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A simple and highly efficient system for holographic recording based on a polyvinyl alcohol matrix is presented. A linear polyol is added to the solution made with the polymer and a xanthene dye in order to increase the photoinduced crosslinking rate. The optimization of the film fabrication process is determined experimentally. It is found that the length of the assistant unit, the position and the number of the attached hydroxyl groups is related with the saturation level and the shelf lifetime of the holographic planar grating. Also, host matrix and photoinitiator attributes are significant in the photosensitive medium response.
©2006 Optical Society of America
1. Introduction
One of the most known polymeric matrices for holographic recording is polyvinyl alcohol (PVA), which is a water-soluble polymer that undergoes crosslinking when illuminated with UV radiation. The polymer is found with different molecular weights and hydrolysis degrees, which determine its optical properties [1]. In order to use a visible wavelength for holographic purposes, a photoinitiator of the crosslinking mechanism is inserted. As a consequence, PVA has been doped with different photoreactive centers in looking for a good information storage system, among them the chromium salts [2–5] and the organic dyes [6,7]. Particularly, high diffraction efficiency in holographic recording is obtained when the photosensitive film is prepared with a mixture of PVA, acrylamide, an organic dye and an electron donor [8–11]. Nevertheless, in this case, PVA is not involved in the recording mechanism as it acts only as a host of the acrylamide monomer, which is polymerized by the process derived by the photogeneration of ion radicals.
Recently, it has been reported that the crosslinking rate induced by UV light in a PVA film can be increased with the assistance of some aromatic alcohols using as initiator a photoacid generator [12]. In contrast, in this paper we proposed a PVA visible light sensitive film containing a xanthene dye as the photoinitiator of the polymer crosslinking and an aliphatic alcohol with two or three hydroxyl radicals like the assistant agent in the process. The xanthene dyes has previously been used as photoinitiators but using dichromate ammonium and an amine as an electron donor [6,13]. Here, the number of elements in the photosystem proposed is less, then to explain the mechanism to crosslink the PVA chains is simpler. Note that the use of electron donors has been avoided because, as it has been investigated [4], they do not always favor the redox process. Also, an investigation of the efficiency, taking into account several parameters of the medium constituents, in recording a plane holographic sinusoidal grating is presented.
2. The photosensitive plates
The general procedure to obtain the solution was to dissolve first the PVA in deionized water at 70°C. Then, the dye and the alcohol were incorporated until get a homogeneous mixture. Next, a portion of the resultant liquid was spread out uniformly on a clean glass substrate laid on a leveled table. The photosensitive films were obtained when the solution solidified after some hours at room conditions (25°C and 40% RH).
In the experiments, several concentrations of PVA, alcohol, and dye were test in order to find the film with the best optical performance. Initially, because different molecular weight and hydrolysis degree of PVA are offered in the market, we studied the optical response in similar conditions for some of them. The results shown in this paper correspond only to the lowest molecular weight (13,000–23,000) available in our lab, which has been observed to be the best for our goal. It may be connected with the lower water content, which reduce the distances between the photoinitiator, the alcohol and the polar part of PVA. The nominal film thickness at the central part of this samples was found to be (18±1)μm.
In addition, different alcohols and acid dyes were tested in order to find the best photosensitive system. The alcohols, which are the assistants in the crosslinking process, were 1,2-ethanediol, 1,3-propanediol, 1,4-butanediol, 1,3-butanediol, and 1,2,3-propanetriol. These linear chains, with different length, have a different number of hydroxyl groups attached in different positions, see Fig. 1. The xanthene dyes, the photoinitiators of the crosslinking process, are Rose Bengal, Erythrosin B and Eosin Y. These acid dyes, which have shown a behavior as proton acceptors [14–16], have been selected because they are fluorescein derivatives and have a similar molecular structure. Typical absorption spectra for the different dyed-films are shown in Fig. 2.
