Laser-induced configurational transitions in liquid crystals

Nematic liquid crystal director reorientation in a spatially varying temperature field has been investigated experimentally and theoretically. Both planar and homeotropically aligned E7-D27 samples have been studied. The nonuniform temperature field is produced by focusing a He-Ne laser on the dye doped cell. A physical model is proposed, where reorientation results from the coupling between the flexo-electric polarization and the DC electric field induced by the order-electric effect. The predictions of the model are compared with the result of experiment. The calculation of the free energy shows that this configurational transition is second order. The effect of director reorientation on absorption and on the far field diffraction pattern are discussed.; When a photon is absorbed by a molecule the angular momentum of the photon is transferred to that molecule, exerting a torque. The effect of such torques are observed in dye doped nematics when the incident laser beam is circularly polarized. A simple model is proposed to describe the resulting distortion. From the calculated director field the far field diffraction pattern is simulated. An experiment has been designed to observe this far field pattern.; Due to the large birefringences of nematics, a small director reorientation can produce large effect on the propagating light. In a high intensity optical field, the transverse modulation of the liquid crystal director has been investigated. The far field pattern observed consists of diffraction spots and has a periodic structure. The linear stability of the system has been examined. The measured transverse modulation threshold intensity is in good agreement with the predicted value.; Laser induced director reorientation is the source of the "giant" optical nonlinearity which is important for optical power limiting and optical information recording. In this research we have explored the potential applications of laser induced director reorientation. We have found that laser writing is possible in polymer stabilized cholesteric texture (PSCT) materials, with line widths as small as 5{dollar}mu m.{dollar} Laser writing was observed in both planar and focal conic states, with long term stability. These materials may therefore be useful for optical information storage applications.