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Photon diffraction questions

  1. Nov 24, 2005 #1

    dlgoff

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    Have there been any "one photon at a time" edge diffraction experiments? And does polarization of the photons have any effect on the interference patterns? Can you even consider polarization of single photons?

    Thanks

    Don
     
  2. jcsd
  3. Nov 24, 2005 #2
    i suppose u need to find out about single photon detection before u can observe single photon diffration and then would'nt u need a 100% transmitting polarizer??? and in order to edge lets say 1 x500nm photon u gunna need some pretty expensive alignment equiptment why got some interesting ideas don??

    ubiq
     
  4. Nov 25, 2005 #3
    I don't think there has been.
     
  5. Nov 25, 2005 #4

    dlgoff

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    When doing the double slit experiment, does polarization have any effect?

    Regards
     
  6. Nov 25, 2005 #5
    light polarization is involved only if you try to execute the experiment with two sources normally polarized. In such a case you can't see any fringe for the lack of interference conditions.
    If you polarize light only after beam splitting you get the same effect: no interference.
    Light must be able to interact with itself
     
  7. Nov 25, 2005 #6

    dlgoff

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    I understand that polarization wouldn't matter for one photon to interfer with itself, but I'm wondering if the orientation of the e-field relative to the slits(or edge) makes any difference. I quess you have no way of knowing the orentation for a single photon?
     
  8. Nov 25, 2005 #7
    Hello dlgoff,
    I don't think that the photon's polarization plays a role. At least in calculations I've seen, the polarization doesn't play a role.
    It is possible to know the orientation of a single photon when you measure it. You could for example put a "horizontal" polarizer in front of the double slit and make sure that only "horizontal" polarized photons pass through the double slit.
    This can be done with special plastic foils, that are strechted, such that the molecules are oriented in a certain direction. (see here:
    see here: http://www.stereoscopy.com/library/w05-16.gif.
    Or you could use so-called "beamsplitter cubes", where the horizontal polarized photons can pass and the vertical polarized photons are reflected (see here:
    http://www.newfocus.com/images/support/polarizing_cube.gif)
    One more thing: A single photon has a polarization. Physicists try to use polarized photons for quantum communication, maybe you've already heard
    about that.
    What exactly do you mean with edge?
     
  9. Nov 26, 2005 #8

    dlgoff

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    "What exactly do you mean with edge?"

    Light passing any well defined sharp edge like say a razor blades edge.
     
  10. Nov 26, 2005 #9
    Ah, ok. I think conducting an experiment with single photons passing an sharp edge wouldn't be so difficult. All you'd have to do is take a "normal" laserbeam and try to align it such that it touches the edge. Then, if you want "single photons", you must dim the laserlight, that is, you must lower the intensity. For example, take two polarizers and rotate them to each other such that only few photons can pass. And you need a sensitive CCD camera that can detect single photons.
    Here some websites:
    http://www.physics.brown.edu/physics/demopages/Demo/modern/demo/7a5520.htm
    http://web.phys.ksu.edu/vqmorig/tutorials/online/wave_part/Single Photons.html
     
  11. Nov 26, 2005 #10

    dlgoff

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    Thanks for the links Edgardo. I understand how the two slit experiment is preformed.

    What I've been wondering if this edge type of one photon at a time experiment has been done. The pattern on the light side of the screen should be what you would expect. But what would be the explaination for the photons that manage to get to the shadow side on the screen? Do these photons interfer with each other or do they just arrive there? i.e. from what patterns I've seen, on the dark side you just see a "smearing" of brightness not bands.

    Regards
     
  12. Nov 27, 2005 #11
    I guess Huygens principle gives an account for this.

    Let me add another interesting configuration that I have in mind.
    Suppose you have a sharp edge (half plane x=0, y < 0 and z real). Now consider another half plane located according to the rules (x = 10, y > 0, z real).

