Diagrams identifying the sign of the refractive index according to the signs of the real and imaginary parts of the constitutive parameters. Blue (dark) areas refer to positive-index dissipative media (n′ > 0 and n″ > 0) supporting forward waves, while pink (light) areas refer to negative-index dissipative media (n′ < 0 and n″ > 0) supporting backward waves. White areas refer to amplifying media (n″ < 0) that are not considered here [17]. (a) Diagram for acoustic metamaterials with complex-valued bulk modulus κ (= κ′ + iκ″) and mass density ρ (= ρ′ + iρ″). (b) Diagram for electromagnetic metamaterials with complex-valued relative permittivity ε (= ε′ + iε″) and relative permeability μ (= μ′ + iμ″).