Molecular Reorientations in Anisotropic Liquids

In Equation (8.13) the angle brackets containing the factor (cos G — -j) signify that is a macroscopic parameter and an ensemble average it can be expressed in terms of the (induced) order parameter Q = ( -cos 0 — y) by 

From Equation (8.16) one can see that the molecular orientational nonlinearity in the isotropic phase of a liquid ciystal is directly proportional to the laser-induced order parameter Q. In typical anisotropic liquids (e g., CS2 or liquid crystals at temperatures far above Tq), the value of Q may be obtained by a statistical mechanics approach. In the completely random system, the average orientation is described by a distribution function G)  

Besides these molecular motions, an optical field could also induce other types of orientation effects in liquid crystalhne systems, for example, nuclear reorientation caused by the field-induced nuclear orientational anisotropy. This process is sometimes referred to as the nuclear optical Kerr effect as it results in an optical intensity-dependent change in the optical dielectric constant. Such effects in liquid crystals have been investigated by Deeg and Payer. In general, these nuclear 

For liquid crystals, owing to pretransitional effects near Tq, the indnced ordering Q exhibits interesting correlated dynamics and temperature-dependent effects. 

In general, short intense laser pulses are required to create appreciable molecular alignment in liquids. To quantitatively describe the pnlsed laser-induced effect, a time-dependent approach is needed. In this regime /(0) obeys a Debye rotational diffusion equation  

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