    Can you see that, looking at this half planes from the view point, for example, (-10, 0, 0) you will see something like a slit ? But there is no slit, but two sharp edges in series.
    So, what pattern will arise if you send photons one at a time, along the x direction (say, from negative x to positive x) ?

    Thanks

    DaTario
     
  13. Nov 27, 2005 #12

    dlgoff

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    And what would happen if in the double slit experiment you placed a double slit in place of the detection screen?
     
  14. Nov 27, 2005 #13
    Hello dlgoff,

    I've found this website that shows an experimental setup for the "knife edge diffraction".

    Although you don't see any bands and only a smearing, I think it's an interference effect that lets the photons "enter the shadow side", though I can't give you any reference for it. The photons interfere with themselves. (If I remember correctly I've seen bands for the razor edge diffraction, but that doesn't matter.)


    @DaTario and dlgoff:
    If you both are interested in how to calculate the diffraction pattern, you can use the so-called Fraunhofer approximation. It's described in the book "Optics" by Eugene Hecht. Basically you have to calculate an integral (a Fourier Transformation), and if you a program like Mathematica it's not very hard to calculate.
     
    Last edited: Nov 28, 2005
  15. Nov 27, 2005 #14

    dlgoff

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    Now this is what I'm looking for. After encountering the edge, do the photons' in the "shadow area" remain in the same orienatation (polarization)?
    Regards
    Don
     
  16. Nov 28, 2005 #15
    Personally, I don't think that diffraction changes polarization. I just don't see any reason why it should for example due to an effect.

    Examples that I know where polarization of a photon is changed:
    Using a special polarizer foil (that plastic foil in the post I've mentioned above), that makes a measurement on the photon.
    Or using a "birefringent" crystal.

    However, that's what I think.
     
  17. Nov 28, 2005 #16

    dlgoff

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    Edgardo
    "Examples that I know where polarization of a photon is changed:
    Using a special polarizer foil (that plastic foil in the post I've mentioned above), that makes a measurement on the photon."
    Do you have the link handy? I'd like to see what they say about the plastic foil.

    Thanks
    Don
     
  18. Nov 28, 2005 #17
    hi, there have been 1-photon-at-a-time interference experiments.

    by 1 photon-at-a-time, i mean an antibunched photon source, as in P. Grangier, G. Roger and A. Aspect. \emph{Europhys. Lett.}, \textbf{1} (1986), 173.

    since antibunched photons do NOT display interference effects, one can get them to display wave effects again by putting them through a mach-zehnder interferometer, for example.

     
  19. Nov 29, 2005 #18
    I just looked it up. The plastic foils are called Polaroid sheets.
    See here: http://www.stereoscopy.com/faq/vectographs.html

    Good website:
    http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/polabs.html#c3

    It's an article about LCD's:
    http://electronics.howstuffworks.com/lcd1.htm

    Besides, you have those sheet polarizers (or foils) in your calculator. Take apart a cheap calculator (not your expensive one from Casio or Texas Instruments!) and examine the LCD. You will find a glass and a foil. On the back of the glass, take off the gray sticker. Then take the foil and the glass, look through them, rotate them to each other and notice what happens. In some calculators, you can find two foils I think. In that case, rotate the two foils to each other.
     
  20. Nov 29, 2005 #19

    dlgoff

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    Yes. But has there ever been 1-photon-at-a-time experiment by shooting photons at a sharp edge? Not slits or beam splitters. I know what the pattern should look like. I'm just curious if its been done.
    Regards
    Don
     
  21. Sep 15, 2009 #20
    Michaelson Interferometer question

    I want to tool up to experiment in this area by making my own Michaelson interferometer since they are so expensive.
    One little problem:
    The interferometer I had in undergraduate class had screw adjustments for the mirrors that I could easily move a single wavelength of light with about a tenth of a turn. I did not see any special mechanism for reducing the screw motion, but it is hard to believe that the screw pitch was so fine since I could see the threads were about 1/10 mm. Do you have any idea how this is done?

    Why isn't there a major topic for Optics?
     
    Last edited: Sep 15, 2009
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