Agar is a natural polysaccharide which, when doped with dichromate ammonium, can be considered as a promising
light sensitive material used for real time hologram recording. The volume transmission gratings were recorded with a
Kypton laser at 413 nm and they were read in real-time with a He/Ne laser at 632.8 nm contrary to dichromated gelatin.
The so obtained holograms formed were phase holograms due to a refraction index modulation. The optimisation of
chemical and physical parameters was investigated in order to form high quality holograms. It was demonstrated the
crucial role played by the remaining water in the final film on the value of the diffraction efficiency. In the optimal
conditions, a maximum diffraction efficiency of 37 % was attained. Both on-off experiments and the storage of the
exposed materials at room temperature and in the dark reveal that the holograms were stable. An attempt to rationalize
the set of results in terms of chemical structure of the polymeric matrix and of its ability to stabilize chromium (V) is
A comparative investigation of the photochemical behavior of dichromated gelatin (DCG), dichromated polyacrylic acid (DCPAA) and dichromated polyvinylalcohol (DCPVA) gave evidence for the similarity of the photochemical behaviour involved when recording holograms in DCG and DCPVA. Chromium (V), the chromium species resulting from the
photoredox process between the starting chromium (VI) (dichromate) and the polymeric chain, was stable likely strongly complexed in gelatine and polyvinylalcohol. The quantum yields of the reaction are high and similar for both reactions. On the contrary, even though chromium (V) was also formed during the primary process in DCPAA, this species was quite unstable and disappeared as soon as the irradiation was stopped. The behavior observed in DCG can be assigned to the presence of OH groups on the polymeric chain, these groups being the only groups present in polyvinylalcohol. Actually, the stability and the state of complexation of the different chromium species was completely opposite in the two matrices: the stabilization of chromium (V) resulting from the photochemical charge transfer in DCPVA, by complexation with PVA is in contrast to what was observed in DCPAA where chromium (V) was highly instable. Regarding chromium (III), the final reduction chromium species, it was complexed in PAA and not in PVA. The experiments performed on films of DC (PVA+PAA) with only a few percents of PAA gave evidence for the strong influence of the presence of the carboxylic groups on the photochemical behavior: the rate of the formation and the
stability of chromium species that reflects the chelating properties of the polymeric matrix. The photochemical results were correlated to the results obtained when recording holograms in the corresponding material: a very nice agreement appeared between the two sets of results. An innovative approach combining the monitoring of the structural modification of the polymeric matrix and the fate of the various chromium species ((VI), (V) and (III)) was then implemented. For the first time, it was established that chromium (V) was at the origin of the cross-linking implied in the hologram formation by acting as a bridge between hydroxyl groups of the polymeric chains. A second unanswered question was also elucidated. The improvement brought by ammonium dichromate with respect to potassium dichromate involves amide groups as additional chelating sites for chromium (V) resulting in the increase of the matrix cross-linking.
The photochemical behavior of dichromated polyvinylalcohol (DCPVA) films was analysed upon exposure at 365 nm in connection with the hologram quality recorded in such a photosensitive material. The evolution of both involved species, chromium and polyvinylalcohol, were quantified by implementing an innovative approach. This approach combines the monitoring of the structural modification of the polymeric matrix and the fate of the various chromium species ((VI), (V) and (III)). For the first time, it was established that chromium (V) was at the origin of the cross-linking implied in the hologram formation by acting as a bridge between hydroxyl groups of the polymeric chains. A second unanswered question was also elucidated. The improvement brought by ammonium dichromate with respect to potassium dichromate involves amide groups as additional chelating sites for chromium (V) resulting in the increase of the matrix cross-linking.
Irradiation of dichromated polyvinyl alcohol and dichromated polyacrylic acid DC(PVA-PAA) at 365 nm was
investigated in order to understand the involvement of the polymeric matrix in the reduction process of the Cr(VI)
leading to the formation of the hologram. The photochemical evolution of the matrix was directly correlated to the
disappearance of the absorbing species, chromium(VI). A special attention was paid to the absorption profile of the
films. Due to the impermeability of the polymer, no oxidation proceeded. The reduction of Cr(VI) into Cr(V) induced the
formation of carboxylate species perfectly correlated with the consumption of carboxylic groups. Besides the acido-basic
reaction undergone by the carboxylic groups, the reduction of Cr(VI) provoked the cross-linking of the polymer. Each
polymer appears to play a specific role in the mechanism.