Super- and subradiance in dilute disordered cold atomic samples: observations and interpretations

When a photon is sent onto an atomic ensemble, it interacts collectively with the $N$ atoms of the sample and not simply with one of them. This results in measurable modifications in the scattering rate, the emission diagram or the temporal dynamics. Among these collective effects, here, we study in detail the temporal dynamics of the decay of the scattered light after switching off the driving laser. Right after the switch-off, the decay can be faster than for a single atom, whereas at later time, the decay becomes slower. We refer to this behavior as superradiance and subradiance, respectively. We present in this Chapter our investigations of super- and subradiance in disordered cold atomic ensembles, mainly with low-density samples (dilute regime) and a weak excitation (linear-optics regime), but we also studied the first corrections beyond these limiting cases. We emphasize the different possible interpretations of these phenomena, with in particular the presentation of an optical description of these effects, which sheds a new light on cooperative scattering in disordered samples and provides a more intuitive understanding of the physical mechanisms at play.