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Nonlinear Ultics Of Liquid Crystals: Collective Reorientation Effects

The largest contribution to the nonlinear susceptibility of liquid crystalline materials arises from the effects of optical fields on orientational order. One mechanism of fundamental importance is the collective reorientation of the molecules by the radiation field which couples to the orientation via the polarizability anisotropy. The reorienting torque of the optical field is in general opposed by surface and elastic torques, and hence the response of the system depends on the sample geometry and surface anchoring effects. We consider the details of the interaction between the radiation field and the liquid crystalline medium and examine the nature of optical field induced elastic deformations. These reorientation effects can give rise to technologically important phenomena such as optical bistability and optical field induced scattering. We compare the predictions of theory with the results of recent experiments on homogeneous liquid crystals and on composite polymer dispersed liquid crystal (PDLC) films.