Frequency mode coupling: minimal value of the effective refractive index n_eff as a function of the side length a_e of the electric resonator, namely, the frequency of the electric mode is varying. Insets: real (purple) and imaginary part of (green) the effective index n_eff; the shaded area denotes the bandwidth of negative effective index. f₁₁ and f₁₂ are the resonances frequencies of the individual resonators (see the text).