Imaging mechanismof the holographic recordingmaterial dichromated cellulose triacetate

Kuoping Wang, Lurong Guo, Lan Zhou, and Jianhua Zhu

Dichromated cellulose triacetate ~DCCTA!, a new, to our knowledge, holographic recording film, ispresented. The material has some special properties, such as good environmental stability, a strongerreal-time effect, strong relief modulation, a stratified sensitivity without the use of any coating technique,a light weight, flexibility, easy fabrication in a large area, and more. By the systematic study of thephysicochemical changes of the microstructure of DCCTA in photochemical reaction processes with anelectron paramagnetic resonance spectrometer, an infrared spectrometer, an UV spectrophotometer, andinterferometric microscopy, we found that, as a dichromated light-sensitive system, DCCTA has a novelimaging mechanism: Light irradiation causes not only the formation of the cross links between Cr31

and molecular chains but also the decomposition of the main molecular chains of the film. In thereal-time imaging process of DCCTA holograms the former plays a dominant role after the holograms arepostprocessed; however, the latter is more important in the imaging process, and the holograms exhibita positive etching property, which differs completely from that of other well-known dichromated mate-rials, such as dichromated gelatin and dichromated poly~vinyl! alcohol. © 1996 Optical Society ofAmerica

Key words: Holographic recording material, dichromated cellulose triacetate, imaging mechanism.

1. Introduction

Dichromated light-sensitive systems such as dichro-mated gelatin ~DCG! and dichromated poly~vinyl! al-cohol ~DCPVA! all play important roles inholography, holographic optical elements, etc.1–9The detailed imaging mechanisms of these types ofmaterial, however, are not yet understood thor-oughly. The accepted scheme10 is that light irradi-ation reduces the sensitizer Cr61 to Cr31 first, andthen Cr31 cross links polymeric molecules, which en-hances the refractive index, reduces the solubility ofthe exposed regions, and creates the visible hologramimage. This means that DCG and DCPVA arenegative etching materials. Dichromated cellulosetriacetate ~DCCTA! is another dichromated light-sensitive system developed by the authors.11,12

K. Wang is with the Department of Physics, Wuhan University,Wuhan 430072, China. L. Guo, L. Zhou, and J. Zhu are with theInformation Optical Institute, Sichuan Uniton University,Chengdu 610064, China.Received 21 February 1995; revised manuscript received 5 Feb-

ruary 1996.0003-6935y96y326369-06$10.00y0© 1996 Optical Society of America

With its special properties, such as strong relief mod-ulation, a stratified sensitivity, a strong real-time ef-fect, etc., DCCTA has obtained some preliminaryapplications in holographic optical elements.13,14The real-time imaging mechanism of the hologramsin the holographic recording process has been re-searched.15In this paper we investigate the physicochemical

changes of this material during photochemical reac-tion through the use of electron paramagnetic reso-nance ~EPR! spectrometry, infrared spectrometry,UV spectrophotometry, and interferometic micros-copy, so as to further understand the imaging mech-anisms of dichromated light-sensitive systems and todevelop their wider application.

2. Experiment

A. Materials

DCCTA is fabricated from commercial cellulose tri-acetate film ~First Film Factory of the Chemical Min-istry, Baoding, China! through preprocessing anddichromated sensitization techniques ~for details seeRef. 11!. The preprocessed material has the sameunit structure as that of the cellulose molecule,shown in Fig. 1. The units are linked with COOOC

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bonds and constitute the long molecular chains of thefilm. The hydroxyl group ~OOH! in each unit is acoordination group. When the preprocessed DCCTAfilm is sensitized in a aqueous ammonium dichro-mate solution, dimethyl sulfoxide ~DMSO! is intro-duced into the sensitization solution as an electrondonor to improve the sensitivity of the film.

B. Spectra

The sensitizer of dichromated light-sensitive sys-tems, Cr61, is usually considered to be reduced toCr31 through the intermediate Cr51 and Cr41 ionicstages. Cr41 is very labile and transfers to Cr51

quickly.10 Cr51, which has one unpaired d electron,has a stable EPR signal at room temperature.Therefore, one can use the EPR technique to study itsformation and change under different experimentalconditions, so as to understand the reducing processof chromium from Cr61 to Cr31 during the holo-graphic recording process. The EPR signals of Cr51

in DCCTA film are recorded with a Model ER-SRC200DEPR spectrometer ~BRUKER! and are shown inFig. 2. Curves 1, 2, and 3 correspond to the sensi-tized films exposed to irradiation from a location 100cm away from a 50-W high-pressure mercury lampfor 1 min, 8 min, and 10 min, respectively ~the irra-diated areas measure 0.5 cm2!, with a g factor of1.97866 0.0002. As is well known, the peak absorp-tion area of the EPR signal is proportional to theconcentration of unpaired electrons16; here it is pro-

Fig. 1. Unit structure of the cellulose molecule in DCCTA film.

