Photorefractivity in polymer-stabilized nematic liquid crystals

The charge generation and transport properties of nematic liquid crystals doped with electron donors and accepters are shown to be strongly modified by the addition of low concentrations of a polymeric electron acceptor. These new liquid crystalline composites exhibit significantly enhanced photorefractive properties. The new composites are produced by polymerizing a small quantity of a 1,4:5,8-naphthalenediimide electron acceptor functionalized with an acrylate group in an aligned nematic liquid crystal. Photopolymerization creates an anisotropic gellike medium in which the liquid crystal is free to reorient in the presence of a space-charge field while maintaining charge-trapping sites in the polymerized regions of the material. The presence of these trapping sites results in the observation of longer lived, higher resolution holographic gratings in the polymer-stabilized liquid crystals than observed in nematic liquid crystals alone. These gratings display Bragg regime diffraction. Asymmetric beam coupling, photoconductivity, and four-wave mixing experiments are performed to characterize the photophysics of these novel materials. The mechanism of charge transport in the polymer-stabilized liquid crystals is also discussed.