Geometric metasurface fork gratings for vortex-beam generation and manipulation

Abstract

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA. In recent years, optical vortex beams possessing orbital angular momentum have received 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 provide 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. Among various types of metasurfaces, the geometric phase metasurface has shown great potential for robust control of light- and spin-controlled wave propagation. 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. The proposed metasurface fork gratings, combines phase singularity of the fork pattern and linear phase gradient of the metasurface together. It acts as both a polarization beam splitter and an OAM generator for light. The OAM states of an incident optical vortex can be converted to new modes by adding additional topological charges of the metasurface, while the SAM states of light determine their propagation directions.