  Hyperbolic materials are artificially engineered composite media which allow
innovative optical properties that could lead to significant advances in
numerous breakthrough technologies. However, these properties are
conventionally studied by means of an effective medium theory. But this one is
specifically well adapted to describe the optical behaviour of composite
materials in the far field region. Here, we adress the question of the validity
of such an approach in the near field region. We show that the presence of
localized modes such as resonant surface waves drastically limits the validity
of the effective description and requires revisiting the concept of
homogenization in near-field. In particular we demonstrate from exact
calculations that one can find surface modes in spectral regions where the
effective medium theory predicts hyperbolic modes only. Hence, the presence of
surface modes which are not accounted for in the effective medium theory can
lead to physical misinterpretations in the description of hyperbolic materials
and their related properties.
