Hello,
I am observing a nearly periodic background spectrum for a wide variety of SERS substrates. I see this response (attached below) for bare Au films, Cu films, and Au nanoparticles on Silicon and glassy carbon. I do not observe it on silicon and glassy carbon itself. Also I only observe this response when using a 50x objective (we have two) but not with a 10x. I am suspecting there is some optical issues within the instrument but I do not know what. Any advice would be helpful. Thank you.
If a Raman background is close to periodic, interference effects may have played a role. Typically the layer thickness may be the reason behind that and you can actually use Bragg's law (modified by the refractive index) to measure thicknesses by Raman.
If you are using structured substrates, maybe the surface structure also has some sort of size effect.
Do you have access to a different Raman spectrometer? Then you could exclude (or verify) the substrates as the cause.
Jürgen Weippert Thank you for your response.
I do not have easy access to another Raman at the moment, but because of the many different substrates I have tested that show this effect (various thicknesses of Au thin films, Cu foil) and then due to the fact I see the periodic background when adding Au nanoparticles to relatively flat substrates like glassy carbon and silicon, I feel confident I can rule out the substrate as the direct cause. Is it possible that the bandpass filter or notch filter need to be adjusted?
If it's not the substrate, then it's going to be fun [not]. That means you can have an interference basically anywhere in the optical path. First, check your lenses, mirrors etc. for visible impurities, maybe you're lucky and you'll find one of them coated. If you have an optical fiber, the transition into or out of that is often a good candidate, as well.
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