  The collective absorption and emission of light by an ensemble of atoms is at
the heart of many fundamental quantum optical effects and the basis for
numerous applications. However, beyond weak excitation, both experiment and
theory become increasingly challenging. Here, we explore the regimes from weak
excitation to inversion with ensembles of up to one thousand atoms that are
trapped and optically interfaced using the evanescent field surrounding an
optical nanofiber. We realize strong inversion, with about 80% of the atoms
being excited, and study their subsequent radiative decay into the guided
modes. The data is very well described by a simple model that assumes a
cascaded interaction of the guided light with the atoms. Our results contribute
to the fundamental understanding of the collective interaction of light and
matter and are relevant for applications ranging from quantum memories to
sources of nonclassical light to optical frequency standards.