Fig. 2. EPR spectra of Cr51 in DCCTA film at room temperature,with exposure times of 1 min ~curve 1!, 8 min ~curve 2!, and 10 min~curve 3!.

6370 APPLIED OPTICS y Vol. 35, No. 32 y 10 November 1996

portional to the Cr51 concentration. The growth ofthe spectrum ~from curve 1 to curve 2! indicates areduction in chromium from Cr61 to Cr51, and thenthe fall of the spectrum, from curve 2 to 3, the furthertransformation of Cr51 to Cr31.

C. UV Spectra

Cr61, Cr51, and Cr31 have their own absorptionpeaks. Table 1 lists17 the maximum absorptionwavelengths labs and the extinction coefficients E ofCr61, Cr51, and Cr31. Therefore, the UV spectra ofDCCTA films under different irradiation conditionscan be used to confirm the formation of Cr51 and Cr31

in the film. Figure 3 shows the absorption spectra ofDCCTA films recorded with a Model UV-2100y2100SUV-visrecording spectrophotometer ~Shimadzu!.Curves 1 and 2 correspond to unexposed and exposedDCCTA films, respectively. Curve 1 reveals that asensitized DCCTA film is generally sensitive toviolet–green irradiation in the visible region, whichis due to the absorption of Cr61 at 454 nm ~see Table1!. The exposed film, however, appears to have aweaker absorption band in the green–red region~curve 2!. With reference to Table 1, we infer thatabove absorption band of the exposed film is due tothe absorption peak of Cr51 at 590 nm.15 In addi-tion, although it is rinsed with flowing water, the

Table 1. Visible Spectra of Various Chromium Oxidation States

OxidationState

labs ~maximum!~nm!

E@Ly~mol cm!#

Cr61 454 4600Cr51 590 250Cr31 575 15.5

Fig. 3. UV spectra of a DCCTA film without ~curve 1! and with~curve 2! light irradiation.

Fig. 4. Infrared spectra of a DCCTA film with sensitization ~curve 1!, exposure ~curve 2!, and postprocessing ~curve 3!.

surface of the exposed film still displays a light bluecolor. We ascribe this surface color to the color ofCr31 and to the cross links formed between Cr31 andthe polymeric molecules.

D. Infrared Spectra

To research the possible changes of the molecularstructure in DCCTA film during the reduction pro-cess of chromium ions from Cr61 to Cr31, we recordthe infrared spectra of the films under different irra-diation conditions and postprocessing conditions witha Nicolet Model 5MX Fourier infrared spectrometerand show the spectra in Fig. 4. Curves 1, 2, and 3correspond to the spectra of a sensitized film, an ex-posed film ~exposed to light from a location 20 cmaway from a 50-W high-pressure mercury lamp for 60min!, and a postprocessed film ~processed with a 5%aqueous sodium hydroxide solution for 30 s!, respec-tively. From Fig. 4 it can be seen that, near thevibration frequency of 1050 cm21, which representsthe intrinsic frequency of a COOOC bond, the infra-red spectrum of the sensitized film has a strongerabsorption peak ~curve 1!. When the sensitized filmis then exposed to light irradiation, this spectral peakfalls to some extent ~curve 2!, which indicates thatlight irradiation causes the decrease of the number ofCOOOC bonds in DCCTA molecules. After the ex-posed film is further processed with sodium hydrox-ide, the absorption peak of the spectrum becomesweaker ~curve 3!, i.e., the number of COOOC bondsbecomes smaller.In addition, after the sensitized film has been ex-

posed to light irradiation and processed with sodiumhydroxide, the intrinsic vibration frequency of OOHnear 3410 cm21 falls as well. We ascribe it to thetransformation ofOOH to COOOCr ~see Fig. 7, be-low!.

E. Holograms and Their Surface InterferometricMicroscopy Photographs

Aswas pointed out in Section 1, DCG andDCPVA arenegative holographic materials. When we exposethese materials to light irradiation, the exposed re-gions have greater refractive indices and a lower sol-ubility than those in unexposed regions because ofthe formation of the cross links between Cr31 andgelatin or poly~vinyl! alcohol molecules in exposedregions. Although it is also a dichromated light-sensitive system, DCCTA possesses an etching prop-erty similar to that which the positive photoresistsexhibit. When a DCCTA film is exposed to lightirradiation, the exposed regions of the film have agreater solubility than that of the unexposed regions,and the polymeric materials in exposed regions areremoved in postprocessing.To investigate the etching properties and relief-