Fig. 2. Absorbance in (1,2-ethanediol)-containing PVA films doped with different xanthene dyes. The concentrations of PVA/dye/polyol are 8.0/0.05/4.4% by weight, respectively. Excitation and reading wavelengths are shown.
It has to be point out that the water temperature to dissolve PVA is related with the performance of the resulting film. Samples obtained after dissolving the lowest molecular weight PVA in water at room temperature show an inferior performance. It may be related with a structural ordering of PVA molecules when cooling. Also, the temperature of the solution at the moment of adding the polyol is important. So, we had to try several conditions to obtain the better results as described in section 4.2. Finally, remnant water content in the photoreactive samples depends on the polyol used and, consequently, it changes their optical features.
3. Photoinduced crosslinking mechanism
The dyed PVA samples were illuminated using a 30mW semiconductor laser emitting at 532nm. This wavelength is in the absorption band of all the dyes and induces fluorescence. When the dye molecule absorbs a photon, it ionizes as an acid agent. Then, in the excited state, it takes protons from the hydroxyl groups of the alcohol resulting bleached and leaving an ionized alcohol. This ion radical interacts with the side-hydroxyl pendant groups of the PVA chains and serves as a bond for two of them. In this way, the photocrosslinking is obtained. A schematic of the proposed photochemistry is indicated in Fig. 3, where only two hydroxyl pendant groups are indicated for the alcohol.
The bleaching rates for samples prepared with the three different dyes are shown in Fig. 4. The power of the illumination beam is 30mW and it is not expanded. The concentration by weight of PVA, dye and alcohol in the solution was 8.0, 0.4, and 4.4%, respectively. The selected alcohol for this comparison had been 1,2-ethanediol. From these measurements it is concluded that in the Eosin Y-containing sample the dye molecules are reduced faster, and the final transmittance is higher than the others. This difference in the quantum yield is probably related with the sites where this molecule, in the excited state, can accept protons from the crosslinking assistants [17]. Finally, polyol-free samples were exposed under identical conditions observing no dye bleaching. This result ensures that the first part of the proposed photochemistry is correct, thus, the alcohol molecule is related with the reduction of the xanthene dye.
Besides, a spectrophotometric comparison between the Eosin Y-doped samples prepared using the different polyols has been achieved. It has been observed that spectra are basically the same, detecting only a very small broadening of the absorption band for the (1,2-ethanediol)-containing sample. It may be caused by an insignificant aggregation of the dye. Then, for practical purposes, the environment conditions given by the different alcohols are alike. This similitude was also observed when the bleaching rate was measured under identical experimental conditions. Final transmittance and bleaching speed was similar for all the samples.
Fig. 4. Bleaching rate in (1,2-ethanediol)- containing films using different fluorescein derivatives as photocrosslinking precursors
4. Holographic recording
It has been reported [18] that the optical recording in a xanthene dye-PVA film is poorly efficient. The induced holographic grating has both absorptive and refractive contributions, but the excitation wavelength in that case was also used as the reading one. In this work, the holographic characterization was performed recording an interference pattern, too, which was obtained using the optical arrangement shown in Fig. 5. The excitation and probe wavelengths are 532nm and 635nm, respectively. The intensity ratio of the recording beams was kept equal to 1, having each a power density of ≈10mW/cm2. Red light, 2mW power, is used to monitor exclusively the refractive index changes because it is weakly absorbed by the photosensitive system even after bleaching, and, consequently, it does not alter the phase grating formation. The spatial frequency of the resulting unslanted holographic grating is 150 lines/mm, and then it is a plane grating. The time behavior of the diffraction efficiency in one of the first-order diffracted beams (η +1) has been taken as a measure for the crosslinking rate. Light on the detector has been pass through a red filter to avoid strayed light, which is also the reason to used the probe beam behind the photoreactive sample. Also, in order to determine the real performance of the sample, reflection at the interfaces and film absorption were not taken into account to calculate η +1.
Fig. 5. Holographic set-up used in the optical recording characterization. S=photoreactive sample, L1=recording laser, L2=reading laser; M1, M2, M3=mirrors, BS=beamsplitters, P1, P2=photodetectors.
