Holographic optical elements recorded in silver halide sensitized gelatin emulsions. Part I. Transmission holographic optical elements

Transcription

1 Holographic optical elements recorded in silver halide sensitized gelatin emulsions. Part I. Transmission holographic optical elements Jong Man Kim, Byung So Choi, Sun Il Kim, Jong Min Kim, Hans I. Bjelkhagen, and Nicholas J. Phillips Silver halide sensitized gelatin SHSG holograms are similar to holograms recorded in dichromated gelatin DCG, the main recording material for holographic optical elements HOE s. The drawback of DCG is its low sensitivity and limited spectral response. Silver halide materials can be processed in such a way that the final hologram will have properties like a DCG hologram. Recently this technique has become more interesting since the introduction of new ultra-high-resolution silver halide emulsions. An optimized processing technique for transmission HOE s recorded in these materials is introduced. Diffraction efficiencies over 90% can be obtained for transmissive diffraction gratings. Understanding the importance of the selective hardening process has made it possible to obtain results similar to conventional DCG processing. The main advantage of the SHSG process is that high-sensitivity recording can be performed with laser wavelengths anywhere within the visible spectrum. This simplifies the manufacturing of high-quality, large-format HOE s Optical Society of America OCIS codes: , , , , Introduction Silver halide sensitized gelatin SHSG holograms are similar to holograms recorded in dichromated gelatin DCG. Holographic optical elements HOE s are often recorded in DCG, which offers high diffraction efficiency combined with low noise. The drawback of DCG is its low exposure sensitivity and limited spectral response. Therefore considerable attention has been directed at silver halide materials that are processed in such a way that the final hologram will have properties like a DCG hologram. This can be achieved by special processing techniques. The results are rather good, which means that low-scatter HOE s of high efficiency can be produced. In addition, the SHSG hologram is printout free. However, during the recording of the HOE on J. M. Kim, B. S. Choi, S. I. Kim, and J. M. Kim are with the Display Laboratory, Advanced Institute of Technology, Samsung, P. O. Box 111, Suwon, South Korea. H. I. Bjelkhagen and N. J. Phillips are with the Centre for Modern Optics, De Montfort University, Hawthorn Building, The Gateway, Leicester LE1 9BH, United Kingdom. The address for H. I. Bjelkhagen is hansholo@aol.com. Received 4 May $ Optical Society of America a silver halide emulsion, light scattering will take place, which differs from the recording of a hologram on DCG, where little or no scattering occurs. Up until now most of the SHSG techniques have been developed for materials such as Agfa, Kodak, or Ilford holographic materials. Recently this technique has become more interesting since the introduction of ultra-high-resolution silver halide emulsions manufactured by, e.g., Slavich 1 in Russia and Holographic Recording Technologies 2 in Germany. When such materials as these are employed, HOE s can be manufactured with a quality more or less equal to HOE s recorded on DCG materials. Briefly, the technique of generating a SHSG hologram is to expose a silver halide emulsion and then process it in such a way that local tanning near the silver sites will occur within the emulsion. In most cases, tanning dichromate bleach is used. Then the material is fixed to remove all the silver halides from the emulsion, leaving only gelatin. The last step of the processing is to dehydrate the material with isopropyl alcohol in the same way that DCG holograms are processed. A dry SHSG hologram must be sealed hermetically to prevent moisture penetrating into the emulsion, which would destroy the image. The difference between the SHSG process and the 622 APPLIED OPTICS Vol. 40, No February 2001

2 DCG process is that in the latter case hexavalent chromium is reduced photolytically to the trivalent state, whereas in the SHSG process the same is done chemically. The developed silver in the SHSG emulsion reduces the chromium during bleaching in the dichromate bleach solution: 6Ag 0 Cr 2 O H 3 6Ag 2Cr 3 7H 2 O. (1) Maximum tanning occurs in the regions of maximum bleaching. Cross-link bonds between neighboring gelatin molecule strands cause a local hardening of the gelatin. Transmission HOE s are easier to record and process than HOE s of the reflection type. In this paper we describe primarily techniques that work well for transmission HOE s. In a future paper we will present the recording and processing of reflection HOE s recorded in ultra-highresolution silver halide materials. A summary of existing SHSG processing techniques has been published by Bjelkhagen History of Silver Halide Sensitized Gelatin Processing The SHSG process has been known since the beginning of the 1970 s. Pennington et al. 4 used a complicated, 21-step chemical process that included dichromating and reexposing to create the first SHSG holograms. However, in the early 1980 s the process became more manageable in practice. A simpler process intended for Kodak 649F plates and based on the Kodak D-19 developer and the R-9 bleach was presented by Graver et al. 5 Chang and Winick 6 published a slightly different processing method to create SHSG holograms on 649F plates. The developer was the same but instead of the reversal Kodak R-9 bleach, the authors used a modified version of the Kodak R-10 bleach that was a rehalogenating dichromated bleach bath. SHSG processing for Agfa holographic materials was reported by