  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.

Optics