Spin angular momentum of light

The spin angular momentum of light (SAM) is the component of angular momentum of light that can be associated with the wave's circular or elliptical polarization.

An electromagnetic wave is said to have circular polarization when its electric and magnetic fields rotate continuously around the beam axis during the propagation. The circular polarization is left (${\displaystyle L}$) or right (${\displaystyle R}$) depending on the field rotation direction (but be careful that both conventions are used in science, depending on the subfield).

When a light beam is circularly polarized, each of its photons carries a spin angular momentum of ${\displaystyle \pm \hbar }$, where ${\displaystyle \hbar }$ is the reduced Planck constant and the ${\displaystyle \pm }$ sign is positive for Left and negative for Right circular polarizations (this is adopting the convention most commonly used in optics). This SAM is directed along the beam axis (parallel if positive, antiparallel if negative). The above figure shows the instantaneous structure of the electric field of left (${\displaystyle L}$) and right (${\displaystyle R}$) circularly polarized light in space. The green arrows indicate the propagation direction.

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