Fimia et al. 7 In general the thinner and harder Agfa emulsion was regarded to be more difficult to use than the Kodak 649F plates. For the SHSG process, the authors used the Kodak D-8 developer, a modified Kodak R-10 bleach, and the nonhardening F-24 fixer. By use of a washing bath of a slightly-higher-than-room temperature 30 C, as well as a dehydration bath at an elevated temperature 60 C, the diffraction efficiency was increased compared with that obtained for room-temperature solutions. However, the signalto-noise ratio decreased with temperature. In three other publications from the Spanish research group, Boj et al. 8 and Fimia et al. 9,10 revealed some interesting results. The influence of the bleach bath temperature was investigated. The diffraction efficiency obtained for a transmission grating was higher for HOE s processed at 50 C 80% than at 20 C 40%. For reflection holograms, the so-called phenidone ascorbic acid and sodium phosphate (PAAP) developer 11 was recommended. With the PAAP developer better resolution could be obtained. The performance at high spatial frequencies was improved, which was important for reflection SHSG holograms. Ferrante 12 investigated the spatialfrequency response of the Agfa 8E75 high-definition HD emulsion used for SHSG transmission HOE s. Employing the processing technique by Graver et al., 5 Ferrante obtained experimental data points that fitted a theoretical modulation transfer function for an emulsion with an apparent grain size of approximately 70 nm. The exposure needed for SHSG holograms developed in Kodak D-19 was quoted as between 80 and 200 J cm 2. The influence of the temperature of the processing solutions was also investigated by Hariharan. 13 In addition, he compared the difference between using a tanning bleach dichromate and a nontanning bleach ferricyanide. Hariharan wanted to prove that tanning could actually be performed during development instead of during bleaching. Moreover, use of a developer, which is classified normally as a nontanning developer, can actually produce a hardening effect if the material is brought subsequently into contact with an oxidizer even nontanning. A metol quinol developer containing a large amount of sodium sulfite will not harden gelatin. During development, oxidized developer products are bound to the gelatin immediately surrounding the developed silver. No cross linking will take place during the development described above, but the oxidized developer molecules constitute available bridges whose free ends can react if they come into contact with an oxidizer. This effect has been described by Tull. 14 Hariharan s experiment confirmed that SHSG holograms could be obtained on the basis of this effect. Angell 15 introduced a 13-step processing scheme intended for the Kodak 649F material. Later the process was modified slightly. 16 The complicated and time-consuming procedure contained some interesting aspects of SHSG processing. So far the main tanning bleaches used were the dichromate bleaches Kodak R-9 or R-10. Angell claimed that potassium chlorochromate Peligouts salt used as a bleaching agent was an improvement. This is part A of the Kodak chromium intensifier Kodak CIA. In addition, he claimed that a fixer with a hardener could improve the dynamic range and the signal-to-noise ratio. Angell s processing scheme contained some new details, such as stabilization and emulsion protection, for example. In particular, use of an organosilane coupling agent Dow Corning Z-6020 in the fixing step makes it possible to maintain the emulsion thickness after the processing, which is important for many HOE applications. The organosilane coupling agent N- 2-aminoethyl -3- amino-propyltrimetohoxy silane was recommended because it was attainable to the gelatin matrix. The emulsion thickness controlling technique makes it possible to obtain a permanent chemical way of controlling emulsion thickness in holographic emulsions. However, the actual method depends on the emulsion type, the silver halide solids loading, the exposure level, the bleaching technique, etc. For example, for 10 February 2001 Vol. 40, No. 5 APPLIED OPTICS 623

3 the Kodak 649F emulsion, Angell found that adding 2 4% of the coupling agent to the fixer resulted in an emulsion thickness of 16 m after drying, which was equal to the original thickness. Another option is to use the Kodak C41 stabilizer that consists of two parts. Part A is a formaldehyde hardener and part B an organosilicon agent. The stabilizer can be utilized in the SHSG process to control emulsion thickness. Weiss and Millul 17 and Weiss et al. 18 published a simpler SHSG processing method for the 649F emulsion. Based on Hariharan s 13 experiments with tanning developers, the authors introduced the CW-C2 developer 19 to the SHSG process. They tested both reversal and rehalogenating dichromate bleach solutions. Weiss and Millul compared the D-19 developer with the CW-C2 developer and found that CW-C2 gave significant higher signal-to-noise ratios. The CW-C2 developer is therefore often a better choice, in particular for reflection HOE s. For the bleaching part of the process, the authors tested a variety of bleaching agents and found that only the reversal or the rehalogenating ammonium dichromate bleach could produce high diffraction efficiency. A theoretical SHSG model was presented by Weiss and Friesem, 20 which explained the storage mechanism in silver halide emulsions. Phillips et al. 21 were able to obtain high-quality holograms using a new SHSG processing technique. The traditional view stating that the DCG system hardens gelatin, thus preventing solubilization of the material, must be balanced against other