The interaction between light and an ensemble of scatterers can be significantly modified if the scatterers interact strongly with each other. For example, dipole-dipole interactions in ensembles of atoms, ions, plasmonic nanostructures, etc., can produce striking collective phenomena including modified decay rates (superradiance and subradiance), lineshifts, Fano resonances, and near-perfect optical reflection. A common interpretation of this collective behaviour involves considering the collective eigenmodes of the system, the eigenvalues of which determine the decay and resonance properties. However, I will present results from a recent experiment and numerical simulations which challenge this simple eigenmode picture of scattering. After initial pulsed excitation of a dense cloud of atoms, the ensuing fluorescent flash decays faster than the eigenmodes would predict (‘hyperradiance'), whilst (contrary to the typical linewidth-decay-rate correspondence) the linewidth experiences effectively no broadening.

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