The spin-orbit decomposition of the angular momentum of free electromagnetic fields is artificial
In the standard decomposition of the total angular momentum (AM) J(ro)=L(ro)+S of a free electromagnetic field the vector S represents an intrinsic angular momentum also refered lIS the photon spin. In 1936 Beth  performed a famous experiment in which a intense beam of circularly polarized light reaching a quartz half-wave plate emerged on the opposite side with the inverse polarization. The mechanical torque on the plate could be measured in their apparatus and it coincides in sign and magnitude with what would be expected if one consider a variat ion of ±2ħ per photon due the polarization inversion. That experiment is sometimes referred to as the measurement of the spin AM of the photon. In atomic theory is very well know that a absorption or emission of a photon circularly polarized changes the z component of the atomic AM of ±ħ in perfect concordance with the image of a intrinsic AM carried for photons. The uncountable number of experiments carrying similar interpretations turned the concept of photon spin undisputable.