In the bleaching analysis, it was observed that the oxidation rate for Rose Bengal and Erythrosin B dyes was slower and the saturation level smaller than for Eosin Y. In consequence, there is a higher population of oxidized dye molecules that means an also higher number of crosslinked chains. This was confirmed because η +1 for the system was also higher for the Eosin Y-PVA-alcohol system using similar concentrations. Then, the results shown from now on correspond only to the Eosin Y photosensitive systems with a polyol incorporated. The comparative analysis of optical recording efficiency has been divided in three sections, each corresponding to the different characteristics that alcohol chains present. Also, grating recording evolution curves are shown for samples with different concentrations in order to make evident their connection.
4.1 Number of hydroxyl groups
In order to determine the dependency on the number of hydroxyl groups attached to the alcohol chain, films were prepared with 1,2-ethanediol and 1,2,3-propanetriol as crosslinking intermediates. It has been observed that the time behavior of η +1, in both cases, is almost equivalent. In Fig. 6, the concentrations of PVA, dye and polyol were 8.0, 0.16, and 2.2%, respectively. Nevertheless, the induced grating in the films made using the triol vanishes after some hours. In contrast, the diol-containing films keep the refractive index modulation for several weeks under room conditions with nearly the same amplitude. The possible explanation for that is the following. The tested polyols can interact strongly with water by means of the hydrogen bonds. So, it is possible that the third hydroxyl group in 1,2,3-propanediol, which is probably unbounded, breaks the crosslinking between the vinyl chains and the index modulation disappears. It is also observed that the dye remains oxidized in the triol-containing film, but the absorption modulation has vanished. This last result indicates that there is not dye reduction involved in the loss of PVA crosslinking.
4.2 Diol chain length
Next, the optical recording using films prepared with different chain length diols was compared. 1,2-ethanediol, 1,3-propanediol and 1,4-butanediol were the selected crosslinking intermediates. In Fig. 7, the evolution of diffraction efficiency is shown. The concentrations of PVA, dye and alcohol were 8.0, 0.08, and 4.4%, respectively. Again, the higher attained refractive index modulation was observed in the film containing 1,2-ethanediol. Inclusively, in films prepared with a higher content of dye and polyol, the η +1 was an order of magnitude less and the recording was not permanent. Moreover, as it has been observed in the (1,2,3-propanediol)-containing films, the dye states bleached but the absorption grating vanishes.
Fig. 6. Temporal evolution of diffraction efficiency using (1,2,3-propanetriol)- and (1,2-ethanediol)-containing Eosin Y-PVA films when a 150lin/mm holographic grating is recorded
Fig. 7. Same as in Fig. 6, but using (1,2-ethenediol)-, (1,3-propanediol)-, and (1,4-butanediol)-containing Eosin Y-PVA films
However, the η +1 of (1,3-propanediol)-containing samples was increased after modifying the film fabrication process. This adjustment has to be done because the diol needs refrigeration and probably the sudden change of temperature affects it. So, the tailored procedure is the following. Once the PVA and dye were dissolved in the deionized water, the solution was left to reach the room temperature. Then, the diol at 4°C was added and the mixture stirred until get a homogeneous product. After the spreading out on the substrate and the water evaporation overnight process, the films were introduced in an oven at 60°C for 1 hour. The obtained η +1 when the phase grating was induced increases in one order of magnitude. Additionally, this refractive index modulation was permanent.
In the other hand, the optical response of the (1,4-butanediol)-containing film has not been satisfactory, even when the fabrication process was changed. From these observations it can be said that induced by light crosslinking, magnitude and time stability, is influenced by remnant water content in the film as well as by the alcohol chain length.
