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SUMMARY:Photorefractive effect and two beam energy exchange in hybrid liqu
 id crystal cells - Pinkevych\, I (Taras Shevchenko National University of 
 Kyiv)
DTSTART:20130523T110000Z
DTEND:20130523T120000Z
UID:TALK45482@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION:Hybrid organic-inorganic photorefractive cells possess many ad
 vantages among photorefractive systems. In such cells a liquid crystal (LC
 ) is sandwiched between two photorefractive layers. The interfering incide
 nt 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 d
 iffracts from the induced grating\, leading to energy gain and loss within
  each beam.\n\nWe developed a theoretical model to describe two-beam energ
 y 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 theo
 ry\, 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 describe
 d in the Bragg regime.\n \nThe theory has been compared with results of an
  experimental study on hybrid cells filled with the LC mixture TL 205. Exp
 erimentally the energy gain is maximal at much lower grating wave numbers 
 than is predicted by nave theory. However\, if the director reorientation 
 is cubic rather than linear in the space-charge field term\, then good agr
 eement between theory and experiment can be achieved using only a single f
 itting parameter. We provide a semiquantitative argument to justify this n
 onlinearity in terms of electric-field-induced local phase separation betw
 een different components of the liquid crystal.\n \nHybrid cholesteric sys
 tems exhibit extra features not observed in hybrid nematic cells. Specific
 ally\, the gain coefficient changes sign as a function of grating spacing.
  Following the paradigm used for hybrid nematic cells we developed a theor
 y for the optical gain characteristics of hybrid cholesteric cells. Theore
 tical results for exponential gain coefficients have been compared with ex
 perimental results for hybrid cells filled with cholesteric mixtures TL205
 /CB15 and BL038/CB15. In order to reconcile theory and experiment\, we req
 uire that near the cell surface\, nematic ordering must dominate. Under th
 is supposition\, we are able to fit experimental data to theory for both c
 holesteric mixtures\, subject to the use of some fitting parameters. This 
 provides good evidence that the key physics of the system has been correct
 ly identified.\n \n*(joint work with Victor Reshetnyak\, Tim Sluckin\, Gar
 y Cook\, and Dean Evans)\n
LOCATION:Seminar Room 1\, Newton Institute
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