page_title: Angular momentum of light
section_title: Q-Plate
hierarchical_section_title: Angular momentum of light / Exchange of spin and orbital angular momentum with matter / Q-Plate
caption_attribution_description: English: The q-plate effect for left and right-hand circular polarizations.
context_page_description: The angular momentum of light is a vector quantity that expresses the amount of dynamical rotation present in the electromagnetic field of the light. While traveling approximately in a straight line, a beam of light can also be rotating around its own axis. This rotation, while not visible to the naked eye, can be revealed by the interaction of the light beam with matter.
  There are two distinct forms of rotation of a light beam, one involving its polarization and the other its wavefront shape. These two forms of rotation are therefore associated with two distinct forms of angular momentum, respectively named light spin angular momentum and light orbital angular momentum.
  The total angular momentum of light and matter is conserved in time.
context_section_description: Another method for generating OAM is based on the SAM-OAM coupling that may occur in a medium which is both anisotropic and inhomogeneous. In particular, the so-called q-plate is a device, currently realized using liquid crystals, polymers or sub-wavelength gratings, which can generate OAM by exploiting a SAM sign-change. In this case, the OAM sign is controlled by the input polarization.
caption: The q-plate effect for left and right-hand circular polarizations.
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