  In recent years, optical vortex beams possessing orbital angular momentum
have caught much attention due to their potential for high capacity optical
communications. This capability arises from the unbounded topological charges
of orbital angular momentum (OAM) that provides infinite freedoms for encoding
information. The two most common approaches for generating vortex beams are
through fork diffraction gratings and spiral phase plates. While realization of
conventional spiral phase plate requires complicated 3D fabrication, the
emerging field of metasurfaces has provided a planar and facile solution for
generating vortex beams of arbitrary orbit angular momentum. Here we realize a
novel type of geometric metasurface fork grating that seamlessly combine the
functionality of a metasurface phase plate for vortex beam generation, and that
of a linear phase gradient metasurface for controlling the wave propagation
direction. The metasurface fork grating is therefore capable of simultaneously
controlling both the spin and the orbital angular momentum of light.
