Effective-medium approach to planar multilayer hyperbolic metamaterials: Strengths and limitations

We express the optical properties of multilayered hyperbolic metamaterials (HMMs) in terms of the Fresnel reflection coefficients at the boundary between the metamaterial and the ambient medium. Formation of a band of bulk propagating modes in HMMs located far outside the lightcone of homogeneous isotropic media is demonstrated. Exotic behavior of HMMs, such as the broadband Purcell effect and suppressed outward scattering, is reproduced. Conditions under which a metal-dielectric multilayer can be approximated by a homogeneous effective medium with extreme anisotropy (indefinite medium) are derived. It is shown that real multilayer HMMs usually have a smaller Purcell factor than the corresponding effective medium; however, the reverse scenario is shown to be possible due to an intermediary role of short-range surface plasmon excitations in the outermost metal layer in a metal-dielectric multilayer.