From localized to delocalized plasmonic modes, first observation of superradiant scattering in disordered...

Our study proposes a new way to observe and explain the presence of extended plasmonic modes in disordered semi-continuous metal films before the percolation threshold. Attenuated total reflection spectroscopy allows us to follow the transition of plasmon modes from localized to delocalized resonances, but also reveals unobserved collective plasmon modes. These bright modes with out-of-plane polarization are transverse collective plasmonic resonances. By increasing the density of metallic nanoparticles in a wavelength scale, we observe an angular squeezing and spectral broadening of these modes. This behavior can be explained considering that transverse localized surface plasmon resonances of each nanoparticle, all resonant, interact in a collective and coherent way via a common confined light mode: the evanescent wave. These many-body resonances, which have never been clearly identified in such disordered semi-continuous metal films, can be described by analogy with atomic physics as superradiant modes. Our first simulations, using dyadic Green's formalism, demonstrate the existence of this mode for a dense array of plasmonic systems. In this regime, the radiation rate of the superradiant mode increases with the number of tied dipoles. This explains the spectral broadening observed in our work and constitutes the first manifestation of superradiance mode in plasmonic random structure.