Michel Bolte

Photochromic materials for holographic data storage

Photochromism and real-time holographic recording were characterized for two principal classes of photochromic compounds: spiropyrans and fulgides. For spiropyran molecules, important thickness and writing intensity effects were observed. Concerning fulgide, in PMMA matrix, the closed form presents a maximum of absorption centered at 525 nm upon irradiation at 365 nm. We have determined the photoreaction rate constants kUV and kvis respectively for the coloring and bleaching process: kUV equals 1.2 X 10-3 s-1 and kVIS equals 11.1 X 10-3 s-1. Photochemical fatigue resistance in different polymer matrices was investigated. We found a loss of 9, 11, 13 and 35 percent respectively in PS, CA, PMMA and PVK. Concerning holographic recording, we obtained diffraction efficiency (eta) equals 0.65 percent in PMMA films 30 micrometers thick.

Holographic characterization of fulgides doped polymer films

Dynamic holographic recording was performed on two fulgides (A540 and A670)/Doped films. Maximum diffraction efficiency of 7% and 6% has been achieved on A540 and A670 doped PEPC/PS films. The effects of the matrix, writing intensity and film thickness on diffraction efficiency were studied. Holographic fatigue resistance in polymer matrix and epoxy resin has been investigated. It was found that holographic fatigue of fulgide doped in epoxy resin is closed to zero at least after 40 Write-Read-Erase cycles (WRE).

Characterization of dye-doped PMMA/PVK films as recording materials

The photoinduced reversible color change in photochromic doped PMMA and PVK films was investigated. Upon UV and visible irradiation, closed form absorbency, in the beginning of the photoreactions, followed first-order kinetic and, then deviated from this order. The rate- constants kuv of the coloring process, and kvis of the bleaching process, were determinated. For the two diarylethenes studied, kuv is much larger in both polymer matrices. However, kvis is stronger in PVK for fulgide Aberchrome 670. Both rates are identical in PMMA. Photochemical fatigue resistance was studied. For fulgide Aberchrome 670 in PMMA matrix, we found 13 percent disappearance after 10 repeated UV and visible cycles.

Characterization of dye-doped PMMA, CA, and PS films as recording materials

The photoinduced reversible color change and in-situ recording of fulgide Aberchrome 670 doped polymethyl methacrylate (PMMA), cellulose acetate (CA) and polystyrene (PS) were investigated. Upon UV and visible exposure, closed-form absorbency followed first-order kinetic. The rate constants KUV and KVIS for respectively the coloring and bleaching process were determined. In PMMA matrix KUV equals 1.2 * 10-3 s-1 and KVIS equals 11.1 8 10-3 s-1, in CA matrix kUV equals 2.7 * 10-3 s-1 and kVIS equals 6.4 8 10-3 s-1 and in the case of PS film kUV equals 2.1 * 10-3 s-1 and kVIS equals 11.9 * 10-3 s-1 were obtained. These results show that, KVIS is much larger than KUV for all matrices. Photochemical fatigue resistance in different polymer matrices was investigated. We found a loss of 9, 11 and 13 percent in PS, CA and PMMA respectively, after 10 repeated UV and visible cycles. The real time holographic recording in fulgide doped PMMA films were studied. We have analyzed the effect of the photochromic concentration, the thickness of the film and the recording intensity on the diffraction efficiency. The highest diffraction efficiency is obtained for the concentration of 5 percent of the fulgide dye in PMMA film with an exposure energy of 10 mw/cm2. For the same sample we have not observed any diffraction beam when the sample was illuminated by an intensity of 3 mw/cm2.

Matrix effects on the photochemical fatigue resistance of fulgide-doped polymers

Fulgide Aberchrome 670-doped polymer films are studied. The closed form of this chromophore presents a maximum of absorption centered at about 525 nm and is formed upon irradiation of its colorless open form at 365 nm. We have determined the photoreaction rate constants, kUV and kVIS, respectively, for the coloring and bleaching processes. It was found that photochemical fatigue of fulgide doped in the mixture of a cyanoacrylate adhesive with 10% of dioctoepoxy monomer is essentially absent after 40 UV/visible irradiation cycles. We thus report, for the first time, a simple and rapid preparation of dye-doped polymer films in a solvent-free process that shows significant improvements in fatigue behavior. Our previous investigation of conventional polymer matrices prepared by gravity deposition of dilute dye-polymer solutions found losses of 9, 11, 13, and 35% after only 8 UV/visible irradiation cycles, respectively, in polystyrene (PS), cellulose acetate (CA), poly(methyl methacrylate) (PMMA), and polyvinyl carbazole (PVK).

