  e present a detailed study of the retrieved optical parameters, electrical
permittivity, magnetic permeability and refractive index, of the coupled
fishnet metamaterial structures as a function of the separation between layers.
For the weak coupling case, the retrieved parameters are very close to the
one-functional-layer results and converge relatively fast. For the strong
coupling case, the retrieved parameters are completely different than the one
unit fishnet results. We also demonstrate that the high value of the figure of
merit (FOM=|Re(n)/Im(n)|) for the strongly coupled structures is due to the
fact that the real part of the negative $n$ moves away from the maximum of the
imaginary part of n (close to the resonance), where the losses are high.
