Retardation achromatic

Retardation plates are frequently used in polarimetry instruments. Quarter-wave and half wave plates are most often used and may be fabricated in either wavelength-dependent or achromatic forms. 

Figure 9.5 Composite laminate of three retardation plates to form a single, achromatic...

Eq. (3.2-16) shows that the frequency dependence of 6 is manifested through the dependence of the refractive index n on the radiation frequency. Since the spectral range in which optical materials are used is far enough from their absorption bands the dependence of n and hence of 6 on the frequency is a slightly and smoothly changing function in this spectral interval. Therefore, totally reflecting retarders are almost achromatic optical devices. 

For the infrared circular polarization devices and techniques are not as readily available as e.g. in the visible range. Suitable retarders exist which exhibit virtually achromatic quarterwave behaviour from well above 4000 cm to about 700 cm (Korte et al., 1988), Sec. 3.2.3. Being based on internal total reflection they deviate the beam, nevertheless they render possible to take Fourier-transform spectra of a sample irradiated with circular polarized radiation as well as to analyze the polarization state of radiation... 

In order to improve the evaluation in the range where the cosine function is quite insensitive, an additional phase shift is introduced by placing a retarder in the reflected beam. Usually an internal-reflection prism is employed to produce a phase shift somewhere in the range from 30° up to 150°. Since the refractive index does not change much within the transparent spectral range, the phase shift produced by such a retarder is almost achromatic (Korte et al., 1988). For limited spectral ranges multilayer devices are also suitable (Rdseler and Molgedey, 1984). Provided a phase retardation of exact 90° is applied, one finds with the quotient Q 2 of 7 (45°) and 7 (135°)... 

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