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Development of pixelated holograms recording strategies and aging study of different holographic photopolymer mechanisms for Augmented Reality applications

We found Holographic materials in a widespread field of applications and particularly in Augmented Reality (AR) area, which has been attracting attention for several years. Scientists have developed various complex holographic optical design for displaying clear and bright images in transparent devices. Holographic materials developed for this technology necessitate stringent optical properties such as photosensitivity, transparency, low cost and robustness. Photopolymer materials offer a reliable solution for these requirements. Our research team has recently presented a unique concept for AR applications that requires evaluating different photopolymers system in order to support our development. Among the photomaterials under test, we have studied in particular a photopolymer formulated in our own laboratory based on du Pont patents that contains N-Phenylmethacrylamide as monomer. This solution is not commercially available and has the advantage of a good transparency and wet chemistry. Another photopolymer under test, based on a different photochemistry mechanism, is the commercial product Bayfol® HX from Covestro available as laminated layers on triacetate cellulose film substrates. Our AR optical concept requires the use of pixelated holograms with a complex recording process that strongly depends on the inhomogeneous properties response of the photomaterial. In this paper, we describe the both photopolymer materials behavior during this holographic recording step. Then, we discuss about writing strategies implemented to improve the hologram homogeneity. In a second part, we evaluate the robustness of the holograms written in our photomaterial and we mesure their spectral stability under thermal stresses in order to extrapolate their natural aging. A comparison is made with the commercial product, it underlines that the robustness strongly depends on the nature of the polymer chemical formulation.