  Recent advances in engineered gradient metasurfaces have enabled
unprecedented opportunities for light manipulation using optically thin sheets,
such as anomalous refraction, reflection, or focusing of an incident beam. Here
we introduce a concept of multi-channel functional metasurfaces, which are able
to control incoming and outgoing waves in a number of propagation directions
simultaneously. In particular, we reveal a possibility to engineer
multi-channel reflectors. Under the assumption of reciprocity and energy
conservation, we find that there exist three basic functionalities of such
reflectors: Specular, anomalous, and retro reflections. Multi-channel response
of a general flat reflector can be described by a combination of these
functionalities. To demonstrate the potential of the introduced concept, we
design and experimentally test three different multi-channel reflectors: Three-
and five-channel retro-reflectors and a three-channel power splitter.
Furthermore, by extending the concept to reflectors supporting higher-order
Floquet harmonics, we forecast the emergence of other multiple-channel flat
devices, such as isolating mirrors, complex splitters, and multi-functional
gratings.
