# Do EM waves transmit spin polarization?

Suppose you have a normal dipole antennae (transmitter and receiver) . Spin polarized current (as opposed to normal current) is sent into the transmitter, it emits an EM wave and the Receiver receives it. Will the charge carriers in the receiver become spin polarized as well? In other words, will the spin polarization of the transmitter current have some effect on the receiver, like for example imposing the spin polarization on the receiver carriers by means of making EM radiation circularly polarized?

I am aware that this effect is possible using certain semiconductors. But I am talking about a normal metal chunk used as the antennae. I am wondering whether the spin polarization of the transmitter current will have any effect on the receiver on a deeper level: using principles of Quantum Field Theory and Quantum Electrodynamics? (I don't know anything about QFT and QED btw)

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If the receiver is made of a non-ferromagnetic conductor, it might not support a spin-polarized current even if the EM radiation does carry angular momentum in its polarisation. The electrons would pick up some net spin at first, but that net spin polarisation could well dissipate in transverse phonons or a similar carrier. I'm not sure about the specifics so this might not answer your question, but this mechanism would depend on the electron-transverse phonon coupling in the antenna material. – Chay Paterson Mar 31 '13 at 16:32
Yes, I beleive spin relaxation is short in non-FMs. What about free charges? I am wondering about the fundamental mechanism of how spin is transferred from a bunch of free charges at the transmitting end to the same at the receiving end. (via radiation of course) – iTunnels Apr 1 '13 at 0:23
I'm not quite sure what you're asking, but the relevant changes in quantum numbers would be: transmitter electron emits photon (electron ms = +1/2 --> -1/2, photon has ms = +1), receiver electron absorbs photon (electron ms = -1/2 --> +1/2). The rates of emission and absorption will depend on DOS in the transmitter and receiver materials. – Chay Paterson Apr 2 '13 at 19:34
The $m_s = -1/2$ electrons just wouldn't absorb those photons. Conservation of angular momentum forces a selection rule there. – Chay Paterson Apr 8 '13 at 12:11
@LubošMotl has a good description in his blog on how classical fields emerge from the quantum substrate, and uses the photons as an example motls.blogspot.com/2011/11/… . I am not able to extend the discussion to polarizations from spins. I suspect that "it ain't simple" and will involve assumptions about spatial coherence of spin operators. – anna v Apr 16 '13 at 4:11

A relevant study: see the following work; Universal spin-momentum locking of evanescent waves by Machelan T.V. and Jacob Z.; Univ. of Alberta, Canada And Purdue Univ.

From the abstract:

We show the existence of an inherent property of evanescent electromagnetic waves:
spin-momentum locking, where the direction of momentum fundamentally locks the polarization of the wave. We trace the ultimate origin of this phenomenon to complex dispersion and causality require-ments on evanescent waves.

From the introduction:

An important signature of the recently discovered quantum spin hall state of matter is the existence of electronic surface states which are robust to disorder (non-magnetic impurities) . This property arises since the spin of the electron is intrinsically locked to the direction of propagation (momentum) and the electrons cannot back scatter unless there is a spin-flip. [emphasis mine] Intriguingly, recent experiments have explored an analogous phenomenon in photonics showing polarization dependent directional propagation of optical modes in spontaneously emitted as well as scattered light. For example, experiments have shown that spontaneous emission from atomic transitions is preferentially uni-directed along a fiber depending on the magnetic quantum number of the excited state. On the other hand, surface plasmon polaritons excited with circularly polarized light have also demonstrated unidirectional propagation. [...]

A quantum field theoretic treatment has also recently shed light on the interesting spin properties of evanescent waves.[...]
In analogy with the behavior of electrons in the quantum spin hall effect, we call this phenomenon "spin-momentum locking".

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Once more, you just copied the content of the link you give at the end without any personal effort to answer the question. That's not an answer. – ACuriousMind Mar 2 at 16:47
When one is referring to a very narrow area work its difficult to interject but the info was useful therefore i presented it otherwise i could have given the reference in comment section. if you advise i c an do the same-give the link in comment section. – drvrm Mar 2 at 17:03