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The trans-cis isomerization of azo dyes in liquid crystalline hosts is studied. It is shown that the full set of parameters governing the isomerization process can be deduced from polarized pump-probe transmission measurements. The results indicate that the dye order parameter for the trans isomer is relatively high and is strongly related to the liquid-crystalline order. The cis isomer exhibits a much lower dye order parameter and is not strongly dependent on the liquid-crystalline phase. The barrier to the thermal cis-trans relaxation is found to be relatively unaffected by the liquid-crystalline phase, while changes are observed in the pre-exponential factor.

Optical reorientation measurements in dye-doped nematic liquid crystals are reported. The dye-induced optical torque exceeds significantly the normal optical torque for all anthraquinone dyes investigated. With one of the dyes, optical Freedericksz transition in a planar layer is demonstrated.

The optical reorientation of nematic liquid crystals doped with azo dyes is studied. It is shown that in the presence of absorption both the trans and cis isomers enhance significantly the optical torque; they contribute, however, with opposite signs to the net enhancement and tend to cancel each other's effect. The anomalous angular dependence of optical reorientation reported by Zolotko et al. is explained by considering the photoinduced trans-cis equilibrium. ͓S1063-651X͑98͒03510-7͔

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We provide experimental evidence that photoalignment at the nematic liquid crystal (NLC) -polymer interface can not be simply considered as a twodimensional process. Moreover, our experiments clearly indicate that the photoaligning process does not depend on the individual properties of the NLC material and those of the interfacing polymer exclusively. According to our measurements, the polymer and the NLC layer "sense-each-other", i.e., the polymer-liquid crystal interface should be regarded as a coupled system, where the two components mutually influence each other. Furthermore, we show that the temperature induced anchoring transition also has to be taken into account for the complete description of the photoalignment mechanism.

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