Korniychuk P. Surface-mediated Director Photo-reorientation of 4-n-pentyl-4'-cyanobiphenyl NLC in DC-field

Light-induced reorientation of nematic liquid crystal (NCL) 4-n-pentyl-4'cyanobiphenyl (5-CB) in a dc-field is studied. Specifically the results of investigation of light-induced twist-effect, photorefractive effect, hidden grating effect, as well as processes of free-charges' redistribution in the NLC cell and their influence on liquid crystal orientation are presented. We found that laser beam with wavelength of 440 nm and with intensity higher than 10 mW/cm2 decreases the threshold voltage of the twist effect in the cells filled with 5-CB. The light-induced twist effect in a dc-field in such cells was found. To explain the observed effect we propose a model of optical control of surface charges. We have produced optical limiter in the milliwatt range based on the light-induced twist effect. We observed that in homeotropically aligned 5-CB cells with ITO electrodes photorefractive gratings can be written by irradiating them by intensity-modulated light. Main characteristics of the photorefractive gratings (dynamic characteristics, diffraction efficiency dependence on external electric field, incidence angle and polarization of the test beam) are presented. During the irradiation of the grating we observed the coupling of the beams, which indicates that there is a phase shift between the modulation patterns of light intensity and refractive index. The direction of the energy transfer from one recording beam to the other can be controled by dc-field polarity and orientation of the recording beams. It was found that optimal parameters of photorefractive grating recording correlate with parameters of free charge redistribution in NLC cells, namely time of grating recording is approximately equal to the time of charge redistribution in dc-field; dc-voltage giving maximum of grating diffraction efficiency is equal to dc-voltage of Helmholtz layer formation. This correspondence confirms that surface charges play essential role in recording photorefractive gratings in homeotropic NLC cells.