Polymer-stabilized cholesteric diffraction gratings: Effects of UV wavelength on polymer morphology and electrooptic properties

We compare the performance of polymer-stabilized cholesteric liquid crystal diffraction gratings for two limits of the polymer morphology: a one-dimensional array of polymer walls that extend through the grating thickness ("bulk network"), and a thin layer of patterned polymer fibrils localized at one surface ("surface network"). In each case, the polymer is spatially templated by the liquid crystal orientational order, but the depth of the network is dictated by UV absorbance of the liquid crystal and a choice of wavelength used to initiate photopolymerization. Whereas both polymer morphologies yield robust electrically switchable gratings, the surface-stabilized grating operates at lower field thresholds and shorter transition times. However, the patterning of the surface network significantly reduces the contrast between grating on and off states, a limitation that can be potentially offset by employing Isotropic monomers index-matched to the liquid crystal.