Depth attenuated refractive index profiles in holographic gratings recorded in photopolymer materials

Abstract

Photopolymers are systems of organic molecules that rely on photoinitiated polymerization to record volume phase holograms. Characteristics such as good light sensitivity, large dynamic range, good optical properties and relatively low cost make photopolymers one of the most promising materials for write-one, read-many (WORM) holographic data storage applications. Thus, it is interesting to understand the mechanisms that control the way information is stored in photopolymer materials. Different authors have demonstrated that two processes play the main role in hologram formation: monomer polymerization and monomer diffusion. A number of models based on these two processes have been proposed and their prediction capability has been validated. In this work we extend the capabilities of the existent models by introducing another important characteristic: the attenuation of light through the depth of the material which happens in the recording process. In order to check the validity of the theoretical model that we propose, volume phase transmission gratings are recorded in a PVA/Acrylamide photopolymer with different spatial frequencies. Using the Rigorous Coupled Wave Theory (RCWT) we show that we can obtain information about the higher harmonics in the recorded refractive index modulation. Comparison between simulated and experimental results validates the interpretation provided by the proposed model.