Compensators, polarized light quarter wave plate

The convenhonal spectrometer consisted of a Coderg double monochromator equipped with a cooled PMT and was described in detail by Brooker et al. (1994). A 1 W laser was required to obtain spectra with adequate signal to noise ratio. A half-wave plate controlled the polarization of the incident beam. The 90° scattered light was analyzed with Polaroid films with accepted parallel or perpendicular polarized light. A quarter wave-plate in front of the entrance slit served to compensate for grating polarization preference. 

Compensators are used in this context when the difference between fast and slow components is relatively small. On the Michel-Levy chart, there is a large band to the left where the interference colors are whiti and not very informative. A compensator adds a constant offset to the small offset created by the fiber. The labels on the bottom of the chart correspond to fixed ofifeets that can be added to increase the contrast. A compensator is a crystal that is inserted in the light path after the sample, but before the polarizer. Compensators are labeled by the amount of offset they add. A full wave-plate compensator adds 550 nm, for example, corresponding to the x-axis labels on the Michel-Levy chart. A quarter wave plate would add 137.5 nm. A full wave-plate compensator is also called a first-order red compensator, because of the red color it imparts to the background under crossed polars. 

Imperfections or nonidealities of the optical components in an ellipsometer can cause errors in A and il/. Quantitative analyses of errors have been made by Jerrard for inexactness of the quarter-wave plate, by Azzam and Bashara" for nonideal optical activity of the quarter-wave plate and surface roughness, and by Smith" for finite bandwidths of the monochromatic light source as well as the source polarization. Archer and Shank" and Yolken, Waxier, and Kruger considered the effect of multiple reflection within the compensator plate. It is reported that antireflection coatings on the surfaces of the compensator plate are beneficial. Aspnes" treated in a formal way the first-order effects from... 

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