From localized to delocalized plasmonic modes, first observation of superradiant mode in silver random film

Semicontinous Plasmonic films, such as random silver films obtained by evaporation on a glass substrate, are very attractive due to the confinement of Electromagnetic (EM) field in sub-wavelength area. When the thickness of... [ view full abstract ]

Semicontinous Plasmonic films, such as random silver films obtained by evaporation on a glass substrate, are very attractive due to the confinement of Electromagnetic (EM) field in sub-wavelength area. When the thickness of the film is reduced down to few nanometers, the samples present discontinuities: silver Nanoparticles (NPs) appear. For a specific metallic surface coverage, the film presents very strong EM fields localized in hot-spots near the percolation threshold. Such concentration of field is already used for enhancing the Surface Enhanced Raman Scattering of single molecule [1]. This singular behaviour results from coexistence of localized and delocalized plasmonic modes. In this contribution, the nature and structure of these modes are investigated by measuring the reflectivity of the film.

Following the transition of plasmonics modes from localized to delocalized resonance, our work has revealed a remarkable intermediate behaviour before the percolation: for 6nm film we observe the coexistence of an absorbing (dark) and reflecting (bright) collective modes that can be addressed by changing incident angle. Theses collective plasmonic modes [2] results from the hybridization of localized plasmon resonance. With similar elliptical polarizations, this dark mode seems to be precursor of Surface Plasmon Polariton, observed for higher silver coverage. “Bright” mode with out-of-plane polarization corresponds to a transverse collective plasmonic resonance, usually with uncoupled oscillation [2]. By increasing the density of metallic nanoparticles in a wavelength scale, we observe an angular squeezing and a spectral broadening of this mode. This behaviour can be explained considering that transverse LSPR of each nanoparticle, all resonant, interact in a collective and coherent way via a common confined light mode: the evanescent wave. Such phenomenom has been described as superadiance in atomic physic [3]. In this regime, radiation rate of superradiant mode increases with the numbers of tied dipoles. This could explain spectral broadening observed in our work and constitutes the first observation of plasmonic superradiance effect in silver random film.

References:
[1] Walter et al. Phys. Rev. Lett., 98, 137401 (2007).
[2] Willingham et al Opt. Exp, 19, 6450 (2011).
[3] R. H. Dicke, Phys. Rev. 93, 99 (1954).