Weak superradiance in arrays of plasmonic nanoantennas

A collection of N emitters can exhibit an N-fold broadening of the radiative linewidth resulting from the development of a macroscopic dipole moment. Such a broadening of the radiative linewidth has previously been observed in systems of several nanoparticles and has often been described in terms of superradiant behavior. However, the understanding of the physics behind the observed dependence of radiative linewidth on the number of irradiated nanoparticles is far from complete. In this paper, we present theoretical and experimental results that elucidate this broadening mechanism in plasmonic systems and draw a connection with the phenomenon of Dicke superradiance. We demonstrate that, in the limit where radiative broadening dominates, the extinction linewidth of a planar array of plasmonic nanoantennas scales linearly with the number of nanoantennas contained within a circle of radius equal to the resonant optical wavelength. We explain this classical phenomenon as a weak superradiance effect, which corresponds to the case in the Dicke model where only the ground state and the first collective excited state contribute to the enhanced radiation.