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

Holographic transmission gratings are formed by the anisotropic visible laser radiation curing of a multifunctional acrylate monomer blended with the liquid crystal (LC) mixture E7. This results in an anisotropic spatial distribution of phase-separated LC droplets within the photochemically cured polymer matrix. The morphology of thin films (5-20 mu m) containing the gratings is examined by low-voltage, high-resolution scanning electron microscopy and transmission electron microscopy. Low concentrations of E7 (16% LC) coupled with rapid curing kinetics result in the formation of narrow LC-rich Bragg lamellae without well defined 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 mixture containing the highest LC concentration (34% LC) exhibited lamellae approximately 200-250 nm wide that are separated by polymer lamellae that also possess a small fraction of phase-separated LC droplets. The other variable examined in detail, laser writing intensity, has little effect on the morphologies exhibited in these films. The morphology is related to the performance (diffraction efficiency, transmission, switching times and fields) through a simple model.