BRAGG GRATINGS IN AN ACRYLATE POLYMER CONSISTING OF PERIODIC POLYMER-DISPERSED LIQUID-CRYSTAL PLANES

We report on the results of holographic studies in an acrylate photopolymer system containing a nematic liquid crystal. We demonstrate that Bragg gratings can be formed in this system and that the liquid crystal greatly enhances the diffraction efficiency. For a liquid crystal loading of 16.5 wt %, we have observed diffraction efficiencies approaching 100 %. With careful control of liquid-crystal loading and rapid postcuring, we have obtained optically clear gratings of high diffraction efficiency (>80 %) and narrow angular selectivity (<1-degrees) which are stable over a period of months. Low-voltage scanning electron microscopy reveals a unique morphology for these gratings, showing that they consist of periodic polymer-dispersed liquid-crystal planes. The liquid-crystal microdroplets are confined to the Bragg planes, having a size approximately equal to one-half the grating spacing. We also observe droplet elongation and coalescence along the Bragg planes.