Dynamic Holographic Gratings Recorded by Photopolymerization of Liquid Crystalline Monomers

Dynamic liquid crystalline gratings composed of alternating permanent and alignable regions were prepared by photopolymerization of a liquid crystalline monomer mixture with interfering laser beams from an argon ion laser (488 nm). The 6-μm-thick gratings were reversibly switched to states with modulated orientation or modulated order by application of an electric field or by heating, respectively. The structure of the gratings, with periods ranging from 10.8 to 0.35 μm, was probed through holographic diffraction of 633-nm laser light. Several diffraction orders were observed from thin-regime gratings (period = 10.8 μm), and simulation of the observed intensities indicated that the grating refractive index profiles were between sine and square wave in shape. Images of the gratings obtained through optical microscopy also provided structural information and allowed direct evaluation of spatial homogeneity. The diffraction experiments also demonstrated the potential utility of the liquid crystalline media for holographic recording, which offer clean on-off switching of diffracted light (on-off intensity ratios in the 10²- 10³ range), minimal degradation after 10⁵ switching cycles, and holographic efficiencies up to the theoretical limit for thin gratings (about 34%).