Optical Kerr Effect and Transient Laser-Induced Molecular Reorientation

In contrast to nematics, a helical twist of the molecular director takes place in the chiral nematic phase. Studies of the spin-lattice relaxation in chiral nematics have shown that the relaxation mechanisms are essentially the same as in pure nematics [141, 142]. At high Larmor frequencies the relaxation is diminished by molecular self-diffusion and by local molecular rotations, whereas director fluctuations determine the relaxation rate at low Larmor frequencies. This can be easily understood because the spin-lattice relaxation rate in the MHz region is dominated by orientational fluctuations with wavelength much smaller than the period of the helix. The influence upon the rotating frame spin-lattice relaxation time Tip of the rotation of the molecules due to diffusion along the helix, an effect specific for twisted structures, has not been observed in COC [143]. 

7 Optical Kerr Effect and Transient Laser-Induced Molecular Reorientation 

The first observation of optical-field induced ordering in the isotropic phase of a nematic liquid crystal was reported by Wong and Shen [74]. They have used a linearly polarized Q-switched, 50 kW ruby laser pulse to induce molecular ordering in the isotropic phase of a nematic liquid crystal. The in- 

Frost and Lalanne [77] have performed similar experiment in MBBA. The molecular theory of orientational fluctuations and Kerr effect can be found in the work of Flytzanis and Shen [76]. The optical Kerr effect in the alkoxyazoxybenzene homologous series is reported by Hanson et al. [78] and the same technique was used by Coles [79] in the studies of the pretransitional dynamics of alkyl cyanobiphenyl homologs. 

In the nematic state, the optical field of the pump beam couples to the anisotropy of the index of refraction An and exerts a torque on the molecules. This results in several interesting nonlinear phenomena, such as the optically induced Fredericksz transition [144, 145] and transient induced molecular reorientation [146]. The dynamics in the vi- 

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Quasielectric Light Scattering and Order Fluctuations in the Isotropic Phase 174 Nuclear Magnetic Resonance and Order Fluctuations in the Isotropic Phase. 175 Quasielastic Light Scattering and Orientational Fluctuations below Tc. . . 177 Nuclear Magnetic Resonance and Orientational Fluctuations below Tc.. .. 177 Optical Kerr Effect and Transient Laser-Induced Molecular Reorientation.. 181... 

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