Optical Properties of Nematic Liquid Crystals

One of the most striking anisotropic properties of nematic liquid crystals is their optical anisotropy, which is manifested as birefringence. It was this property of liquid crystals that led to their discovery about 100 years ago, and early in the history of liquid crystals attempts were made to determine their birefringence. The first rough estimation was made by Lehmann in 1905 [1], who placed the nematic phase of 4,4 -bis(methoxy)azoxybenzene between a plane glass plate and a lens with a large radius of curvature. The birefringence 

Dorn [2] performed the first precise measurements of the principal refractive indices rif. and for the N and SmA phases of ethyl-4- [4-ethoxybenzylideneamino]-a-methylcinnamate using Abbe s total reflection method. Also later measurements on nematic liquid crystals by Gaubert [3] and Mauguin [4] were based on the determination of the boundary angle of total reflection. In 1950, Pellet and Chatelain [5] intro- 

An essential condition when measuring the birefringence of nematic liquid crystals is 

In Abbe s double prism method a thin ho-meotropically oriented nematic film is placed between the hypotenuse faces of two rectangular prisms. Using light polarized parallel and perpendicular to the optical axis, respectively, the principal refractive indices and can be obtained directly from the critical angle of total reflection, provided that the refractive index of the prism is greater than that of the liquid crystal [2, 8-10]. In some cases the director of the nematic phase was oriented uniformly parallel to the prism planes [11-14]. Special variants of the total reflection method have been described [3, 4, 15, 16]. A total reflection refractometer of the Abbe type has also been used to determine n, n and An in the infrared region between 2.5 and 14 p.m [17]. 

In the Leitz-Jelley microrefractometer the planar-oriented nematic liquid crystal is placed into a small hollow prism. The light passing through the prism is doubly refracted, and two images can be recorded on a scale. The two images are due to the ordinary and extraordinary rays [8, 28-31]. 

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Besides the direct electrical induction of electro-optical effects in liquid crystals, their activation by illumination of photoconductors could be of great technical interest. This method might well permit the electro-optical properties of nematic liquid crystals to be used on a larger scale, because photoconductor activation may eventually be applied to light amplification, optical data processing, and projection display systems, or used for recording phase-type holograms. 

The nonlinear optical properties of nematic liquid crystals have recently been studied by various authors. It has been shown that an intensity-dependent refractive index is due to the optical reorientation of the molecules. 

Non-linear optical properties of nematic liquid crystals... 

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