  The collective interaction via the environmental vacuum is investigated for a
mixture of two deviating multi-atom ensembles in a moderately intense laser
field. Due to the numerous inter-atomic couplings, the laser-dressed system may
react sensitively and rapidly with respect to changes in the atomic and laser
parameters. We show for weak probe fields that in the absence of absorption
both the index of refraction and the group velocity may be modified strongly
and rapidly due to the collectivity.
