Christophe Lafond

Primary photochemical process in films of dichromated gelatin: a quantitative approach

The photochemical behavior of dichromated gelatin films is investigated on irradiation at 365 nm and 491.6 nm. Fourier transform infrared (FTIR) spectroscopy confirms the two-step reduction of chromi- um(VI) into chromium(III). Prior to the irradiation, the molar extinction coefficient ? of chromium(VI) in the film is measured as a function of the pH of the starting solution with the result that e appears to be pH independent in the range 2 to 10 contrary to what is observed in solution. Chromium(V) resulting from a charge transfer between the polymeric matrix and chromium(Vi) species in the excited state appears to be surprisingly stable in the gelatin matrix. Accordingly, the subsequent slow reduction of chromium(V) into the final chromium(III) does not interfer in a significant way in chromium(VI) reduction. As a result, it is possible to determine the UV-visible spectrum of chromium(V) and to quantify its formation. So far no quantitative results were reported in the literature concerning the chromium(V) spectrum and its formation in a film of dichromated gelatin. FTIR spectroscopy confirms the two-step reduction of chromium(VI) into chromium(III).

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.

Recent improvements on mid-IR chalcogenide optical fibers

Fabrication process of arsenic-sulfide (As-S) and arsenic-selenide (As-Se) optical fibers has been improved to enhance the transmission in the mid-IR region. Typical attenuation spectrum of As-S or As-Se optical fibers shows impurities bands, such as S-H, Se-H, O-H, which limit their operation and cause the increase of the attenuation loss in the mid-IR. Precursors purification methods and glass processing were improved to minimise those impurities bands. Regarding As- S fibers, the attenuation around 2.7 μm is 0.12 dB/m and S-H concentration is lower than 0.3 ppm. In the case of As-Se fibers, the minimum of attenuation located at 6 μm is 0.2 dB/m and Se-H concentration is lower than 0.5 ppm. Efforts have been also made to improve the mechanical properties which are usually affected by several parameters such as drawing conditions or heterogeneous inclusions contained in the glass. The double-crucible method gives high quality core/clad interface and consequently increases the strength of the fiber. Inclusions consist mainly of carbon and silica particles. Those impurities enter the glass from initial precursors and are also formed by interaction with the apparatus material. Thanks to the process improvement, impurities particles are minimized and tensile strengths up to 0.32 GPa and 0.41 GPa are reached for As-Se and As-S fibers respectively.

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 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.

Primary photochemical process in chromium doped polymeric material: interactivity with holography

A comparative investigation of the photochemical behavior of dichromated gelatin (DCG), dichromated polyacrylic acid (DCPAA) and dichromated polyvinylalcohol (DCPVA) gives evidence for the similarity of the photochemicalbehaviour 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, is stable, likely strongly complexed in gelatin and polyvinylalcohol. The quantum yields of the reaction are high and similar for both reactions. On the contrary, even though chromium (V) is also formed during the primary process in DCPAA, this species is quite unstable and disappears as soon as the irradiation is 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.