Photorefractive effect and two beam energy exchange in hybrid liquid crystal cells
Seminar Room 1, Newton Institute
AbstractHybrid organic-inorganic photorefractive cells possess many advantages among photorefractive systems. In such cells a liquid crystal (LC) is sandwiched between two photorefractive layers. The interfering incident light beams induce a periodic space-charge field in the photorefractive layers. Space-charge electric fields leak into the adjacent LC, causing director modulation, and hence the diffraction grating. Each light beam diffracts from the induced grating, leading to energy gain and loss within each beam.
We developed a theoretical model to describe two-beam energy exchange in a hybrid nematic cell and calculated the energy gain of the weak beam, as a result of its interaction with the pump beam. In the theory, the flexoelectric mechanism for electric field-director coupling is a more important than the LC static dielectric anisotropy coupling. Thus, flexoelectricity is the main physical mechanism governing the magnitude of the director grating and the two-beam coupling. The LC optics is described in the Bragg regime.
The theory has been compared with results of an experimental study on hybrid cells filled with the LC mixture TL 205. Experimentally the energy gain is maximal at much lower grating wave numbers than is predicted by na´ve theory. However, if the director reorientation is cubic rather than linear in the space-charge field term, then good agreement between theory and experiment can be achieved using only a single fitting parameter. We provide a semiquantitative argument to justify this nonlinearity in terms of electric-field-induced local phase separation between different components of the liquid crystal.
Hybrid cholesteric systems exhibit extra features not observed in hybrid nematic cells. Specifically, the gain coefficient changes sign as a function of grating spacing. Following the paradigm used for hybrid nematic cells we developed a theory for the optical gain characteristics of hybrid cholesteric cells. Theoretical results for exponential gain coefficients have been compared with experimental results for hybrid cells filled with cholesteric mixtures TL205/CB15 and BL038/CB15. In order to reconcile theory and experiment, we require that near the cell surface, nematic ordering must dominate. Under this supposition, we are able to fit experimental data to theory for both cholesteric mixtures, subject to the use of some fitting parameters. This provides good evidence that the key physics of the system has been correctly identified.
*(joint work with Victor Reshetnyak, Tim Sluckin, Gary Cook, and Dean Evans)
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