observations, such as that of the reduction of the bulk index of the gelatin layer and the appearance of gelatin in the processing solutions. The authors proposed that the large values of index modulation in DCG holograms are caused by gelatin hydrolysis in the nodal parts of the image structure. Based on the new ideas about the mechanism behind the DCG process, two new processing procedures for SHSG holograms were formulated. Important Russian research in this field was undertaken by Usanov et al This research was based on the formation of a microcavity structure and is described in the following way. The gelatin in a photographic emulsion is adsorbed on the silver halide grains. In fact, only a part of the gelatin molecules is adsorbed. The molecule chains are also linked within the gelatin mass of the emulsion. The thickness of the adsorbed layer in a dry emulsion is nm. Each silver halide grain is surrounded by gelatin molecules linked at different points by active groups that are able to form complex compounds with silver grains produced during the development. The Russian method is based on the fact that these adsorbed layers are less active and will be more difficult to harden than the surrounding gelatin mass. Variations in hardening between exposed and unexposed areas therefore will occur. After the silver and silver halide grains are removed from the emulsion and the hologram is dehydrated, microcavities will remain that will be responsible for the refractive-index variations. The processing is performed in the following way. After the material has been exposed, developed, and fixed, it is hardened in a potassium dichromate solution or in formaldehyde. The treatment in the hardening solution takes 1hormore in the potassium dichromate bath. The duration of treatment is six times longer in the formaldehyde solution. After that the material is bleached, fixed, and finally dehydrated. Another possibility is to bleach the material before hardening it. After that, it is fixed and dehydrated. A third possibility is to use a reversal bleach after development. Then the material is hardened, and the final steps are fixing and dehydration. The second processing method results in holograms with better spectral selectivity than holograms processed according to the third method. The new SHSG technique was tested with the Russian material PFG-03. High-quality holograms were produced with diffraction efficiencies between 70% and 90% at nm. We note that the D-19 developer, to which is added a large amount of potassium bromide 20 g l, is a better alternative for processing reflection HOE s in the PFG-03 emulsion than the GP-2 developer used previously. 24 Simova and Kavehrad 25,26 introduced a modified processing technique for HOE recordings on the Agfa 8E75 HD emulsion. The processing technique was based on a 10-min hot 75 C wash to soften the hard Agfa emulsion. This was done after the emulsion was developed and fixed in a nonhardening fix solution. Bleaching was performed in a solvent of ammonium dichromate bleach at 50 C. A diffraction efficiency of approximately 80% could be obtained over a rather wide spatial-frequency range. The authors main application was the fabrication of a 4 4 holographic star coupler. Extensive research on SHSG processing has been performed in Spain. Pascual et al. 27,28 have described the advantages of using rehalogenation of the silver image as an important feature of the SHSG process. Because silver is not removed from the developed site when rehalogenation of the silver takes place, the formation of hardened shells around the bleached silver site is enhanced by nonremoval of the contents of each shell, as explained by Phillips et al. 21 An optimized process for the Agfa 8E75 and 8E56 HD materials was published by Fimia et al. 29 in which a modified R-10 bleach at 50 C was used. It was shown that the PAAP developer produces HOE s with lower noise and better diffraction efficiencies at higher spatial frequencies as compared with the D-19 developer. In another publication by Fimia et al., 30 the authors investigated how to optimize the SHSG process by studying the HOE diffraction efficiency in a water tank before dehydration. In addition, they introduced the ascorbic acid and sodium carbonate AAC developer, which avoids the influence of oxidation products during development. By measuring the diffraction efficiency at different steps during the process, they found it possible to obtain, e.g., the optimal potassium bromide concentration in the R-10 bleach. 624 APPLIED OPTICS Vol. 40, No February 2001

4 A diffraction efficiency of 80% was obtained for diffraction gratings 1000 lines mm recorded on Agfa 8E56 HD emulsion at a 514-nm wavelength. The material was processed in the AAC developer, bleached in the optimal R-10 bleach, fixed in Kodak F-24 fixer, and dehydrated in isopropanol in the usual way. It is known that the cross linking of gelatin is not effective in highly acid bleach baths such as the R-10. By interrupting the process after the bleaching step without washing the plate and leaving the plate in high humidity for 24 h, Fimia et al. 31 reported a better spatial-frequency response for SHSGprocessed Agfa materials. The SHSG process for diffuse-object recording has been investigated by Fimia et al The inclusion of noise gratings as a new source of noise in SHSG processing was discussed. In particular, to understand its influence on the signal-to-noise ratio may require new models. The most recent progress in SHSG processing was reported by Beléndez et al and Neipp et al. 39 regarding the new red-sensitive BB-640 emulsion of the ultra-fine-grain silver halide type grain size approximately 25 nm from Holographic Recording Technologies in Germany. The high diffraction efficiency over 90%, the extended spectral-frequency response e.g., 40% for Agfa as compared with 85% for BB-640 at 2800 lines mm, and the high signal-to-noise ratios indicate the importance of SHSG processing for HOE manufacturing in the future. In another publication, Neipp et al. 40 have investigated the influence of the development in SHSG-processed Slavich PFG-01 plates. 