Organic materials for real-time holographic recording

Photoactive organic layers, such as photochromic polymers and photopolymers, are investigated for real-time and in-situ holographic recording. The results with photochromic polymers containing spiropyran and spirooxazine dyes revealed that variations in the UV beam excitation and visible recording beam procedures significantly changed the exposure sensitivity and diffraction efficiency. These effects were exploited for all-optical modulation of the holographic gratings and explained by the photochemical and thermal transformations between photochromic stereoisomers. Copolymers of the photochromic spiropyrans were investigated for optical recording with infrared laser radiation at 10.5 and 10.6 μm. Although the recording primarily occurs by thermal bleaching, some nonthermal IR processes are also involved. Holographic recording in photopolymer layers, based on acrylamide monomers dissolved in polyvinylalcohol, is influenced by chemical additives. Specifically, a superadditive sensitization effect of diphenyl iodonium chloride together with triethanolamine significantly increased the exposure sensitivities at 514 nm, by a factor of more than 3 (to about 15 mJ/cm 2 ). Several formulations produce large enough refractive index modulations so that very high diffraction efficiencies (DE > 90%) are obtained. For certain conditions, under highly asymmetric recording angles, the diffraction efficiency is significantly reduced, and was found to originate from fringe bending due to nonlinear shrinkage. The introduction of crosslinking and gelling agents stabilize the formed grating structures against dimensional distortions.