Anomalous intensity dependence of optical reorientation in azo-dye-doped nematic liquid crystals

Abstract

The intensity dependence of optical nonlinearity in a nematic liquid crystal doped with an azo-dye is investigated. The reorientational part of the nonlinearity changes from self-defocusing to self-focusing character while the intensity passes through the saturation value of trans-cis photoisomerization. This observation, in accordance with previous models, indicates that the optical torque generated by the trans-isomers is of opposite sign than the torque generated by the cis-isomers. At very low intensities a further reorientational nonlinearity was found, which is attributed to light-induced orientational redistribution of adsorbed molecules at the surface.

Light-Induced Director Reorientation in Nematic Liquid Crystals Doped with Azobenzene-Containing Macromolecules of Different Architecture

“…First of all, we notice that the phenomena observed for the lowmolar-mass dyes 1 0 and 2 0 are quite similar to those observed previously for azobenzene dyes [7][8][9][10][11] and can be explained by the photoconformational transitions of dye molecules. In this context, it seems possible that increasing the nonlinearity at complication of the molecular structure (passing from 1 0 to 1 and from 2 0 to 2) is due to suppression of the photoisomerization.…”

“…It was found later [7][8][9][10][11] that the nonlinearity sign (the sense of the light-induced director rotation) can be different for one and the same dye. Namely, the negative nonlinearity can transform into the positive one at decreasing the angle d formed by n and the field E of the extraordinary wave (by changing the angle of light incidence on a cell or passing from the homeotropic to the planar cell) and at increasing the light intensity.…”

Photoinduced gliding of the surface director in azo‐dye doped nematic liquid crystals

“…In studies of bulk reorientation, we have found that interpretation of photoinduced reorientation required the inclusion of gliding on the rubbed polyimide surface [11,12]. A drift in baseline measurements between experiments, which could not be associated with thermal drift, has also been common.…”

Magnetically induced Freedericksz transition and relaxation phenomena in nematic liquid crystals doped with azo-dyes

“…It is well known that when the number of cis isomers is increased, a corresponding reduction in the nematic order arises, eventually giving rise to isotropy [15,16]. Janossy et al [17] have shown experimentally and explained theoretically that the isomers trans and cis induced opposite light torques into a nematic matrix (E63 and Dispersed Orange 3).…”