I am trying to build a homemade SPR device. So far I have:
1. right-angle BK7 glass prism (Edmund optics NT30-575) coated with 50nm sputtered gold
2. laser diode with built in focussing lens (3mW, 670nm, RS194-026)
3. linear glass polariser (Edmund optics NT43-783)
I don't know the axis of polarisation of laser or polariser. As the laser is 80:1 polarisation ratio, I assumed that, as an initial starting point, I could work with the laser and prism only. I had planned to simply use the SPR effect to determine optimum polarisation axis of the laser diode (i.e. by finding the dark band and then rotating the laser until the contrast was optimised).
so far I have tried various angles and rotations, and can see no dark band whatsoever.
OK, so I’ve managed to find the polarisation angle using an LCD screen (I’m assuming the screen is polarised either left-right or top-bottom rather than 45degrees or some random angle - this may be a weak assumption!).
For the moment, I’m shining polarised laser light through a prism coated with ~50nm gold and projecting the output light onto a white wall in a darkened room (For future work, I’m designing a CCD camera set-up to more accurately quantify the light levels).
After trying a range of angles from 30-70 degrees, I still haven’t found the dark band. I'm wondering if my eyes are simply not sensitive enough to pick out the slight difference in intensity at this wavelength of light, and I should concentrate on setting up CCD detection?
I have to say that i'am not familiar with how exactly the construction of a SPR setup should be.
However to have SPR (dark band) you must have a coupling between the light wave, and the gold surface.
This depends on several parameters:
-- metal film : 50 nm gold in you case
-- wavelength: 680 nm
-- angle of incidence:
-- refractive index of media on either side of the metal film: BK7 glass prism and ?
The light should be p-polarized (polarized in the plane of the surface) to obtain a sharp dip.
Because I 'am not a material scientist i don't know how to test the p-polarization.
But maybe you can search some literature because a lot of people try to build one of their own.