Q-plate

A q-plate is an optical device that can form a light beam with orbital [angular momentum of light|orbital angular momentum] from a beam with well-defined Spin angular [momentum of light|spin angular momentum]. Q-plates are based on the SAM-OAM coupling that may occur in media that are both anisotropic and inhomogeneous, such as an inhomogeneous anisotropic birefringent waveplate. Q-plates are also currently realized using total internal reflection devices, liquid crystals, metasurfaces based on polymers, and sub-wavelength gratings.
The sign of the OAM is controlled by the input beam's polarization.