Refractive chiral nematics

Note 3 Chiral nematic mesophases exhibit Bragg scattering of circularly polarised light at a wavelength proportional to the pitch P (Xr = P, where is the mean refractive index). 

As described earlier, the spectacular optical properties of chiral nematics are determined by the helicoidal pitch, the birefringence (and refractive index), the direction and polarization of the incident light, and the arrangement of the helix axis. For normally incident light the direction of the helix axis gives rise to the three classical textures depicted in Fig. 2, and typical photomicrographs taken with crossed polarizers are shown in Fig. 6 these are ... 

The refractive index data for n and n, calculated from n and (Eq. 11), may also be used to calculate the order parameter (5) in the chiral nematic phase using the Haller [56] technique with the Vuks local field correction factor [57] from... 

The change in the refractive index dn caused by the laser-induced temperature rise dT, described by the term (0 /6p) p in Equation (9.42), is often written in terms of the index gradient dn/dT. For most organic liquids, including isotropic liquid crystals, dn/dT is on the order of 10 " K. Figure 9.12 shows the temperature dependence of the liquid crystal TM74A (from EM Chemicals) as measured in our laboratory. The liquid crystal is a mixture of four chiral nematic materials in the isotropic phase. The measured dn/dT of the material is about 5X10 ... 

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