Photoinduced dichroism and light induced torque in absorbing liquid crystals

It has been shown that polarized light exerts a torque on the director of a nematic liquid crystal via interaction with light absorbing dye molecules dissolved in a liquid crystal matrix. Two guest-host systems were studied, one with a positive and the other with a negative dichroism. The former contains an azobenzene bridge. In the held off planar state the light induced dichroism was shown to be due only to the thermally induced decrease in the orientational order parameter. When the initial planar orientation is disturbed by an a.c. electric field exceeding the Frederiks transition threshold the additional photoinduced torque exerted on the director can be observed which is directed outward the light electric victor. The magnitude of the torque has been estimated.