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Journal of Optics B Quantum and Semiclassical Optics

Highly nonlinear atomic medium with steep and sign-reversible dispersion*

A. M. Akulshin, Alberto Cimmino, Andrei I. Sidorov, Russell McLean, Peter Hannaford

Published Aug 1, 2003• 27 citations
Publisher

Abstract

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.

DOI
10.1088/1464-4266/5/4/353

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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 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.
We show that ground-state Zeeman coherence prepared by two-photon Raman transitions in alkali atoms results in steep controllable and sign-reversible dispersion. Pulse propagation with small negative as well as positive group velocity of light $(\ensuremath{-}c/5100$ and c/41000)c/41000)c/41000) in a Cs vapor cell is reported. Energy exchange between copropagating light components through long-lived Zeeman coherence with enhanced absorption or transmission has been observed.
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.
Summary form only given. The propagation of light in a dispersive medium is characterized by the group velocity /spl nu//sub g/=c(n+/spl nu//sup dn//d/spl nu/)/sup .1/ where n is the refractive index and /spl nu/ the optical frequency. Depending on the sign and magnitude of /sup dn//d/spl nu/ in the medium, the group velocity can be positive, infinite or negative. The group velocity corresponds to the speed of the propagation of the maximum of an optical pulse travelling into the medium. This remains true whether the dispersion in the medium is normal (/sup dn//d/spl nu/>0) or anomalous (/sup dn//d/spl nu/<0). It has been demonstrated that coherently prepared atomic systems can exhibit steep dispersion leading to small absolute values of the group velocity (slow light). In recent work, we have demonstrated that narrow resonances arising from the coherent interaction of two optical fields with a closed atomic transition between two degenerate levels may correspond to a steep anomalous dispersion and thus to slow and negative group velocity. We report on the experimental study of the propagation of an amplitude modulated wave in a medium presenting negative group velocity.
We have observed the propagation of an approximately 35 ns long light pulse with a negative group velocity through a laser-cooled 85Rb atomic medium. The anomalous dispersion results from linear atom–light interaction and is unrelated to long-lived ground-state coherences often associated with fast light in atomic media. The observed negative group velocity (−c/360) in the Rb magneto-optical trap for a pulse attenuated by less than 50% is in good agreement with the value of dispersion measured independently by a rf heterodyne method. The spectral region of anomalous dispersion is between 15 and 40 MHz, which is an order of magnitude wider than that typically associated with ground-state coherences.
Summary form only given. The field of atom manipulation using laser light has experienced a rapid growth in the recent years. By the interaction of the atoms with laser light it is possible to design media with desired optical properties such as transparent media showing negative or positive dispersion. Negative dispersive transparent media can be used e.g. to realize an optical cavity with large build up but broadband response (white light cavity) or to demonstrate superluminal light propagation. It has also been demonstrated that positive dispersive media can exhibit steep dispersion leading to small absolute values of the group velocity (slow light, see e.g.) or reducing e.g. the line width of a cavity relative to the line width of the empty resonator.We report on recent experiments on investigation the absorptive and dispersive properties of a coherent prepared media, that shows electromagnetically induced transparency on extremely narrow dark resonances. We have carried out the experimental study of a two-photon excitation via quantum interference in a three level Cs atomic system. The experiments were done on a Cs atomic beam.