  In many optical systems, including metal films, dielectric reflectors, and
photonic crystals, electromagnetic waves can experience evanescent decay. The
spatial length scale of such decay defines the penetration depth. The ability
to reduce the penetration depth is important for a number of applications in
free-space and integrated photonics. In this paper, we consider a ultrathin
metafilm structures consisting of alternating regions of metal and dielectric.
We show that the penetration depth of such metafilm can be significantly
smaller as compared to that of a corresponding metal thin film. The reduction
of the penetration depth arises due to the enhanced effective mass in the
photonic band structure. This effect can be used to enhance the reflectivity of
ultrathin reflectors, and to increase the packing density of subwavelength
plasmonic waveguides.
