Negative refractive index response of weakly and strongly coupled optical metamaterials

We present a detailed study of the retrieved optical parameters, electrical permittivity epsilon, magnetic permeability mu, and refractive index n 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 from the one-unit fishnet results. We also demonstrate that the high value of the figure of merit [FOM = vertical bar Re(n)/Im(n)vertical bar] 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.