Energy, photons, angular momentum and polarization

In summary, a linearly polarized photon passing through a quarter-waveplate that is free to rotate and aligned at 45 degrees will transfer energy to the plate, causing the photon to lose energy. When the experiment is repeated with a fixed plate, the same change from linear to circular polarization will occur, but there will be negligible energy transfer due to the large moment of inertia of the plate. These experiments have been successfully carried out with 1/2 wave plates and molecule clusters, and while access to the relevant papers may be restricted, the results demonstrate the transfer of angular momentum through photon interaction.
  • #1
alfredblase
228
0
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 acquire 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
 
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  • #2
alfredblase said:
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 acquire 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?

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.

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

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
thanks vanesch,

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.

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

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.

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,,,
 
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  • #4
alfredblase said:
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 acquire an angular momentum,,, [Walborn, Cunha, Padua and Monken 2002, Physical review A, 65, 033818-1]

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],,

2. 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
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
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
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.

sorry that argument doesn't really wash
 
  • #7
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
 
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  • #8
bump bump bump
 

1. What is energy and how is it related to photons?

Energy is the ability to do work or cause change. Photons are particles of light that carry energy. The energy of a photon is directly proportional to its frequency, meaning that higher frequency photons have more energy than lower frequency photons.

2. What is angular momentum and why is it important in physics?

Angular momentum is a measure of an object's rotational motion. It is important in physics because it is a conserved quantity, meaning that it remains constant unless acted upon by an external torque. This makes it a useful tool in understanding the behavior of rotating systems, such as planets in orbit or spinning objects.

3. How is angular momentum related to polarization?

Polarization is a property of light that describes the orientation of its electric field. When light is polarized, its electric field oscillates in a specific direction. Angular momentum is related to polarization because the direction of the electric field determines the direction of the photon's spin, which contributes to its angular momentum.

4. What is polarization and how does it affect light?

Polarization is a property of light that describes the orientation of its electric field. It can be linear, circular, or elliptical, and the direction of the electric field determines the direction of the photon's spin. The polarization of light can affect how it interacts with other materials and can be used in various technologies, such as 3D glasses and polarized sunglasses.

5. How do photons behave differently in different mediums?

Photons can behave differently in different mediums due to interactions with the material's atoms and molecules. This can result in effects such as refraction and absorption, which can change the direction and intensity of light. The speed of light can also vary in different mediums, affecting the wavelength and frequency of photons.

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