THE MORPHOLOGY AND PERFORMANCE OF HOLOGRAPHIC TRANSMISSION GRATINGS RECORDED IN POLYMER-DISPERSED LIQUID-CRYSTALS

Holographic transmission gratings are formed by the anisotropic visibl e laser radiation curing of a multifunctional acrylate monomer blended with the liquid crystal (LC) mixture E7. This results in an anisotrop ic spatial distribution of phase-separated LC droplets within the phot ochemically cured polymer matrix. The morphology of thin films (5-20 m u m) containing the gratings is examined by low-voltage, high-resoluti on scanning electron microscopy and transmission electron microscopy. Low concentrations of E7 (16% LC) coupled with rapid curing kinetics r esult in the formation of narrow LC-rich Bragg lamellae without well d efined boundaries. These LC-rich lamellae, with approximate widths of 100 nm, are composed of small droplets measuring 20-50 nm in diameter. Increasing the concentration of LC in the prepolymer mixture results in larger lamellae (canals) of LC-rich material. Films formed from a m ixture containing the highest LC concentration (34% LC) exhibited lame llae approximately 200-250 nm wide that are separated by polymer lamel lae that also possess a small fraction of phase-separated LC droplets. The other variable examined in detail, laser writing intensity, has l ittle effect on the morphologies exhibited in these films. The morphol ogy is related to the performance (diffraction efficiency, transmissio n, switching times and fields) through a simple model.