Effect of spatial mode coupling: effective electromagnetic parameters (real and imaginary parts): permeability μ_eff (top), permittivity e_eff (middle) and effective index n_eff (bottom). (a) Low mode coupling: lattice period l_p = 360 μm: the minimum value of the effective refractive index is n_effmin < 0.04; (b) strong mode coupling: lattice period l_p = 260 μm. The shaded area denotes the bandwidth of the negative effective index: the minimum value of the effective refractive index is n_effmin = −1.5. The static effective permittivity values yielded by the Maxwell–Garnett formula, e_eff = 2.1 and e_eff = 2.9, are in good agreement with the result of the simulation.