# 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, 2021 at 16:55
• Optical field means electromagnetic wave.
– Jack
Mar 26, 2021 at 0:41

This has already been asked in several forms in Physics Stack Exchange. Within the semi-classical 'electric dipole' approximation, only the electric field of light interacts with the electron. The effect of magnetic field is usually very weak except for a few cases like highly concentrated pulses etc. Magnetic field can flip spins, in a process called 'spin-flip'. There are experimental cases reported, the most famous being the 21cm astronomical limit. I wrote a more comprehensive answer here.

well, you certainly can change the spin of an electron through acting on its orbital motion with the electric field. It is called "spin-orbit coupling" (SOC) and a lot of magneto-optical and opto-magnetic (inverse) effects completely rely on it. You might want to look at papers discussing all-optical magnetization switching with circularly polarized light in various materials using, e.g. inverse Faraday effect. Further, spin-orbit coupling is the only way to couple spin and light in a hamiltonian. Theorists who calculate interactions of optical fields and spins often calculate (and show in their papers) no-SOC cases where they artificially set the SOC strength to 0 and demonstrate that the effect they were interested in vanishes. Yet, the reason why it is difficult to flip electron"s spin with light is that, well, SOC is usually (in common materials) not very strong. If you go down the periodic table, SOC increases rapidly, and heavy rare-earths might be the systems where optical spin-flip transitions are to be looked for.

I think the claim assumes linear polarization of the light which has zero angular momentum. For circularly polarized light, the spin of the electron should be flipped. (e.g. Appl. Phys. Lett. 114, 041104 (2019)) Indeed, it has a probability issue as explained by @Xivi76.

• @Nope. You may take a look at that paper.
– Jack
Mar 26, 2021 at 1:20
• OK, then I have same question as you. Mar 26, 2021 at 1:33
• I have accepted Xivi76's answer. Is this your paper: aip.scitation.org/doi/10.1063/1.4991913 :)
– Jack
Mar 26, 2021 at 1:43
• Yes. I know people investigating spin relaxations in GaAs usually flip the spin by circularly polarized light. Mar 26, 2021 at 2:19
• Could you cite a paper?
– Jack
Mar 26, 2021 at 2:40