Most carbon phases are anisotropic, and therefore their Raman spectra are sensitive to polarisation of light. Besides, the gratings don't have exactly the same response when light is polarised or not. So, how best using the polarisation kit of the spectrometer for getting very reproducible spectra of carbon materials, irrespective to the grating used ?
In my case, the intensity ratio ID/IG depends on the grating, which is a significant problem. This occurs even after correction for the grating response (the correction being done from the response to a non-polarised white light source), which is known to depend on wavelength. So I suspect a problem of polarisation, as the gratings have different treatments (blazes), whose response to polarisation effects is not the same.
How getting rid of this ? Should the circular polarisation be used for averaging normal and orthogonal polarities ? Should this polarisation be applied to the laser, or to the Raman tool, or both ? Or any combination HH, VV, HV or VH ?
Thank you in advance for your help.
For the incident radiation, circular polarization (via a quarter wave plate that I think Horiba sells as an accessory to their Raman equipments) should be used. That of course does not solve the problem entirely, as the signal radiation can also be biased in polarization. However, you may want to start there and see if most of your problem is solved. With some equipments (I don't know about the Horiba one), there are polarization scramblers on the signal end, to randomize the signal polarization - if you can get your hands on those, then that would address your problem as well. Only problem is that the signal-to-noise ratio will decrease, so you would need to adjust acquisition time accordingly if your signal is weak to start with.
Thank you.
In the software, I can select polarization for the laser or for the Raman, or both together. Which one should be polarized ? Both circularly, or one circular and the other not ?
Alain
I would suggest circular for the laser, and then no polarization filter for the Raman signal side. What you can do then is, for a given sample, to change the sample orientation on your sample holder to a few positions, and see if your Raman spectrum changes appreciably. If it does not, then your problem is solved. If it does change, then you may need to talk to Horiba and think about a scrambler.
The scrambler can't be installed on an XPlora, as the system unfortunately doesn't have the required space for that. I was told that the response of the gratings can be recorded using the lamp at the bottom of the microscope as a white light source, and by rotating the polariser. But it is not clear to me how I should exactly do this procedure, and more, how I might do some corrections once the grating responses are obtained. Can you help ?
Alain
First, I would suggest that you set up the circular polarizer on the laser side, then rotate your sample incrementally, and see if your Raman signal changes appreciably. If your signals (for example, the ratio of a few major peak intensities) aren't changing appreciably, then the circular polarizer should be sufficient.
If you still have problems, then you would have to test the polarization sensitivity of the detector as set up in your spectrometer with your difraction grating. For that, you would rotate the polarizer ("analyzer") on the signal side, incrementally. The white light source (if it is an incandescent light) should be for the most part randomly polarized (although usually there's a slight bias of course, and plus every time you reflect off of an optic in a spectrometer, the polarization can get biased). If it's a fluorescent-type lamp, then there will be particular emissions with possibly certain polarization bias. Assuming it's a source with a randomized polarization, you would want to incrementally rotate the 0->90 degrees on the analyzer (polarizer on signal side). That can give you a map of the bias of your detector-in-system to signal polarizations, and allow you to do a correction if the map is collected sufficiently accurately. ==> If you do things this way, then when you go back to measuring your sample, you would basically have to measure the signal incrementally from 0 to 90 degrees on the analyzer, and then apply the correction factor on each of the obtained spectra.
Thank you so much for your detailed explanations. Still how to do the corrections is unclear to me once the data are collected. Is it possible to do them directly in the software of the instrument, or should the data be treated manually in an independent software such as Matlab or Origin ?
Thank you for your patience ...
Alain
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