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

Abstract

The director reorientation in the liquid crystals (LCs) cells initiated by the trans– cis isomerization of azo-dyes was studied with the aid of pump–probe–beam experiments. Two different types of the samples were used, namely, a planar aligned LC layer, doped with an azo-dye, placed between substrates coated with rubbed poly(vinyl alcohol) (PVA) films, and an undoped LC layer placed between substrates, one of which (input) was coated with an untreated azo-polymer film and other one was coated with rubbed PVA film. The cells were excited with nanosecond pulses of the second harmonic of an Nd:YAG laser. By comparing the reorientation dynamics with the help of dynamic holography method in both samples it was established that the values of development times of the orientational effects were the same. The reorientation develops within 15–17 μs 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. Contrary, 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 reorientation is due to the photoisomerization of the azo-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 influence on the director reorientation.