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].