Refractive index, electric-field-induced anisotropy

The Kerr and Cotton-Mouton experiments stand apart from the others in the sense that it is the electric-field-induced anisotropy in the refractive index which is measured (i.e. the difference in two quantities) rather than a single quantity such as an intensity. This means that, e.g., for the Cotton-Mouton effect, the experimentally-useful quantity is... 

The first observation of natural optical anisotropy was made in 1669 by Bartolinius in calcite crystals, in which light travels at different velocities depending on the direction of propagation relative to the crystal structure. The electrooptic effect, electric-field-induced anisotropy, was first observed in glass in 1875 by J. Kerr. Kerr found a nonlinear dependence of refractive index on applied electric field. The term Kerr effect is used to describe the quadratic electrooptic effect observed in isotropic materials. The linear electrooptic effect was first observed in quartz crystals in 1883 by W. Rontgen and A. Kundt. Pockels broadened the analysis of this relationship in quartz and other crystals, which led to the term Pockels effect to describe linear behavior. In the 1960s several developments... 

An optically isotropic liquid crystal (LC) refers to a composite material system whose refractive index is isotropic macroscopically, yet its dielectric constant remains anisotropic microscopically [1]. When such a material is subject to an external electric field, induced birefringence takes place along the electric field direction if the employed LC host has a positive dielectric anisotropy (Ae). This optically isotropic medium is different from a polar Uquid crystal in an isotropic state, such as 5CB (clearing point = 35.4°C) at 50 C. The latter is not switchable because its dielectric anisotropy and optical anisotropy (birefringence) both vanish in the isotropic phase. Blue phase, which exists between cholesteric and isotropic phases, is an example of optically isotropic media. 

Electric field-induced birefringence, also known as Buckingham birefringence, is used for the experimental determination of molecular quadrupole moment. In the case of infinite dilute solutions, the Buckingham birefringence molar constant, which is related to the anisotropy of the refractive index induced by a field gradient, reads ... 

The role of push-pull molecules in PR systems is to provide refractive index modulation in response to an electric field. In polymers with a low glass transition temperature (Tg), where the molecules are relatively free to rotate due to a higher free volume, the chromophores can reorient in the applied/generated electric field. Because of the anisotropy in polarizability of the chromophores, such a reorientation will induce high refractive index anisotropy termed as orientational birefringence. The ability of a chromophore molecule to induce refractive index change is usually expressed as the figure of merit (FOM) defined by Equation 8.4 [44],... 

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