Laser-induced surface and bulk reorientation of the director in azo-dye-doped liquid crystal cells

The director reorientation in the liquid crystals (LCs) cells initiated by the trans-cis isomerization of ate-dyes was studied with the aid of pump-pr obe-beam experiments. Two different types of the samples were used, namely, a planar aligned LC layer, doped with an ate-dye, placed between substrate s coated with rubbed poly(vinyl alcohol) (PVA) fi;ms, and an undoped LC lay er placed between substrates, one of which (input) was coated with an untre ated ate-polymer film and other one was coated with rubbed PVA film. The ce lls were excited with nanosecond pulses of the second harmonic of an Nd:YAG laser, By comparing the reorientation dynamics with the help of dynamic ho lography method in both samples it was established that the values of devel opment times of the orientational effects were the same. The reorientation develops within 15-17 mus and the efficiency of the orientational gratings is 13-15%. In both kinds of samples the director orientation on the output surface with the rubbed PVA coating was not changed after irradiation. Cont rary, the orientation of the director on the input substrate was changed in both cases. In case of a LC cell with azo-modified surface the reorientati on is due to the photoisomerization of the ate-polymer film, In case of the cell with azo-dye-doped LC layer the photoisomerization of dye in the bulk of LC together with PVA film rearrangement on input substrate exert an inf luence on the director reorientation. (C) 2001 Elsevier Science B.V. All ri ghts reserved.