Space charge-induced reorientation in polymethyne dye-doped nematics underexcitation with nanosecond laser pulses

Photoelectric phenomena and orientational nonlinearity have been studied wh ich are induced in planar liquid crystal layers oriented by SiO and doped w ith polymethyne dyes (0.1 wt.%) under irradiation with nanosecond laser pul ses. It was demonstrated that illumination of LC cells with Q-switched ruby laser pulses results in a drastic molecular reorientation which develops d uring 20 ms. This phenomenon is caused by photo-generation of surface and s pace charges in the LC cell and spatial distribution of space charges along the grating vector and beam propagation direction both with and without a DC field applied. A model is discussed which explains the orientational mec hanism of photo-excitation by emergence of electrohydrodynamic instability at presence of both unipolar injection and anisotropy of conductivity and d ielectric permittivity. (C) 2000 Elsevier Science B.V. All rights reserved.