Photoinduced reorientation of nematic liquid crystals doped with an azo dye: a dynamic and steady-state study of reorientation and loss of liquid crystal order.

We compare photoinduced reorientation of homeotropic and planar aligned nematic liquid crystal 4-pentyl-4-cyanobiphenyl (5CB) doped with trace amounts of azo-dye disperse orange 3 (DO3) by studying the optical nonlinearities of the sample. Theoretical and experimental analyses confirm the proposal that the trans and cis isomers can be treated as independent contributors to the enhancement factor. Dynamic measurements indicate three contributions to photoinduced optical nonlinearities, two of which are isotropic and a third corresponding director reorientation. We also measure a large negative enhancement factor for the trans isomer and a positive enhancement factor for the cis isomer, consistent with previous measurements. The latter indicates that the mean field for the cis isomer is very small. Planar aligned samples demonstrate zenithal gliding whereas homeotropic samples do not. In addition, steady-state and dynamic measurements indicate loss of liquid crystal order associated with absorption as well as possible out-of-plane reorientation.