Generalized effective-medium theory for metamaterials
  We present an effective-medium model for calculating the frequency-dependent effective permittivity $\epsilon(\omega)$ and permeability $\mu(\omega)$ of metamaterial composites containing spherical particles with arbitrary permittivity and permeability. The model is derived from the zero-scattering condition within the dipole approximation, but does not invoke any additional long-wavelength approximations. As a result, it captures the effects of spatial dispersion and predicts a finite effective refractive index and antiresonances in $\epsilon(\omega)$ and $\mu(\omega)$, in agreement with numerical finite-element calculations. 
Brian A. Slovick, Zhi Gang Yu, and Srini Krishnamurthy
physics.optics