3. Silver Halide Materials Agfa is no longer producing holographic materials; however, the company manufactures a lithographic emulsion Millimask that is similar to the former holographic Holotest 8E56 HD material. The recording materials used in our experiments are commercial silver halide materials currently obtainable on the market: Agfa Millimask, Slavich VR-P, and Slavich PFG-03C. The Millimask and VR-P are similar orthochromatic materials, whereas the PFG- 03C is of the ultra-high-resolution panchromatic type. The main differences between the Slavich and the former Agfa holographic materials are the grain size and the silver content in the emulsion. The panchromatic Slavich emulsions have grain sizes as small as 10 nm. The former highest-quality Agfa materials Holotest HD materials had grain sizes of approximately 45 nm. The Slavich silver content is one half approximately 0.25 g cm 3 of the silver content in Agfa materials. To obtain high-quality HOE s, Rayleigh scattering that occurs during the recording has to be as low as possible. In particular, if blue laser wavelengths are used for the recording, an ultra-fine-grain silver halide material is needed. Phillips et al. 41 discussed scattering in silver halide emulsions from the concept of a scattering mean free path. Photographic layers used for commercial holographic materials are normally approximately 7 m thick. The emulsion consists of small silver halide crystals in a gelatin matrix. In 1871 Lord Rayleigh 42 stated that light scattering from these crystals would be proportional to the sixth power of their radius for a given wavelength and would increase with the inverse fourth power of the wavelength as the wavelength decreases. If the radius of the silver halide crystals assumed spherical is a and the number per unit volume of the layer is N, then 4dN a 3 m AgH, (2) 3 where denotes the density of silver halide, m AgH is the mass of the silver halide unit area, and d is the thickness of the emulsion. The atomic weight of silver is 108 and that of bromine is 80. Here we note that the mole fraction of Ag in AgBr is Then m AgH m Ag, m Ag 5g/m 2 for Agfa materials, AgBr 6.47g cm 3. A change in grain size usually means that the same amount of silver bromide would be shared between fewer but larger grains, which in turn means that Na 3 is constant. For the former Agfa materials we have 8E materials: grain diameter 2a 44 nm 3 N 8E grains cm 3, 10E materials: grain diameter 2a 90 nm 3 N 10E grains cm 3. For the Slavich material we have PFG-03C: grain diameter 2a 16 nm 3 N PFG-03C grains cm 3. The scattered intensity I s of light off these small particles is S Nda 6. But because Na 3 is constant, I s is therefore a 3, which means that light scattering varies with the cube of the grain size. Phillips et al. 41 have introduced a parameter denoting the ratio of the scatter mean free path to the emulsion thickness, which can be used as a figure of merit to describe the holographic recording layer, namely, 1 N RS d, (3) where N denotes the number of grains per unit volume, d is the emulsion thickness, and RS is the Rayleigh scatter cross section, which is RS 12 2 n 4 G a n H n G a n H n G 2 2 6, (4) where a is the wavelength of the light in air, n G is the refractive index of gelatin n G 1.54, n H is the 10 February 2001 Vol. 40, No. 5 APPLIED OPTICS 625

5 refractive index of halide grain n AgBr 2.236, and 2a is the grain diameter. The emulsion thickness for the Agfa materials as well as the Slavich materials is approximately 7 m. We performed the calculations using the sodium line wavelength 589 nm. The numerical values of are Agfa 8E materials of approximately 3, Agfa 10E materials of approximately 0.3, and Slavich PFG-03C materials of approximately 55. The 8E material is just on the border of acceptability for recording holograms, the 10E is not really acceptable for high-spatial-frequency recordings. The parameter of Slavich emulsion indicates that this material is considerably better than the Agfa emulsions. Neipp et al. 39 found the parameter for BB-640 grain diameter of 22 nm to be approximately nm. The Slavich material is similar, and considerable improvements are obtained when such ultra-high-resolution materials are used. The Agfa Millimask material that is similar to the 8E emulsion has dyes in the emulsion to reduce scattering, which may help in the recording of transmission HOE s. Transmission HOE s can be recorded in both Millimask and the similar Slavich VR-P emulsion. Reflection HOE s are more critical and require the lowest light-scattering silver halide materials for highest possible diffraction efficiency. For refection HOE s more details will be covered in a forthcoming paper. 