Freedericks transition

Depending on the magnitude of the optical field, the growth of the reorientation process can be slow or fast. If we define an optical Freedericks transition field Ep,... 

E. C. Khoo, ""Optically induced molecular reorientation and third order nonlinear process in nematic liquid crystals. Phys. Rev., vol. A23, pp. 2077-2081, 1981 see also 1. C. Khoo and S. L. Zhuang, " Nonlinear light amplification in a nematic liquid crystal above the Freedericks transition, Appl. Phys. Lett., vol. 37, pp. 3-5, 1980 I. C. Khoo, ""Theory of optically induced molecular reorientation and quantitative experiments on wave-mixings and self-focusing of light, Phys. Rev., vol. A25, pp. 1636-1644, 1982 1. C. Khoo, ""Reexamination of the theory and experimental results of optically induced molecular reorientation and nonlinear diffraction in nematic liquid crystals spatial frequency and temperature dependence, Phvs. Rev., vol. A27, p. 2747, 1983. 

R. M. Herman and R. J. Serinko. ""Nonlinear optical processes in nematic liquid crystals near a Freedericks transition. Phvs. Rev.. vol. A19. pp. 1757-1769. 1969. 

I. C. Khoo, Wavefront conjugation with gain and self-oscillation with a nematic liquid crystal film, Appl. Phys. Lett., vol. 47, pp. 908-910, 1985 I. C. Khoo and S. L. Zhuang, Nonlinear light amplification in a nematic liquid crystal above the Freedericks transition, Appl. Phys. Lett., vol. 37, pp. 3-5, 1980. 

The Freedericks transition for nematic films in magnetic fields H. At low fields the molecules are parallel to the easy-axis of the waU. For fields higher than the threshold the molecules (at least those far from the surfaces) rotate parallel to the magnetic field, (a) splay, (b) twist and (c) bend. 

This agrees with the sample thickness dependence as observed by Freedericks. From the definition of the magnetic coherence lengtii (see (7.20)) it is seen that, at the threshold for the Freedericks transition, the magnetic coherence length is i(Ff,i) =. ... 

It is also easy to see that Freedericks transition can be induced by electric field, too. In analogy with (7.30) one can get the threshold for the transition as ... 

Splay and bend electric Freedericks transitions in (a) homogeneous and (b) homeotropic nematic cells with (a) positive and (b) negative dielectric anisotropies. 

The measuring field strength should be in the linear regime, i.e., the field should not cause observable reorientation in the liquid crystal. In nematics, it means voltages below the Freedericks transition (see Chapter 4), which practically means U < 1 V. In every serious measurement, care should be taken to verify that the measurements are in the linear regime. 

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