Superradiant properties of Collective plasmon modes in ultra-dense film of silver nanoparticles

Random metallic films, before the percolation threshold, present a concentration of electromagnetic field at the nm scale, below the diffraction limit [1], These generated hot spots, induce huge enhancement of light-matter... [ view full abstract ]

Random metallic films, before the percolation threshold, present a concentration of electromagnetic field at the nm scale, below the diffraction limit [1], These generated hot spots, induce huge enhancement of light-matter coupling and these substrates thus constitute performing systems for enhanced spectroscopies. However, the nature of the plasmon modes is still unclear at the percolation threshold  and this manuscript presents experimental and theoretical investigations of these plasmon modes. Films are synthetized by evaporation of silver glass. By varying the effective thickness, one goes from isolated metallic structure to continuous thin film. Then, the nature of the plasmon mode, evolving from Localized Surface Plasmon Resonance (LSPR) to Surface Plasmon Polariton (SPP), is probed by Attenuated Total Reflection measurements (ATR). Our ATR study shows that before the transition to conductive state, in TM excitation, two drastically different behaviors are present at the same wavelength but for different angles of incidence. For supercritical angles, the specular reflection is totally extinguished while at critical angle, the specular reflection is maintained for a large part of the visible spectrum due to the presence of bright plasmon modes. These latter modes present unexpected spectral broadening and an angular squeezing close to the percolation threshold. At this level of deposition, film can be seen as a ultra dense array of plasmonic nanosystems. The actual model of the hybridization for coupled plasmon modes  is not sufficient to explain our experimental results. We explain this behavior by considering that transverse Localized Surface Plasmon Resonances of each nanoparticle, interact in a collective and coherent way via a common confined light mode: the evanescent wave. Using Dyadic Green formalism [2], our model confirms the existence of these collective modes for a dense array of plasmon systems and explains the observed spectral broadening and angular squeezing. This superradiance phenomenon, increasing with the numbers of involved particles, is maximum before the percolation threshold where the density of large nanoparticles is maximum and disappears after where nanoparticles coalesce.

[1] M. I. Stockman et al PRL, 87, 167401 (2001).

[2] J.J. Choquette et al PRA , 82, 1 (2010).