4. Materials Used in This Investigation The orthochromatic Agfa-Gevaert Millimask HD FL5 emulsion has its color sensitivity peaked at 525 nm. It is a high-contrast material with a gamma of approximately 5. The grain size is approximately 45 nm for the HD material. The HD emulsion has an improved line edge gradient, which means a sharper edge to the lithographic lines. This feature is important for microlithography research, which is the main application for the Millimask materials. Antihalation action is provided by a volumetric dye that forms part of the emulsion. The orthochromatic Slavich VR-P emulsion is similar to the Millimask material. Mainly it is intended for recording holograms of the transmission type. The emulsion s spectral sensitivity S is useful from 488 to 532 nm. The grain size is between 35 and 40 nm. Antihalation action is provided by an antihalation coating on the back of the material. The sensitivity S of VR-P measured at 514 nm is S 75 J cm 2. The panchromatic Slavich PFG-03C emulsion is intended for the recording of color holograms. The emulsion s useful spectral sensitivity is from 450 to 700 nm. The grain size is between 10 and 20 nm. There is no antihalation coating because the material is intended for the recording of both transmission and reflection holograms. The sensitivity S 2 3 mj cm 2 measured at 514 nm. The D-log E curves for these emulsions are shown in Fig. 1, which we obtained by recording amplitude Fig. 1. D-log E curves of investigated holographic plates. diffraction gratings with two beams interfering with each other at an angle of 30 by use of s-polarized light from an argon-ion laser nm. To avoid internal substrate reflections, an indexmatching fluid Dekalin solvent, n was applied between the emulsion substrate and a lightabsorbent black glass plate underneath it. Note the dramatic saturation effects shown in the D-log E curves. Our understanding is that these effects are caused by the transition of development from the formation of filament black silver to the formation of a colloidal brown silver as the development process evolves. We obtained the optical density using a photographic densitometer X-Rite 310. The procedure of making an amplitude hologram is as follows: 1 exposure incident intensity of 100 W cm 2, with various exposure times ; 2 develop 1:4 diluted Agfa G282c, lithographic developer, for 3 min at 20 C ; 3 wash 3 min in running water; 4 fix 1:4 diluted Ilford Hypam fixer with rapid hardener, diluted 1:40, for 2 min at 20 C; note that Ilford rapid hardener contains aluminum chloride ; 5 wash 5 min in running water; and 6 dry1hinhigh-humid desiccator with in excess of 60% relative humidity. 5. Reversal and Rehalogenating Bleaching in Silver Halide Sensitized Gelatin Processing There are two different bleaching techniques that can be applied to SHSG processing. After the emulsion has been developed, it can be bleached with a solvent reversal bleach, which means that the developed silver is dissolved during this process see Fig. 2. The other possibility is that the developed silver in the emulsion is rehalogenated with a bleach that normally contains potassium bromide see Fig. 3. The two techniques have their advantages and disadvantages, and which technique to apply depends on the results one wants to obtain. For example, to maintain emulsion thickness, the rehalogenating tech- 626 APPLIED OPTICS Vol. 40, No February 2001

6 Fig. 2. Schematic diagram of the typical solvent bleach SHSG process. nique is recommended. However, in both processes there is a fixing step applied in which material is removed from the emulsion. If the hardening of the gelatin is not sufficient, shrinkage of the emulsion may occur as a result of the fixing step. However, the selective hardening of the emulsion during bleaching and the overall hardening applied to the emulsion during processing will affect the results and make it possible to control emulsion thickness. 6. Gelatin Hardening and Its Role in Silver Halide Sensitized Gelatin Processing A. Effect of Gelatin Hardening before and during Processing In DCG holograms, the refractive-index modulation occurs in two phases, i.e., the rearrangement of gelatin chains and the formation of voids formed in the Fig. 3. Schematic diagram of the typical rehalogenating bleach SHSG process. unhardened areas. Unlike the situation for DCG processing, the SHSG process is based on the fact that microvoids formed during processing remain in the gelatin after processing, which constitute the refractive-index modulations. The microvoid structure that remains after processing 共Fig. 2兲 can collapse easily during or after processing, during the dehydration steps. The essential SHSG processing problem is how to substitute the molecules of water and isopropanol with air without emulsion shrinkage or collapse of the microvoid structure in the processed emulsion. To achieve this, the dry emulsion must be of an appropriate hardness, which means that it can withstand the wet processing without collapse of the microvoid structure. Hence the key to the success in SHSG processing is how to obtain and maintain a uniform and firm structure of microvoids in the pure gelatin layer after processing and drying. It is well known that the hardening, swelling, and drying techniques are the most important parts of the processing steps in both transmission and reflection HOE s obtained by SHSG processing.23 The main parts of our present investigation are 共1兲 to find the optimal hardening process, 共2兲 to find the dehydration technique that yields uniform and high-efficiency results, and 共3兲 to optimize the SHSG process for the new silver halide emulsions. B. Emulsion Characteristics As a Function of Hardening The significance of the gelatin hardening process is that it reduces the tendency of gelatin to swell in water and aqueous solutions and increases the range of temperature allowed for further processing. In general, the hardness of holographic emulsion affects the HOE characteristics in SHSG processing. It greatly depends on the initial hardness of the emulsion. The new silver halide emulsions are softer than, for example, the earlier Agfa emulsions as well as the Millimask plates. The emulsion swells during the wet processing, and the degree of swelling depends on the initial hardness of the dry emulsion. Swelling of gelatin depends on a number of factors, e.g., internal and external osmotic effects and viscoelastic effects. In addition, swelling will depend most importantly on ph, which can influence these effects. Minimum swelling occurs at or near the solution s isoelectric point. The swelling varies with the acidity or alkalinity of the solution. The isoelectric point is that ph where the negative and positive electric charges balance out. For lime-processed gelatin the isoelectric point is at a ph of approximately 4.7. In the performance of SHSG processing of the former holographic Agfa 8E75 HD emulsion, it was recommended that the plates be presoaked in a warm water solution before exposure to obtain the desired results.10 For the new silver halide emulsion BB640, hypersensitization was needed before exposure to obtain the best results.38 If the Slavich plates, which have a rather soft emulsion, are processed without prehardening, the emulsion could be dam10 February 2001 兾 Vol. 40, No. 5 兾 APPLIED OPTICS 627

7 aged during the washing or drying process. New hardening baths have been formulated that are suitable for the investigated emulsions. During the hardening process of the SHSG holograms the emulsion structure is converted into a pure gelatin structure of appropriate hardness. Such conversion possibly is caused by the fact that the reaction of gelatin molecules adsorbed on particles of silver or silver salts is less than that of surrounding molecules of gelatin mass. That is, the difference in hardening of gelatin at regions of the bright and dark zones formed in the emulsion during the exposure. These intensity variations are converted into a structure of differentially swelled emulsion layer regions. 23 The degree of gelatin swelling depends on the salt concentration of the processing solutions and the constitutes of the holographic emulsion. Only a limited amount of swelling takes place in the developer and fixer in normal hologram processing. In general, further swelling takes place during the bleach or rinse steps. C. Influence of ph on Gelatin Hardening A number of factors that influence the rate and degree of hardening or tanning of proteins are ph dependent. If chromium is introduced in the gelatin in a bleach bath of high acidity, the tanning effect is low. In aqueous solution basic chromate salts aggregate gelatin to a high degree in the ph range. For the chrome-hardening agents that are usually employed, there is a certain upper limit to the ph values of the solutions. This is caused by the fact that the incorporation of too many hydroxyl groups in the chromium complex will lead to a high degree of aggregation, which then makes the tanning compound too bulky for effective emulsion permeation and hence hardening. What is normally taking place in an acid bleach bath is that dichromate is absorbed into the bleach bath and reduced to trivalent chromium by the developed image silver. It has been suggested that the tanning can occur only in a subsequent washing bath that has a ph value of approximately 5. 5 The ph of the bleach solution is so low that the swelling of emulsion is considerable, and the rinse in distilled water exacerbates the problem of swelling as well. We focused on the hardening process before development and during the bleach procedure. Initially, various hardening agents were tested, such as, e.g., chrome alum, potassium alum, and formaldehyde solution. The ph value was adjusted somewhere between 4 and 6. Unfortunately, good results could not be achieved with these chemicals, although they are regarded as good hardeners for photographic emulsions. The degree of hardening with chrome alum is greater than that with potassium alum. However, the former solution was unstable, and thus it was difficult to control the degree of hardening. Other auxiliary hardening solutions that we tested Fig. 4. Diffraction efficiency of PFG-03C emulsion hardened in formaldehyde solution. contained aluminum nitrate, chromium III chloride, and ammonium chloride. The formaldehydehardening solution was found to be the best when we were producing SHSG holograms using the PFG-03C plates. The ph value was adjusted to approximately 5 to obtain maximum hardening with a formaldehyde bath. The results of the prehardening tests are shown in Fig. 4. We prepared the PFG-03C plates in a formaldehyde solution, varying the treatment time. The SHSG processing was according to the procedure described in the following. These results show that high diffraction efficiency can be obtained only when the holographic plate possesses appropriate hardness before and during the SHSG processing. If the gelatin is too soft, the microvoids formed in the gelatin structure will collapse or be destroyed. On the contrary, if it is too hard, the amplification of microvoids does not occur during the drying process. Given to this fact, it is important to understand fully the behavior of the silver halide emulsion. These aspects will be studied further when we investigate SHSG-processed HOE s of the reflection type. D. Effect of Hardener Added in the Bleach Solution The chrome ions, Cr 3, formed during the bleaching process harden the gelatin molecules by cross linking them. However, the acidity of bleach solution ph of approximately causes a serious emulsionswelling problem. If we adjust the ph to approximately 5, the chromium ions reduce in number and the bleaching will not proceed. We investigated the effect of hardening to find the optimum hardening condition for the SHSG process applied to the Agfa Millimask and the Slavich VR-P plates during processing. When additional hardener is added to the bleach solution, the process can be improved. The results are presented in Fig. 5. By our adding between 1% and 1.5% of hardener to the bleach solution, the diffraction efficiencies of the plates were improved up to 5% compared with normal processing. This phenomenon is still under investigation. It is not completely clear whether the hardener in the bleach solution really improves the overall HOE 628 APPLIED OPTICS Vol. 40, No February 2001

