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Optics Express

Steep atomic dispersion induced by velocity-selective optical pumping

Alexander Akoulchin, Mandip Singh, Andrei I. Sidorov, Peter Hannaford

Published Sep 16, 2008• 8 citations
Publisher

Abstract

We demonstrate a method of preparation of broadband sign-reversible dispersion in alkali vapour based on velocity-selective optical pumping. The refractive index in Rb vapour has been measured using a heterodyne method. The magnitudes of the normal and anomalous dispersion, which are almost constant over a spectral region of approximately 40 MHz, can lead to a reduced (V(g) approximately c/230) or negative (V(g) approximately -c/27) group velocity of light.

DOI
10.1364/oe.16.015463

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arXiv:quant-ph/0512260•Dec 30, 2005•1 citation
We demonstrate a method of preparation of broadband sign-reversible dispersion in alkali vapour based on velocity-selective optical pumping. The refractive index in Rb vapour has been measured using a heterodyne method. The magnitudes of the normal and anomalous dispersion, which are almost constant over a spectral region of approximately 40 MHz, can lead to a reduced (V(g) approximately c/230) or negative (V(g) approximately -c/27) group velocity of light.
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.
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.
Steep dispersion of opposite signs in driven degenerate two-level atomic transitions have been predicted and observed on the D2{D}_{2}D2​ line of 87Rb{}^{87}\mathrm{Rb}87Rb in an optically thin vapor cell. The intensity dependence of the anomalous dispersion has been studied. The maximum observed value of anomalous dispersion ( $\mathrm{dn}/d\ensuremath{\nu}\ensuremath{\simeq}\ensuremath{-}6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}{\mathrm{Hz}}^{\ensuremath{-}1}$) corresponds to a negative group velocity ${V}_{g}\ensuremath{\simeq}\ensuremath{-}c/23000$.
We have recently proposed [9], the use of fast-light media to obtain ultrahigh precision rotation sensing capabilities. The scheme relies on producing a critically anomalous dispersion, in a suitable dispersive medium, which is introduced in the arms of a Sagnac interferometer. We present here an experimental investigation of the anomalous dispersion properties of bi-frequency Raman gain in rubidium vapor, with the goal of using this medium for producing the critically anomalous dispersion condition. A heterodyne phase measurement technique is used to measure accurately the index variation associated with the dispersion. The slope of the negative linear dispersion (or group index) is experimentally varied by more than two orders of magnitude while changing the frequency separation between pump fields, responsible for producing gain. Using this result, we have identified the experimental parameters for achieving a null value of the group index, corresponding to the critically anomalous dispersion condition necessary for enhanced rotational sensitivity.