an atomic medium gives rise to a giant nonlinearity of the atomic susceptibility, in a similar manner to the dark coherence responsible for the well-known electromagnetically induced transparency. The intensity dependent refractive index of Cs vapour with a ground state Zeeman coherence in the vicinity of the D2 resonance line has been measured at low light intensity. The Kerr nonlinearity in the bright coherent state (n2 approx 1.0 × 10-6 cm2 mW-1) is higher than that in a dark state under the same experimental conditions. Higher order nonlinear components of the refractive index have also been estimated for both coherent states.
Experimental investigation of the optical properties of atomic media with bright and dark long-lived light-induced Zeeman coherence is reported. We have obtained a small negative value for the group velocity of light pulses in Cs vapour ( V g ≃− c /6000), which agrees with direct anomalous dispersion measurements. The intensity dependent components of the refractive index of an atomic vapour with long-lived Zeeman coherence has been estimated. We have shown that such coherence can significantly enhance four-wave mixing, leading to the appearance of a multi-frequency comb-like spectrum at relatively low light intensity.
We present the results of experimental studies of the propagation of light pulses through a specially prepared atomic medium with steep anomalous dispersion and enhanced Kerr nonlinearity. The observed negative group velocity is consistent with the principle of causality. The long-lived bright Zeeman coherence that is responsible for the steep anomalous dispersion gives rise also to a giant nonlinearity of the atomic susceptibility, which contributes substantially to the pulse propagation process. The enhanced atomic Kerr nonlinearity in the bright coherent state has been estimated.
The Kerr nonlinear index of refraction for rubidium atoms is measured by using an optical ring cavity with and without electromagnetically induced transparency (EIT). Significant enhancement and inhibition of the Kerr nonlinear index is observed near resonance with EIT. The nonlinear index of refraction is measured as functions of probe and coupling frequency detunings, respectively, with and without the presence of EIT. A simple theoretical calculation including Doppler broadening is presented and is found to be in good qualitative agreement with the experimentally measured results.
We report an experimental observation of large Kerr nonlinearity with vanishing linear susceptibilities in coherently prepared four-level rubidium atoms. Quantum coherence and interference manifested by electromagnetically induced transparency suppress the linear susceptibilities and greatly enhance the nonlinear susceptibilities at low light intensities. The measured Kerr nonlinearity is comparable in magnitude to the linear dispersion in a simple two-level system and is several orders of magnitude greater than the Kerr nonlinearity of a conventional three-level scheme under similar conditions.
Spontaneous transfer of coherence has proven to be a good interpretative scheme for the explantation of the spectra of a probe laser probing a closed degenerate two-level systems. With its aid it was possible to give an explanation for the emergence of electromagnetically induced absorption and also to make predictions on the associated coupling laser spectra. Here we extend our work on the role of the coupling laser in electromagnetically induced absorption presenting new measurements of the probe and coupling laser absorption and dispersion spectra - taken in the D2 line of caesium - which reassert the importance of spontaneous transfer of coherence in the generation of electromagnetically induced absorption spectra. All measurements were performed with linearly polarised coupling and probe laser of orthogonal polarisation, acting on a perpendicularly propagating caesium atomic beam to minimise the Doppler broadening of the lines. For sake of completeness we compared the electromagnetically induced absorption spectra with electromagnetically induced transparency spectra obtained in another two-level system within the same line. The measured probe spectra were used to calculate the refractive index of caesium in the presence of electromagnetically induced transparency and absorption. On that basis we could calculate the effect of Kerr nonlinearities and measure nonlinear Kerr coefficients of the order of n2 ≈ 10-5 cm2/mW with absorption coefficients of the order of &agr; ≈ 0.1 cm-1.