Absorption-induced optical reorientation in nematics doped with azo-dyes

Abstract

Dye-induced optical reorientation in nematic liquid crystals (NLCs) has generated a lot of research in recent years. It was found that the addition of small amounts of absorbing dyes to NLCs (typically less than 1%) significantly increases the optical torque, in some cases by more than two orders of magnitude. Molecular models have been worked out in order to explain this effect. These models have a common starting point: the fact that in the presence of absorption the orientational distributions of the ground-state and excited-state dye molecules are not axially symmetric around the director. Due to this asymmetry, the molecular mean field associated with the dye molecules exerts an effective torque on the director. In Phys. Rev. E 49, 2957 (1994) the change of the dye-host interaction energy upon excitation was considered as the main source of the absorption-induced torque. In Phys. Rev. E 56, 1765 (1997) it was shown that the difference between the rotational mobility of the ground-state and excited-state molecules can contribute also to the enhancement of the optical torque.
In this paper, we study this absorption-induced mechanism using a planar aligned guest-host mixture composed of a NLC doped with an azo-dye. The absorption-induced optical torque introduces an index change in the sample and transmission measurements are used to monitor changes in the orientation of the liquid crystal director. A qualitative magnitude of the optical torque is then evaluated with respect to dye content of the samples.