Light spin angular momentum

The spin angular momentum of light (SAM) is the component of angular momentum of light that is associated with the quantum spin and 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 propagation. The circular polarization is left (${\displaystyle L}$) or right (${\displaystyle R}$) depending on the field rotation direction and, according to the convention used: either from the point of view of the source, or the receiver. 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 (SAM) 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 from the point of view of the receiver 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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