modulation strength of the DCCTA holograms, werecord the DCCTA holographic gratings with acontact-printing method first and then observe therelief shapes and the groove depths of the hologramsby means of interferometric microscopy. The copy-ing master is an amplitude holographic grating madefrom a silver halide plate; the spatial frequency of themaster was 20 linesymm, and the exposure patternand the postprocessing conditions are the same asthose used in Subsection 2.D. The exposure timeand the processing time are 15 and 5min, respectively.Figure 5 gives the interferometric microscopy photo-graphs of the relief shape of the DCCTA gratings withand without post-processed. The photograph shownin Fig. 5~b! reveals that, when the relief-modulationstrength of the postprocessed gratings is stronger,the depth of the surface groove can reach 6 mm.In contrast, DCCTA holograms have a stronger

10 November 1996 y Vol. 35, No. 32 y APPLIED OPTICS 6371

real-time effect.18,19 Figure 6 displays the relation ofthe real-time diffraction efficiencies of the DCCTAholographic gratings versus the exposure time ~thediffraction efficiency is defined as the positive andnegative first-order diffraction intensities divided bythe incident-light intensity!, and the peak diffractionefficiencies of the gratings are seen to reach 60%.Figure 5~a! shows that the groove depth of the real-time holograms is shallow, and the relief modulation

Fig. 6. Relation of the real-time diffraction efficiencies of DCCTAholograms versus the exposure time: concentration of DMSO insensitization solution: filled circles 2.5%, open circles 5%, crosses10%, and open squares 15%.

Fig. 5. Interferometricmicroscopy photographs of the relief shapeof DCCTA holograms ~a! without and ~b! with postprocessing.

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of the holographic gratings is low and can be ne-glected. Therefore, we attribute the higher real-time diffraction efficiencies of these holograms to therefractive-index modulation. Additionally, afterrinsing with flowing water, an exposed film still dis-plays a light blue color on its surface, which indicatesthat the Cr31 ions have combined with polymericmolecules. Therefore, we suggest that therefractive-index modulation of the real-time DCCTAholograms may result from the cross links betweenCr31 and polymeric molecular chains.For the purpose of obtaining a higher-spatial-

frequencyDCCTAhologram, the following processingprocedure can generate less noise and a better grooveshape in the postprocessed hologram: After expo-sure, first bake the hologram at 70–90 °C for 40–50min. Then process the hologram in a 3% ~weightyvolume! sodium hydroxide solution for no more than3 min at room temperature. Third, soak it in a 10%~volumeyvolume! ethylidene lactic acid solution for10 min, and finally rinse the hologram with flowingwater for approximately 30 min and dry it in flowingair. With above processing procedure, we can recorda DCCTA holographic grating, which has a spatialfrequency of 500 linesymm, with a light wavelengthof 441.6 nm from irradiation with a HeCd laser andan exposure of approximate 350 mJycm2, and we findthat the holographic grating possesses a small, butacceptable, amount of noise. The groove depth ofthe hologram, however, reaches only approximately594 nm, which is much shallower than the groovedepth of 6 mm possessed by a lower-spatial-frequencyDCCTA holographic grating. Additionally, we alsocopy a rainbow hologram in a DCCTA film with amaster silver halide hologram and a HeCd laser.The spatial frequency of the copying master is ap-proximately 1100 linesymm. After the hologramhas been postprocessed, we find that the holographicimage is clearer and the noise is not more severe thanthat of a phase silver halide hologram bleached withan aqueous mercuric chloride solution.It should be pointed out that a DCCTA hologram

exhibits a higher real-time diffraction efficiency.Therefore, we believe that, with further improve-ments in sensitivity, this material is better suited tobe applied in real-time holography fields.

3. Imaging Mechanism

In Subsection 2.B the EPR signals of Cr51 underdifferent conditions were shown to reveal that theredox processes taking place in the holographic re-cording process of DCCTA films have characteristicssimilar to those exhibited in DCG and DCPVA.That is to say, light irradiation reduces the sensitizerCr61 to Cr31 through the intermediate ion Cr51.The absorption band that appeared in the green–redregion of the exposed film ~curve 2 from Fig. 3! andthe light blue color that emerged on the surface of theexposed and rinsed film indicate the presence of Cr31

and the combination of Cr31 with the polymeric mol-ecules. Moreover, as has been shown in Fig. 5~a!,the relief modulation of real-time DCCTA holograms

Fig. 7. Schema of the physicochemical processes that occur in DCCTA film.

