In a new report published the Thursday 26 October in the journal Science Advances, a team of physicists based in the UK, Germany, New Zealand and Canada describe how new research into "optical angular momentum" (OAM) could overcome current difficulties with using twisted light across open spaces.
... https://www.gla.ac.uk/news/headline_555908_en.html
What is "Optical angular momentun" ? Is it the same concept as "orbital angular momentun" ? ‘orbital’ angular momentum is not unique to light and occurs (same concept) for other fields , including electrons, neutrons, and atoms (no massless) ?
Light is an electromagnetic wave, which is described as a massless spin-1 field, the electromagnetic field. Electromagnetism a la Maxwell is the paradigmatic example of a (classical) relativistic field theory, and you cannot in a unique and gauge-invariant way split the total angular momentum of the electromagnetic field into spin and orbital parts. Only total angular momentum makes sense. The physical notion that comes next to spin in non-relativistic quantum theory for (necessarily massive!) particles is helicity, i.e., the projection of the total angular momentum to the direction of momentum. In the quantum electrodynamics a single photon can have helicity +1 or -1. For the classical em. wave this corresponds to the circularly left and right-polarized plane-wave modes.
You can expand the single-photon states and also the classical em. field also in terms of angular-momentum eigenstates, i.e., in vector spherical harmonics (most elegantly formulated in terms of Debye potentials). "Twisted light" are electromagnetic waves with higher angular-momentum. In the scientific paper
If the total angular momentum of a photon cannot be split into spin and orbital angular momentum (OAM) parts in a gauge invariant way, thus this expression, isn't gauge invariant ? Or S and L are not genuine AM (Angular Momentum) operators ?
Well, as I said, I think that there is a lot of confusion of this in the literature. Judging from the abstract, the following open-access article seems to discuss it pretty well. I've still to read the paper carefully, however:
