Fresnel rhomb

Figure 9.4 (a) Fresnel rhomb for quarter-wave retardation, (b) Double Fresnel rhomb... 

Achromatic half-wave plates can be easily made from the combination of two quarter-wave Fresnel rhombs shown in Figure 9.3b. This geometry has the nice feature of removing the translation of the light beam that is associated with a single rhomb. 

Figure 2. Cotton s experimental apparatus for measuring absorption coefficients for left-and right-circularly polarized light. p1 incoherent source Lj, L2, focusing lenses Nj, N2, Nicol polarizing prisms M2, limiting apertures Fj, F2, quarter-wave Fresnel rhombs S, sample cell p2, location for light detection (visual). From ref. [2].

Cotton also investigated the use of ellipticity measurements for CD studies. The combination of a linear polarizer and a Fresnel rhomb was used by Cotton to produce polarized light which could vary in its orientation from circular to elliptical. A second linear polarizer was then used to monitor changes in the orientation of the major axis of the ellipse which occurs as a result of the CD. In addition, the apparatus could also be used to measure optical rotation (circular birefringence). The experimental simplicity of this approach relative to his other system allowed more sensitive measurements to be made. This experimental simplicity also served as an impetus for other researchers in the field, and the ellipticity approach dominated the available technology from the time of Cotton s first efforts to the 1960 s, when electronically modulated systems were developed. 

Some attention has to be given to the degree of circular polarization y and thus to the creation of cpl. Two experimental principles are common the use of a quarter wavelength plate and the use of a Fresnel rhomb or a Solcil-Babinet compensator in a few experiments, radiation from a cyclotron after passage through a polarizing undulator was used. The degree of circular polarization is easily jeopardized when a X/4 plate is employed, as this is an interference device... 

As a first step towards the measurement of single molecule effects, Schrader and Korte (1972) reported the measurement of the infrared rotatory dispersion of carvone in liquid crystalline solution. They used a modified commercial spectrometer. They observed a huge effect which is not the result of the carvone itself but of the liquid crystal in which a helical arrangement (cholesteric state) is induced by the chiral solute (Sec. 4.6.4). In this case the liquid crystal acts as a kind of molecular amplifier which allows the absolute configuration of tiny amounts of solutes to be determined reliably. At about the same time Dudley et al., (1972) measured the infrared circular dichroism of (-)-menthol in a liquid crystal. Their equipment consisted of a normal infrared spectrometer supplemented by a Fresnel rhomb made from sodium chloride. 

Thus, the polarization of both the incident and scattered light must be controlled. A typical argon laser produces linearly polarized light. The incident polarization can be adjusted to be either vertical or horizontal with respect to the scattering plane with a double Fresnel rhomb polarization rotator. The scattered light can be analyzed with a Glan-Thomp-son prism polarizer. 

The basic apparatus that is used for measuring circular dichroism is shown in figure 1.2 It consists of a light source, a linear polarizer, a Fresnel rhomb that converts the linear polarized light to circularly... 

The rotation of the linear polarizer 45°, to the appropriate Fresnel rhomb axis, induces the generation of left or right polarized light. 

One such device for such resolution is called a Fresnel rhomb and depends on correctly choosing angles of a rhomboid to allow internal reflection of the plane-polarized light beam from two glass-air surfaces. More modern devices depend on electrical induction of mechanical stress. 

Optical retarder plates, of appropriate thicknesses, made of birefrin-gent material, normally crystalline quartz or mica, can be used to rotate the plane of polarization for linearly polarized light or to produce circularly polarized light. To rotate linearly polarized light by an arbitrary angle, a double Fresnel rhomb is very useful (Fig.6.46). 

The experimental apparatus is depicted in Fig. 3 and is described in detail elsewhere [14]. It consists of a molecular beam scattering chamber, pulsed frequency-doubled dye laser, double fresnel rhomb polarization rotator, fibre bundles to coUect fluorescence from excited states, and time-gated photon counting and boxcar averaging equipment. 

---