is very small and can be neglected. Therefore, weinfer that the stronger real-time effect of DCCTA ho-lograms is due to the refractive-index modulation.Although the decomposition of the polymeric mole-cules in exposed regions may produce somerefractive-index difference from that of the unexposedregions, we deduce that the presence of Cr31 and theformation of cross links between Cr31 and the mole-cules of the DCCTA film are predominant in causingthe greater refractive-index modulation of the holo-grams ~Section 2.C!. On the other hand, the fall ofthe infrared absorption peak of the COOOC bondnear 1050 cm21 ~Fig. 4! means that light irradiationresults in not only the presence of the above-described redox processes and the formation of crosslinks but also in breaking the COOOC bonds anddecomposing the polymeric molecules. If the ex-posed film is processed with sodium hydroxide, thespectral peak near 1050 cm21 will fall further, i.e.,the molecular chains will decompose further. Ingeneral, the smaller the length of the polymeric mo-lecular chains, the greater the solubility of the poly-mer in some solvents. A sensitized, but unexposed,DCCTA film has low solubility in sodium hydroxide.But, from Section 2.E, one can see that a DCCTAgrating exhibits a strong, positive, photoetching prop-erty, i.e., the polymeric materials in exposed regionsof the holograms have higher solubility and are re-moved in the postprocessing solution. That is to say,irradiation decomposes the larger polymeric mole-cules to smaller ones. Therefore, the interferometricmicroscopy surface photograph of the processed DC-CTA hologram @Fig. 5~b!# further confirms the decom-position of DCCTA molecules caused by irradiation.Besides, in an exposed and postprocessed film, thelight blue color on its surface disappears, and it hasthe same absorption spectrum as that of the unsen-sitized film. This result indirectly illustrates thedissolution of the polymeric material in the film’sexposed regions.

Therefore, we illustrate the imaging mechanism ofDCCTA holograms in Fig. 7. When light irradiationreduces Cr61 to Cr31 through Cr51 and also decom-poses the longer molecular chains of the DCCTA filmto shorter ones, the Cr31 also forms cross links withthe shorter molecular chains and generates a three-dimensional net structure in the exposed regions.This structure possesses certain differences in refrac-tive index, light absorption, etc., from that in unex-posed regions, and, as a result, causes the real-timeeffect of DCCTA holograms. These results meanthat the modulation type of real-time DCCTA holo-grams is a mixed refractive-index and absorptionmodulation. On the other hand, because the mainCOOOC bonds break and the polymeric moleculesdecompose, light irradiation brings about increasedsolubility of the exposed film in a sodium hydroxidesolution. Therefore, the postprocessed DCCTA ho-lograms have stronger positive relief modulation@Fig. 5~b!#. In addition, because the postprocessedfilms have the same absorption spectra as those of theunsensitized films, we deduce that the modulationtype of the postprocessed DCCTA holograms is aphase-only relief modulation.One may suggest that the cross links between Cr31

and the cellulose molecules have some othercoordination-link constructions. For example, apossible scheme is that presented in Fig. 8. By an-alyzing the stability of different coordination links,however, we can exclude the imaging mechanism ofDCCTA holograms from the above guess.As is known, the coordination link illustrated in

Fig. 8 constitutes an unstable eight-atom ringtypechelate. Because the cross links between Cr31 andthe cellulose molecules are the origin of the real-timeDCCTA holograms, the holograms formed accordingto above-described mechanism should have poor im-age stability. However, experimental results bearout that DCCTA holograms have good environmental

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stability.20 It is evident that above guess disagreeswith the results.Another possible coordination pattern that consti-

tutes a seven-atom ringtype chelate can also be ex-cluded for the same reasons presented in the abovediscussion. Conversely, a coordination link formedas is illustrated in Fig. 7 constitutes a good stablechelate. We thus deduce that the holograms formedaccording to the scheme illustrated in Fig. 7 can havebetter image stability. This deduction coincideswith the experimental results.

4. Conclusion

By systematically studying the physicochemicalchanges of the microstructure of a new dichromatedholographic recording material, dichromated cellulosetriacetate ~DCCTA!, in photochemical reaction pro-cesses through a series of testing means, such as elec-tron paramagnetic resonance spectrometry, infraredspectrometry, UV spectra spectrophotometry, and in-terferometric microscopy, we found that, as a dichro-mated light-sensitive system, DCCTA has a novelimaging mechanism: light irradiation causing notonly the formation of cross links between Cr31 andpolymeric molecules but also the decomposition ofmainmolecular chains. In the real-time imaging pro-cess of the holograms, the former is predominant, andthe modulation of the holograms is a mixed refractive-index and absorption type. After the holograms arepostprocessed, however, the latter plays amore impor-tant role in the imaging process, and the hologramimaging type is a phase-only relief modulation.

K. Wang gratefully acknowledges support for theChenguang project from the Science and TechnologyCommission of Wuhan City, Hubei, China.

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Fig. 8. Possible schema of the physicochemical processes thatmay occur in DCCTA film.

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