A q-plate is an optical device that can form a light beam with orbital angular momentum (OAM) from a beam with well-defined spin angular momentum (SAM).
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.
[1] The sign of the OAM is controlled by the input beam's polarization.
[2][3] This optics-related article is a stub.