8 Fig. 5. Effect of added hardener in the bleach solution. quality. However, it does improve the diffraction efficiency of the HOE s. 7. Drying Methods in Silver Halide Sensitized Gelatin Processing with Various Solvents The most important variables in the SHSG process, which directly influence the characteristics of the SHSG hologram, are the degree of emulsion hardness during the wet processing as well as the condition of the drying process. In particular, both the temperature and the speed of drying affect the characteristics and the uniformity of the SHSG hologram. The overall task is to substitute the molecules of isopropanol and water contained in the emulsion during the wet processing with air molecules in the last dehydration step. During the final drying step, i.e., when the plate is brought out of the hot isopropanol bath, a sudden transition was observed, which is like a sudden extraction of vapor from the gelatin layer. This occurs only when the temperature of the hot isopropanol bath is close to its boiling point. Only at that temperature does the vapor pressure of isopropanol exceed the atmospheric pressure, and then the isopropanol vapor can be extracted from the microvoid in the gelatin layer. The sudden extraction causes the amplification of microvoids. The effect of the drying process temperature was investigated. The obtainable diffraction efficiency as a function of the final drying bath temperature is shown in Fig. 6. When the isopropanol temperature reaches the boiling point, the highest diffraction efficiency is obtained. These results are in agreement with the results of Usanov et al. 23 When solvents are used in the SHSG drying process, it seems that the saturation vapor pressure participates in the amplification of the microvoids. Other solvents, such as methanol and ethyl-methylketone butan-2-one, have been examined, as well as different drying procedures. Although the saturation vapor pressure of methanol is higher than that of any other solvents examined, the diffraction efficiency of a SHSG hologram processed in methanol is lower than that of holograms processed in isopropa- Fig. 6. Diffraction efficiency as a function of the final drying bath temperature. nol. When methanol was introduced in any sequence of the drying process, ripple defects affected the surface of the gelatin. The saturation vapor pressure of ethyl-methyl-ketone is much lower than that of methanol; the results of our using ethylmethyl-ketone was better than methanol, although not as good as when we used isopropanol. Ethylmethyl-ketone tends to attack film substrates, which means that this solvent is not applicable for processing holographic emulsions coated on film substrates, such as, e.g., triacetate. 8. Improved Silver Halide Sensitized Gelatin Wet Processing Various developers, bleaching, and fixing baths were tested to find the maximum-obtainable diffraction efficiency of the VR-P, PFG-03C, and Millimask materials. First, the difference between two nontanning developers was investigated. The AAC developer does not generate any oxidation products during development. The other is the lithographic Agfa G282c developer. The optical density of the plates developed in the AAC developer is slightly higher than that of the plates developed in G282c. For practical SHSG processing it may be an advantage to use a standard developer, such as the G282c the developer for high-speed reversal processing of Agfa Millimask plates. Such nontanning developers usually contain halide solvents and encourage sharp developed image edges. In addition, it actually endows the emulsion with a notable speed gain when compared with normal high-contrast development. The bleaching process was investigated carefully. We studied the capability of attacking silver in gelatin layers and hardening of gelatin by chrome and aluminum ions. Various chemicals, such as, e.g., chromium chloride, aluminum chloride, and aluminum nitride, were added in the bleach to enhance the hardening process. The gelatin molecule crosslinking process depends on the number of the Cr 3 ions and in particular the ph of the bleach solution. The acidic nature of the bleach solution tends to move the ph to the acidic side of the optimum for the hard- 10 February 2001 Vol. 40, No. 5 APPLIED OPTICS 629

9 Fig. 7. Diffraction efficiency of Millimask plates as a function of the fixing speed. ening process ph of approximately 5. If the bleach is adjusted to a ph of 5, then the dichromate ions will no longer do their job and the bleaching process will not proceed. We introduced extra Cr 3 ions by adding, for example, chromium III chloride or chromium III nitrate to the bleach solution. We can also introduce trivalent aluminum ions by adding, e.g., aluminum III chloride or aluminum III nitrate to the bleach. Trivalent ions derived in this way are not dependent on the local density of developed silver and thus are not known to enhance the differential hardening effects of the tanning bleaches. The former of these compounds is used commonly in hardening fixer solutions for photography. The obtained diffraction efficiency results for HOE s that were produced in augmented tanning bleach are presented in Fig. 5. The addition of extra cross-linking ions has an