  We consider the problem of light transmission from a high refractive index
medium into a low index environment. While total internal reflection severely
limits such transmission in systems with smooth interfaces, diffractive
metasurfaces may help out-couple light that enters an interface at blazing
angles. We demonstrate that the profile of the structured interface can be
numerically optimized to target a particular emission pattern. Our study
suggests that while metasurfaces can help to out-couple light from a range of
incident directions, there exists a universal limit for total transmission
efficiency that depends only on the dielectric properties of the two materials
and is independent of the profile and the composition of the metasurface
coupler.
