Dielectric function of aggregates of small metallic particles embedded   in host insulating matrix
  The optical properties of clusters with metallic spherical particles embedded in an insulating matrix are studied. A theoretical approach is proposed for the calculation of the macroscopic dielectric response for a collection of spheres at random positions embedded in a homogeneous medium. While accounting for the dipole interaction between particles, we have considered the frequency dependence behavior of the imaginary part of the effective dielectric constant in this system with two kinds of particles of different sizes. 
Leonid G.Grechko, Vitaly N. Pustovit and Keith W. Whites