# Why can't we change the spin angular momentum of electrons with an external optical field?

I have read this paper many times: Coupled Spin and Valley Physics in Monolayers of MoS2 and Other Group-VI Dichalcogenides. Interestingly, this paper is also discussed many times in this community, such as:

One of the most important conclusions is the spin-dependent optical transition. However, the authors claim that [optical field couples only to the orbital part of the wave function and spin is conserved in the optical transitions]. Therefore, the spin-dependent transitions are realized by the spin-valley coupling that I have understood.

Here my confusing part is about why we can't change the spin angular momentum of electrons with the external optical field? For example, assume that the left circular polarization light, carried a spin angular momentum $$\hbar$$, is absorbed by an electron with spin angular momentum $$-\dfrac{1}{2}\hbar$$, then the energy of the photon is eaten by the electron, but where is the spin angular momentum of the photon if we think the electron spin is conserved? Is it transferred to the orbital angular momentum of the electron? How?

• What is "optical field"? An electromagnetic field? If yes, the only component than can change the spin is the magnetic one but it is not stronger enough. One thing is about the electric field change the spin and other one is the optical transition (that have to be with the wavefunction symmetry). – Camps Mar 25 at 16:55
• Optical field means electromagnetic wave. – Jack Mar 26 at 0:41