4.3 Position of the hydroxyl groups
Finally, the position of the hydroxyl group in the alcoholic chain is important in the crosslinking process. To prove this, films containing 1,3-butanediol were prepared using a PVA, dye and alcohol concentrations of 8.0, 0.4 and 4.4%, respectively. The η +1 measured in this case, Fig. 8, was in the same order of magnitude than the one for (1,2-ethanediol)-containing films. The higher η +1, ≈5% for 1,3-butanediol and ≈15% for 1,2-etheanediol, and the longer exposure are derived from an increment in dye and diol concentration with respect to the above cases. This recording was also permanent for several weeks. The similarity between both crosslinking processes induced by light may be in the value of the highest occupied molecular orbital energy. Nevertheless, a detailed investigation on this fact must be carried out.
Fig. 8. Same as in Fig. 6, but using (1,2.ethanediol)- and (1,3-butanediol)-containing Eosin Y-PVA films
5. Discussion
As many of the PVA-based photosensitive films, the proposed system does not need a post-exposure treatment. The recording is stable for a long time in the laboratory conditions but crosslinking is not strong enough to overcome a wet treatment. The hydrophilic character of the linear diols used in the experiments can explain the breaking of the photoinduced bonding. Even though, using the expression for thin holograms
where J 1(x) is the first-order Bessel function, λ2 is the reading wavelength and d is the film thickness, the highest refractive index modulation amplitude calculated in Eosin Y-(1,2-ethanediol)-PVA film shown in Fig. 8 was Δn≈5×10-3. This figure of merit is comparable with those reported for other holographic materials in the literature [19–21]. Nevertheless, in those cases, the recorded gratings are in the thick hologram regimen and the recording energy is higher. A further analysis to determine the modulation transfer function is needed to do a more realistic comparison. Even though, the advantage in using diol-containing PVA films is its simplicity in preparation and the risk reduction in the handling of the chemicals. Also, it was found that aromatic polyols like pyrogallol are inadequate in PVA-xanthene dye system. They caused a crystallization of the film even at low concentrations. The drawback of the proposed photosensitive films, based on the results presented, may be its poor holographic sensitivity (S≈7×10-3cm2/J). Nevertheless, the use of a writing source with a higher power, for example an Ar laser, may solve this problem.
Because of the bonding properties shown by aliphatic alcohols, linear diols are used frequently in the fabrication of polyurethane derivatives as difunctional chain extenders and 1,2,3-propanetriol as a polymer crosslinker. In general, while 1,4-butanediol is the preferred chain extender; 1,2-ethanediol and 1,3-propanediol are used for softer elastomers. Nevertheless, this investigation has demonstrated the possibility to photocrosslink PVA-chains effectively using the alcohol molecules. Also, the immunity to temperature of the exposed part has been tested by introducing the obtained holographic grating in an oven at 100°C for an hour and observing no change in the diffraction efficiency. Then, this material can be used for the fabrication of holographic optical elements placed in hot environments. Finally, it was found that the film fabrication process was affected by water temperature only for the (1,3-propanediol)-case. It is possible that, when added at 70°C, the interaction of the diol and the elements of the mixture lead to an undesirable product. A careful research on this fact must be carried out.
6. Conclusions
It has been found that the holographic recording in the system Dye-Alcohol-PVA depends on the attributes of each component. In order to find an optimized concentration of the components, the influence on the photocrosslinking rate of molecular weight of the matrix, acidity of the photoinitiator, chain length of the crosslinking intermediate and, finally, the number and position of the hydroxyl group has been investigated. The best photoreactive film was found to be that containing the lowest PVA molecular weight available (13,000–23,000), the shortest chain length diol (1,2-ethanediol) and the fluorescein derivative with the higher acidity (Eosin Y). In addition, the proposed photochemistry has been validated by the analysis of absorption and refractive index photoinduced changes. From the observations, it can be concluded that the proposed system has an important potential to be used as a permanent optical storage medium.
Acknowledgments
The authors thank the financial support from CONACyT and CONCyTEG under grants SEP-2003-C02-43194 and 04-04-K117-087, respectively. Also, we express our acknowledgment to Dr. Antonio Richa at the Universidad de Guanajuato for the providing of some diols.
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