increasing effect on the diffraction efficiency of HOE s produced by this process. Another important factor in SHSG processing is the speed at which silver halide is removed from the emulsion during fixing. When silver salts are removed, the microvoids may be affected. To minimize this, we investigated the concentration of the fixing solutions used and its influence on diffraction efficiency. Employing weaker fixers means that a longer processing time is needed to remove the silver salts. The slower this process is, the less is its po- Fig. 8. Diffraction efficiency of SHSG-processed plates as a function of exposure. tential negative influence on the emulsion microstructure, provided that the selective emulsion hardening is optimal in previous processing steps. To verify that the fixing speed is important, a standard fix of different dilutions were tested as well as fixing solutions composed of 10-g l and 40-g l sodium thiosulfate hypo. The processing time in these weak fixing solutions may take min to completely remove the silver salts. The influence on diffraction efficiency obtainable on Millimask plates is presented in Fig. 7. A concentration of 10-g l sodium thiosulfate resulted in the highest diffraction efficiency. The new SHSG processing procedure for the investigated materials is summarized in Table 1. The diffraction efficiencies obtained with the new optimized SHSG process are presented in Fig Transmission Electron Microscopy Investigation To show the microstructure of processed SHSG emulsion, transmission electron microscopy TEM investigations of some of the Slavich VR-P plates were performed. Figure 9 shows the representative sizes of the silver halide grains that we obtained by the rehalogenating bleaching method. The size of these grains are between 40 and 70 nm. Remember that Table 1. SHSG Process Procedure Treatment Condition Exposure Argon-ion laser 514 nm Prehardening Formalin bath PFG-03C 7 min at room temperature Developing Agfa G282c 1:4 diluted 3 min at room temperature Washing Running deionized water At least 3 min Bleaching Rehalogenating bleach 5 min at room temperature Washing Running deionized water At least 3 min Fixing Diluted fixer 2 min at room temperature Washing Running deionized water At least 3 min Hot deionized water 10 min at Drying 50%:50%, isopropanol:deionized water 3 min at 30 C 100% isopropanol 3 min at 30 C 100% isopropanol 2 min at 75 C 630 APPLIED OPTICS Vol. 40, No February 2001

10 the VR-P plates have a coated grain size of approximately 40 nm. In Fig. 10 the interference fringes of the recorded diffraction grating are shown at low magnification. In Fig. 11, at higher magnification, the final microvoids are visible, and their sizes are approximately 70 nm. Fig. 9. Cross-sectional TEM image of a bleached VR-P plate processed by the rehalogenating method. Fig. 10. Cross-sectional TEM image of a SHSG-processed VR-P plate at low magnification, which shows the fringes in the emulsion. Fig. 11. Cross-sectional TEM image of a SHSG-processed VR-P plate at high magnification, which shows the microvoids in the emulsion. 10. Conclusions The SHSG process applied to the new improved silver halide emulsions has been investigated, in particular, the influence of hardening, the wet processing steps, and the dehydration process. We have found a working process for SHSG that could be adapted for various types of holographic plates. We believe that this is important because a minor change in processing applied to different materials will cause variation in yield, reproducibility, and reliability of products in mass-production situations. In addition to the Slavich emulsions used in this investigation, there are two red-sensitive emulsions, the PFG-03M emulsion and the PFG-01. The PFG-03M is of the ultra-high-resolution type and is similar to the PFG-03C material. The faster PFG-01 emulsion can be used for SHSG processing of transmission HOE s recorded with red laser wavelengths. The new silver halide emulsions are the most suitable to record high-efficiency transmission HOE s with SHSG processing. Similar results were reported in the investigation of the BB-640 emulsion from Holographic Recording Technologies in Germany by Neipp et al. 39 This research has been supported by the Advanced Institute of Technology, Samsung, Suwon, South Korea under De Montfort Expertise Ltd. contract References 1. Yu. A. Sazonov, and P. I. Kumonko, Holographic materials produced by the Micron plant at Slavich, in Sixth International Symposium on Display Holography, T. H. Jeong, ed., Proc. SPIE 3358, R. Bierenheide, BB emulsion series: current standings and future developments, in Sixth International Symposium on Display Holography, T. H. Jeong, ed., Proc. SPIE 3358, H. I. Bjelkhagen, Silver Halide Recording Materials for Holography and Their Processing, Vol. 66 of Springer Series in Optical Sciences Springer-Verlag, New York, 1993, pp K. S. Pennington, J. S. Harper, and F. P. Laming, New phototechnology suitable for recording phase holograms and similar information in hardened gelatin, Appl. Phys. Lett. 18, W. R. Graver, J. W. Gladden, and J. W. Eastes, Phase holograms formed by silver halide sensitized gelatin processing, Appl. Opt. 19, B. J. Chang and K. Winick, Silver-halide gelatin holograms, in Recent Advances in Holography, T. C. Lee and P. N. Tamura, eds., Proc. SPIE 215, A. Fimia, M. Pardo, and J. A. Quintana, Noise reduction in holographic images reconstructed with blue light, Appl. Opt. 22, P. G. Boj, A. Fimia, and J. A. Quintana, Silver-halide gelatin for the fabrication of holographic optical elements, in Image 10 February 2001 Vol. 40, No. 5 APPLIED OPTICS 631

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