Characterization of the fulgide-doped PMMA films and investigation of photochromic reaction of Langmuir-Blodgett films as recording materials

Photochemical characterization and holographic recording of fulgide Aberchrome 670 and 540-doped polymethyl methacrylate (PMMA) were investigated. Upon UV and visible exposure, closed-form absorbency followed first-order kinetic. The real time holographic recording in fulgides doped PMMA films were studied. The effect of dye concentration, thickness of the film and the recording intensity on diffraction efficiency was reported. We used the Langmuir-Blodgett (LB) technique in order to transferring a compact multilayer of fulgide spread on water surface between two thin films of cellulose acetate (CA). The preliminary results of the surface pressure-area isotherms obtained by LB show the transfer of the fulgide between two CA thin films. Finally, the photochromic reaction of fulgide in LB films was investigated.

Photochemical behavior in dichromated gelatin

The photochemical behavior of dichromated gelatin and of different aminoacids-chromium (VI) systems was investigated. The photoredox process that takes place between chromium (VI) (complexed or not) and the organic moiety leads to the formation of chromium (V) which is then reduced to chromium (III) likely complexed to the medium. In chromium (VI)- aminoacids systems, the quantum yield (Phi) , that measures the efficiency of a photon at a given wavelength, strongly depends on the reductive properties of the aminoacid. (Phi) is proportional to aminoacid concentration. In irradiated DCG films, the formation of chromium (V) is observed and this intermediate species appears to be surprisingly stable in the polymeric matrix.

Photochemical formation of chromium (V) in dichromated materials: a quantitative and comparative approach

We have been working on dichromated photosensitive materials for several years. After investigating the photochemical process in various systems, it appears that the chemical structure of the polymeric matrix plays an essential role in the progress of the reaction that takes place upon irradiation by the laser beams. If the primary photochemical process is always the electron transfer from the matrix to the metallic cation giving rise to chromium (V) and a macroradical, the fate of chromium (V) in the system strongly depends on the chemical structure of the polymer. In dichromated poly(acrylic acid) DCPAA, chromium (V) is an unstable species only detectably by ESR spectroscopy upon irradiation: it disappears after stopping the irradiation very fast. On the contrary, chromium (V) is surprisingly stable in poly(vinyl alcohol) and in gelatin. The first spectral evolution corresponds to the only reduction of chromium (VI) into chromium (V) as evidenced by the presence of an isosbestic point. The subsequent reduction of chromium (V) is a very slow process: the complete transformation into chromium (III) is only achieved after several days. As a result, we were able to estimate the UV-visible spectrum of chromium (V) in the matrix and for the first time, it was possible to quantitatively follow the formation of this species and to draw some conclusion about the complexation sites in gelatin.

Photochemical process in dichromated photosensitive material: dichromated (polyvinyl alcohol-polyacrylic acid)-- a model for dichromated gelatin?

Gelatin (G) can be described as a peptidic chain with different pendent groups among which, hydroxy groups-OH, carboxylic groups-COOH and amino groups -NH2. Works performed on polyacrylic acid (PAA), a poymeric chain with only pendent-COOH group and on polyvinyl alcohol (PVA) with only -OH group revelaed the strong influence of the nature of the chemical structure on the photochemical behavior of dichromated photopolymers, DCG, DCPAA and DCPVA. 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 is complexed in PAA and not in PVA. The primary proces is identical in DCG and in DCPVA, so PVA appears as a good model of the first step of gelatin behavior. But the material obtained after irradiation and treatment of DCG only contains chromium strongly complexed by gelatin. The experiments performed on films of DC 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, mainly chromium (V), involved in the process. Chromium (V) appears to play a key role in the photosensitive properties of dichromated materials. The diffraction efficiencies of holograms recorded in DCPAA or DCPVA resemble the profiles of chromium (V) evolution.

Surface relief gratings in self-developing photopolymer films

Dichromated poly(acrylic acid) (DCPAA) films with dimethyl formamide (DMF) have ben sued to photofabricate surface relief gratings. The formation of those gratings depends on the self-development time in darkness subsequent to the illumination at a wavelength of 442 nm and is obtained without any chemical treatment or wet processing. The modulation depth of those holographic surface relief gratings and the spatial frequency response of the thin DCPAA-DMF films have been chosen to characterize that self- developing photopolymer system. Those holographic characteristics are presented in this paper.