(285f) Prediction of the Effective Medium Properties of Concentrated Multiple Species Plasmonic Composites
Plasmonic materials have been actively considered for light trapping in photovoltaic devices. A metal nanoparticle can sustain a localized plasmon resonance that leads to enhanced absorption and storage of energy. The simplest form of a plasmonic material is a random particulate composite. However, the modeling of optical response of such composites under excitation by an electromagnetic wave is not straightforward. Here, we have developed a new technique for calculating the effective medium properties of concentrated linear plasmonic composites containing multiple metallic (Ag, Au, Al and Cu) monodisperse spherical inclusions in a homogenous absorbing matrix. The underlying physical motif was the separation of the space surrounding any inclusion into two regions, one immediately surrounding the particle with the properties of the matrix (the size of this region depends on the static structure factor) and an effective medium. Self consistent closure relations were found for the conditionally averaged fields by solving the vector Helmholtz equations for a layered sphere in an infinite matrix by utilizing a multipole expansion technique in conjunctions with a Newton-Raphson algorithm for the evaluation of the spectral coefficients. In the quasi-static limit, the permittivity was in an excellent agreement with the classical Maxwell-Garnett theory. For large particle sizes and relatively high concentrations (~2 %), we found that the relationship between the concentration and the attenuation constant is nonlinear and is evidenced by experimental measurements on Ag nanoparticle/ethylene glycol suspensions. Our computations also suggest that at high concentrations (~4%), the plasmonic composites become Fano resonant media with an asymmetric absorption peak. We support this by experimental evidence.
Acknowledgement: NSF CMMI 0757589