Dear Research Community,
I'm doing a research with transmission mode that consist of a laser (1650nm) one polarizer 0 degrees, sample, and other polarizer and the detector. My question is, I can eliminate or reduce the scattering effect by changing the polarization angle?
Thank you
Perhaps you are interested in suppressing reflection using Brewster's angle ?https://en.wikipedia.org/wiki/Brewster%27s_angle
Dear David Chiu , in general, we can assume that scattering happens in all directions, so it would be independent of the polarization angles in your setup. However, because you are interested in transmission of your polarized laser beam through your sample, and assuming your sample is not polarizing the beam itself, then you should get a stronger signal if your second polarizer is placed parallel to the first one (both at 0º), I mean, the scattering would be the same in any direction, but transmission would not, and therefore the best way to maximize your signal would be placing your two polarizers equally oriented.
This answer would be valid only if your sample is homogeneous, non polarizing and produces random scattering (such a sample could be for example an homogeneous dispersion).
If your sample is something like a crystalline solid or a liquid crystal, it could cause polarization of the incoming beam, and for your transmission experiments you should rotate your second polarizer the right angle to register the rotated exit beam. Leaving your two polarizers parallel with a sample which is a polarizer itself would mean that a share of the transmitted beam would be bloqued by the second polarizer, decreasing the intensity recorded by the detector.
Things are really more complicated with scattering, given that it could be elastic or inelastic. Elastic scattering or Raileigh scattering happens when the light photons interact with matter transfering their energy to excite the matter, but shortly after a deactivation happens and the matter reemits a new photon exactly with the same energy as the one from the incident beam, a bit like two billard balls . In some other cases a photon interact with the sample and again, excitates the atoms or molecules to a higher energy state, then they relax but not to the ground state but to some other one, so the released photon would have less energy that the one from the beam, it means scattering at lower energy or larger wavelength. Those would be called Stokes scattered photons. Some photons can hit some atoms or molecules that already are in excited states, so when the atoms or molecules relax to the ground state, they would release a photon with higher energy than that of the incident one, because this photon has the original energy of the impinging photon plus the energy already stored in the sample. So in this case the photons have more energy or lower wavelength. This is an upconversion effect and the scattering is called anti-Stokes.
Hope this helps. Good luck with your experiments and best regards.
David Chiu , you have asked several questions deals with suppressing scattered light, but I think that answers to you would be more informative if you describe optical properties of your case more detailed. In general, audience don't know properties of your system. How strong is scattered light relatively transmitted one? What is the thickness of your sample? Is your sample liquid or powder? We don't know answer on these questions, but such knowledge is important to provide you with useful proposals.
By the way, there are methods how to suppress scattered light, using spatial filtering of transmitted beam. You can put imaging lens into transmitted beam, and detector into image of your sample (cuvette). In such a case lens focal plane draw angular distribution of both transmitted and scattered lights. If you put small circular diaphragm into this plane on the optical axis of your system, you will suppress very significant part of scattered light additionally to suppression by polarization filtering.
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