Polarization scrambler

Both dispersive and nondispersive spectrometers can exhibit preferential transmission of light depending on its polarization. The efficiency of diffraction gratings and the polarization sensitivity of the beamsplitter can cause errors in the observed depolarization ratio, depending on several variables such as experimental geometry and Raman shift region. For this reason, it is often important to place a polarization scrambler between the sample and any polarization-sensitive components of the spectrometer other than the polarization analyzer itself. In addition, it is good practice to measure p for a few known systems to verify accuracy of the apparatus. 

In all spectra recorded, precautions were taken to avoid complications associated with the dichroisra inherent in the optics of the microscope and the monochromator. First, a polarization scrambler was inserted in the path of the Raman scattered light at the coupling between the microscope and the monochromator. Second, we did not change the polarization of the incident light directly but instead used a rotating stage to rotate the sample relative to the plane of polarization of the incident light. 

Figure 2-1 of Chapter 2 shows an experimental configuration for depolarization measurements in 90° scattering geometry. In this case, the polarizer is not used because the incident laser beam is almost completely polarized in the z direction. If a premonochromator is placed in front of the laser, a polarizer must be inserted to ensure complete polarization. The scrambler (crystal quartz wedge) must always be placed after the analyzer since the monochromator gratings show different efficiencies for L and polarized light. For information on precise measurements of depolarization ratios, see Refs. 21-24. 

Figure 8 Schematic layout of a typical 90° Raman depolarization experiment showing the positions of the polarization analyzer and the scrambler. The analyzer may simply be a polaroid sheet, which can be rotated by 90° to allow the parallel ( ) and perpendicular ( ) components of the scattered light to pass through to the detector. The function of a scrambler is to change linear into circular polarization of the light entering the Raman spectrometer slit in order to avoid measurement errors due to the variable spectrometer transmittance of the light polarized in different directions...

Fig. 6. Setup of the electrochemical cell in a 90° collection. WE working electrode, CE counter electrode, RE reference electrode, LA laser, PF pinhole filter, IF interference filter, M mirror, FL focusing lens, CL collection lens, DE degassing, PA polarization analyser, SC scrambler, MO monochromator.

The time-resolved fluorescence anisotropy function, rif), is calculated using Equation 1 in which / (0 and IJJ) are the individual decays collected with the polarization analyzer set parallel and perpendicular to the vertically polarized excitation light. The G factor is included to account for any polarization bias of the detection system. The influence of this term was minimized by arranging the polarization analyzer to be the first element in the detection system and using a polarization pseudo-scrambler (Oriel 28115) immediately prior to the emission monochromator slit. 

The main consideration for the collection lens is that it should have as low an / number as feasible in order to collect as much scattered light as possible. Also, the ratio of the collection lens radius to the distance between the lens and the entrance slit should be equal to the ratio of the radius of the first optical element in the monochromator to its distance to the entrance slit. This condition will ensure that the scattered light will fill the spectrometer optics, giving optimum sensitivity and resolving power. A scrambler, such as a calcite wedge, should be placed in front of the entrance slit to scramble the polarization, since monochromators have different sensitivities to light of different polarizations. For depolarization ratio studies, a polarization analyzer is placed between the collection lens and the scrambler. 

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