Three-Dimensional Dye Distribution in Photo-Oriented Liquid-Crystal Alignment Layers

– The three-dimensional optical anisotropy of photo-buffed dye-doped polymer films and the resulting orientation imparted to a liquid crystal in contact are probed using total internal reflection. Although the linearly polarized writing light generates a uniaxial distribution of dye molecules, the polymer films are biaxial, a result of symmetry breaking by the film surface. Photo-induced anisotropy in dye-doped polymer films has great potential for application in optical storage [1] and in non-linear optics [2]. The dye molecules are attached to the polymer chain or dissolved in the polymer matrix and oriented using a writing beam. An important orientation mechanism is orientational hole burning via photo-induced trans-cis isomerization of the dye molecules [3]. Excitation of trans-isomers, with transition moment along the long axis, and orientational diffusion of the more mobile cis-isomers results in the depopulation of states with orientation along the pump beam polarization. Dye-doped polymer films also have the promise to replace mechanical buffing in the fabrication of LC displays [4]. It has been demonstrated that dye-doped Alignment Layers (ALs) allow the control of in-plane orientation of the liquid crystal with high spatial resolution [5] as well as tilted homogeneous anchoring of the LC [6]. In this letter, we use Total Internal Reflection (TIR) [7] to study both the dye distribution function in a thin polyimide film (tAL = 210 nm) and the resulting orientation of the LC molecules in the vicinity of this AL. The dye distribution is made anisotropic by illumination with green (514 nm) light. The dye molecules used in the experiment are elongated, with both the absorption dipole and the principal axis of the molecular polarizability tensor (corresponding to the largest polarizability) nearly parallel to the molecular long axis. Hereafter, the expression “dye distribution” refers to the distribution of this common axis. In the case