In a recent paper [Phys. Rev. Lett. 94, 197401 (2005)], we introduced a mechanism for creating artificial high refractive index metamaterials by exploiting the existence of sub-wavelength propagating modes in metallic systems. We showed that a perfect metal film with a periodic arrangement of sub-wavelength cut-through slits can be regarded as a dielectric slab with a frequency-independent effective index. Here, we discuss the optical properties of such a system when the perfect metal condition is no longer valid, e.g., in the visible and near infrared wavelength regimes. If the metal obeys a
plasmonic dispersion model, we find that films with a periodic arrangement of sub-wavelength slits support two distinct types of guided modes: a surface mode and a set of effective dielectric slab modes. We show how the behavior of both modes is affected by film thickness and surface properties.