Probing colored noise from the index of refraction of strongly driven two-level atoms

We analyze the effects of a reservoir having a generic spectrum (colored no ise) on the index of refraction of a system of strongly driven two-level at oms (gas) probed by a weak held. We show that for high Rabi frequencies, Om ega, a simple analytic expression results for the susceptibility function ( with respect to the probe) when nu approximate to Omega, where nu is the de tuning between driving and probe fields frequencies. Several distinct featu res are revealed in that function when compared to the one resulting from a reservoir with a white-noise spectrum (atom-vacuum coupling). We also show that the Mach-Zehnder interferometry could permit distinguishing between t he two kinds of spectra and we point out the sensitive dependence of the in dex of refraction of the atomic gas on Omega and find a signature for the n on-Markovian evolution. [S1050-2947(99)01811-9].