Angular momentum of light

The angular momentum of light is a vector quantity that expresses the amount of dynamical rotation present in the electromagnetic field of the light. Indeed, a beam of light, while traveling approximately in a straight line, can also be rotating (or “spinning”, or “twisting”) 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. The total angular momentum of light (or, more generally, of the electromagnetic field and the other force fields) and matter is conserved in time. But there are actually 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 hence associated with two distinct forms of angular momentum, respectively named light spin angular momentum (SAM) and light orbital angular momentum (OAM).

It is well known that light, or more generally an electromagnetic wave, carries not only energy but also momentum, which is a characteristic property of all objects in translational motion. The existence of this momentum becomes apparent in the “radiation pressure” phenomenon, in which a light beam transfers its momentum to an absorbing or scattering object, generating a mechanical pressure on it in the process.

Less widely known is the fact that light may also carry angular momentum, which is a property of all objects in rotational motion. For example, a light beam can be rotating around its own axis while it propagates forward. Again, the existence of this angular momentum can be made evident by transferring it to small absorbing or scattering particles, which are thus subject to an optical torque.

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