# Energy, photons, angular momentum and polarization

1. Feb 18, 2006

### alfredblase

A linearly polarized photon passes through a quarter waveplate that is free to rotate and is aligned at 45 degrees to the polarizaton direction,,,
The wave plate will aquire an angular momentum,,, [Walborn, Cunha, Padua and Monken 2002, Physical review A, 65, 033818-1]

This means that energy is transferred from the photon to the plate, and that therefore the photon loses energy,, Am I correct in saying this?

Next question:

Now the same thing experiment is done but the wave plate is not free to rotate,, the photon will undergo exactly the same change as before, [i,e it will change from linearly polarized to circularly polarized],,

Would I be right in saying that the photon again loses energy by applying a torque to the plate? If not please explain in detail the difference between the first scenario and the second [refering to the physics],,

Help would be much appreciated

Last edited: Feb 18, 2006
2. Feb 18, 2006

### vanesch

Staff Emeritus
Well, if it starts rotating, you will of course not keep your alignment with 45 degrees, so you will end up having -45 degrees, and sending out the OPPOSITE circular polarisation ; the transfer is now in the other direction...

I don't know if you change the frequency of a photon by sending it through a *rotating* quarter-wavelength plate ; that's very well possible, and when the frequency of the photon changes, it has lost (or gained) energy.

It will apply torque to the plate, but as this torque does not do any work, I don't think that there is any energy transfer (and hence a frequency change of the photon).
This is a bit as a ball bouncing on a fixed wall: there's momentum transfer, but no energy loss.

3. Feb 18, 2006

### alfredblase

thanks vanesch,

hmm, im afraid i need a definate answer :shy: Also I presume the plate was not rotating before the photon passed through it

I disagree, I mean, the things fixing the plate will be deformed very slightly no? like when i push my finger against a brick wall the wall deforms very slightly,,,

Last edited: Feb 18, 2006
4. Feb 19, 2006

### Meir Achuz

1. The moment of inertia of the plate is so large compared with that of the photon that the energy given to the plate is completely negligible.
It is the same thing as when you bounce a ball off the floor. The Earth gains momentum, but negligible energy.
2. There is no difference. In the case of the "fixed" plate, it is the Earth that gets the angular momentum but only negligible energy.
In these cases, "negligible" really means negligible.

5. Feb 19, 2006

### Staff: Mentor

Of course, a physically realizable experiment would surely involve a pulse of light containing many many photons, not just a single photon.

It would be interesting to see some numbers for such an experiment: energy of the light pulse, amount of angular momentum transferred to the quarter-wave plate, moment of inertia of the plate, and the amount of rotational kinetic energy gained by the plate.

6. Feb 19, 2006

### alfredblase

sorry that argument doesn't really wash

7. Feb 19, 2006

### alfredblase

I found the link: http://www.spie.org/web/oer/june/jun97/photon.html [Broken]

Ok this website tells us that transfer of angular momentum has been carried out with 1/2 wave plates [Beth 1936], and with molecule clusters,, [Santamato and Shen 1986]

I find it ridiculous that the general public is not allowed access to these papers without paying tho,, is there some way I can read papers from subscription journals for free? I think knowledge should be available to all not just to those with a flush credit card,,

Anyway so apparently wave plates and molecule clusters are rotated thus gaining kinetic energy and so redshifting the photons

Views on this and confirmation/debunking of what I have said would be much appreciated,,

thanks

Last edited by a moderator: May 2, 2017
8. Feb 21, 2006

### alfredblase

bump bump bump