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I wanted to do a quick test with a Kretschmann Surface Plasmon Resonance setup. So I got a glass slide with 50nm coating, a 650 nm, 0.9mW laser diode, and a prism. I essentially place this as a vertical setup with the glass slide on top of the prism with immersion oil in between. I placed it vertically so that gravity keeps the prism, immersion oil, and glass slide in place.

Then I manually move the laser slowly till I see a dip in the reflected beam, but I never see any dips. I even placed a linear polarizer in front of the laser, but I still see no dips in intensity. I was wondering what the issue could be? Or is there a 3D printable vertical SPR setup that I could replicate as I am doing this at home, so a horizontal setup is a bit difficult to do without the oil leaking downwards or some other solutions on top of the gold film.

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  • $\begingroup$ what is your angular resolution? at 650 nm on gold, this resonance can be very narrow (<0.5 degrees). If you move the laser manually, it might be easy to miss. $\endgroup$
    – sleepy
    Apr 12, 2021 at 22:26
  • $\begingroup$ Right now I was just moving the laser by hand at different angles. But I plan on using the 28BYJ-48 stepper motor with arduino to turn the prism to different angles. So its resolution would be 360/4096 = 0.088 degrees. But shouldn't the dip be wide like in this video youtube.com/watch?v=RO6Rq6-cqsY&ab_channel=davecoulter $\endgroup$
    – Jeff Boker
    Apr 13, 2021 at 0:43
  • $\begingroup$ Jeff, the width depends on a few things, and one of them is quality of the gold film. 50 nm looks like a proper thickness though. Vertical or horizontal should not matter at all, just make sure that the polarization is in-plane. Another thing is the oil; when I was building a similar setup a while ago, I tried a few liquids and got the best results with just water (not sure why). Also, before you use the motor, you could try to focus the laser light with a lens. This will effectively broaden the resonance so you will be less likely to miss it. $\endgroup$
    – sleepy
    Apr 13, 2021 at 7:49
  • $\begingroup$ The reason for making it a vertical setup was so that I wouldn't need to create a chamber to hold the slide and liquids together. What did you use to ensure the slide stayed in contact with the prism? Also, I assume you rotated the laser in an arc to change the angle right? $\endgroup$
    – Jeff Boker
    Apr 13, 2021 at 12:40
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    $\begingroup$ 1. I had a golden film on a thin (and light) glass substrate, so a droplet of water was enough to make sure it is attached to the side of the prism. 2. No, I had a prism on a goniometer and rotated the prism and the detector together. Actually, another thing is whether you have a photodiode to detect intensity changes, because if the gold surface is not perfect, the drop in the intensity could be small and hard to notice with the naked eye. Other things to check are whether you don't have the glass slide upside down, and whether there is no oil on top of gold. $\endgroup$
    – sleepy
    Apr 13, 2021 at 13:23

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The best approach is to plate a gold film directly on a flat side of a semi-circular polarizer. The next best approach is to have the gold film surface clamped tightly down on the polarizer. The third best approach is to have a liquid film between the polarizer and the gold surface.

The index of refraction changes as the light exits from the polarizer to air/liquid to the gold surface (and back after reflection). The plasmon resonance depends on the angle incoming to the gold and the index of refraction of the medium above the gold. You must eliminate as many of the intermediate changes as possible. Hence plating the gold film directly on the polarizer. Oil or water physisorb on the gold surface and can cause change in surface plasmon density. Using a non-circular polarizer will cause a change in refraction angle as the light goes into and exits the polarizer.

You do not want to be looking down the path of the reflected laser light to try to "see" this phenomena! You could try using a paper that is coated with a fluorescent dye that is sensitive to the laser wavelength as a sensor for the reflected light. The pattern could reveal itself more strongly that way.

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  • $\begingroup$ I assume that the paper coated with dye is placed on the other side of the prism where the reflected light comes out. Are there papers pre-coated with fluorescent dyes or does this need to be done manually and are there manuals showing how to do it? Also, I noticed that prisms coated with gold directly tend to cost thousands of dollars, so I didn't opt for that solution. $\endgroup$
    – Jeff Boker
    Apr 13, 2021 at 16:42
  • $\begingroup$ I've noted the placement of the dye-coated paper. A quick and dirty approach would be to look for paper that fluoresces at the laser wavelength (e.g. "black light" paper). $\endgroup$
    – Jeffrey J Weimer
    Apr 13, 2021 at 17:43

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