Photoexcited azo-dye induced torque in nematic liquid crystals

We study molecule reorientation initiated by absorption of resonant light, in D2 azo dye doped nematic liquid crystals. The photoexcitation of the dye subsystem creates a torque on the host, that is not collinear with the traditional dielectric torque. The temporal and geometrical characteristics of these torques are also different. Transient three dimensional molecular reorientation is detected through different paths for excitation and relaxation regimes. The complex coupling of the photoisomerizing dye with the nematic host is probably the origin of the observed phenomena. (C) 1998 Elsevier Science B.V. All rights reserved.