I'm using a germanium cell to confine a liquid crystal monomer and study it with FTIR in transmission geometry.
I have use previously CaF2 and ZnSe substrates sputtered with gold obtaining a simple interference pattern of the cell gap (although very low transmission).
When making a cell with germanium, i get 2 interference patterns, one agrees with the 10µm silica spacers used for the cell construction, but i get a second pattern for about 150µm (with n = 1).
Substrates alone don't show interference (so is not between 2 faces of germanium and even if it was, the thickness wouldn't match even using Ge refractive index).
How thick are your Ge plates? What do you mean by "Substrates alone don't show interference"? Does that mean your unfilled cell shows only one type of interference fringe, or one Ge plate alone?
Hello Thomas, sorry for the late reply. With substrate I mean one plate.
The plates are around 1 mm thick.
" Does that mean your unfilled cell shows only one type of interference fringe "
No, unfilled cell show 2 main interferences (actually I found that there around 6 frequencies with FFT).
The filled cell shows the absorption of my compound, the unwanted pattern without modification and the pattern expected from the cell gap, wich decreases in amplitud and almost dissapears when I polymerize (all of this is to follow C=C conversion).
I'm not on my computer right now, will add more spectra when I'm available.
Thanks for your help.
What is the resolution of your spectra? Depending on it it is possible that interference fringes of the Ge are suppressed (as when you record one substrate) or appear (as in case of your cell). I admit that this is my last idea of what is going on and not very probable, even when for Si, substrates can still show interferences for such thicknesses.
One other thing: Using this type of cell you get a very strong electric field standing wave effect inside your cell (even though it is most probably smaller than in case of the CaF2 and ZnSe substrates sputtered with gold), so if you are interested in band ratios you would have to correct them (see e.g. Article The electric field standing wave effect in infrared transfle...
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Dear Dr Thomas: I tried some measurements today of the substrates and the cells with the maximum resolution of our spectrometer (0,25 cm-1) and I indeed can clearly see the interference inside of each germanium substrate and it firs harmonic as you pointed out, however the magnitude is completely different of the "unwanted" fringe.
Also, weirdly, I couldn't get the cell interference pattern for 15 and 30 µm silica spacers, while for 10, 20 and 45 µm the pattern it's clearly visible.
I'll try tomorrow again with those spacers and if possible watch them in the microscope, since they were gifted and don't know how disperse is their size.
I'll give a look at you paper too. As a chemist, I was not aware at all of this "standing wave" optical phenomena.
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Dear Dr Krzysztof: I'm not too sure of their flatness. They had initially a mirror like finishing, but I have made some scratches by using and reusing them.
The scratches are apparently not that deep/thick (they are clearly smaller than the 45 µm silica spacers that we have). However is definitely worth trying. We don't have polishing accessories at our lab, so I'll try to see if some company do that service in our city.
I can also try mixing different pairs of substrates (I have 6 different substrates) and see how the periodicity of the unwanted/unknown fringe changes.
It's also worth noting that this fringe periodicity remains constant even when changing spacers. If I understood correctly, the scratch depth should add to the spacers gap, thus their size should change when changing spacers, or do i misunderstood?
I'm not really sure of the cause of your 2 interference fringe patterns. It appears that you have a beat frequency like that generated from a 2-wavelength source. I would want to determine the frequency of the unknown pattern to be sure it doesn't match the frequency of your electrical supply.
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