  The interaction between light and cold atoms is a complex phenomenon
potentially featuring many-body resonant dipole interactions. A major obstacle
toward exploring these quantum resources of the system is macroscopic light
propagation effects, which not only limit the available time for the
microscopic correlations to locally build up, but also create a directional,
superradiant emission background whose variations can overwhelm the microscopic
effects. In this Letter, we demonstrate a method to perform ``background-free''
detection of the microscopic optical dynamics in a laser-cooled atomic
ensemble. This is made possible by transiently suppressing the macroscopic
optical propagation over a substantial time, before a recall of superradiance
that imprints the effect of the accumulated microscopic dynamics into an
efficiently detectable outgoing field. We apply this technique to unveil and
precisely characterize a density-dependent, microscopic dipolar dephasing
effect that generally limits the lifetime of optical spin-wave order in
ensemble-based atom-light interfaces.
