- Email: [email protected]
Fixation-free rehalogenating bleached reflection holograms recorded on BB-640 plates
1 August 2000
Optics Communications 182 Ž2000. 107–114 www.elsevier.comrlocateroptcom
Fixation-free rehalogenating bleached reflection holograms recorded on BB-640 plates C. Neipp a , I. Pascual b, A. Belendez ´ a
a,)
Departamento de Fısica, Ingenierıa ´ ´ de Sistemas y Teorıa ´ de la Senal, ˜ UniÕersidad de Alicante, Apartado 99, E-03080 Alicante, Spain b ´ Departamento InteruniÕersitario de Optica, UniÕersidad de Alicante, Apartado 99, E-03080 Alicante, Spain Received 30 March 2000; received in revised form 12 June 2000; accepted 14 June 2000
Abstract Fixation-free rehalogenating bleaching is an interesting process for the production of phase reflection holograms. The shrinkage of the emulsion is reduced in comparison with other bleaching methods Žreversal bleaching or rehalogenating bleaching with a fixation step., which is particularly interesting in the case of reflection holograms. In this communication, we present experimental results for fixation-free rehalogenating bleached holograms derived from the novel BB-640 emulsion, a red-sensitive ultra-fine grain emulsion from Holographic Recording Technologies using an R-10 type bleach bath. The influence of the potassium bromide concentrations in the bleach solution on the final quality of the holograms is also studied. The concentrations of the different components of the bleach solution are adjusted to obtain the highest diffraction efficiencies. A high diffraction efficiency of 72% is obtained. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Holography; Holographic recording materials; Photographic emulsions; Photosensitive processing
1. Introduction Bleaching is one of the most popular techniques used to obtain phase holograms with high diffraction efficiencies from silver halide emulsions. This technique was first developed for laser-recorded holograms by Cathey w1x and subsequently several bleaching processes were studied. Upatnieks w2x introduced ferricyanide and copper bleaches, whereas the popular Kodak R-10 bleach was improved by McMahon and Franklin w3x. Concerning phase reflection holograms, Cooke and Ward w4x studied a bleach
) Corresponding author. Tel.: q34-96-590-3651; fax: q34-96590-3464; e-mail: [email protected]
containing an organic oxidizing agent, quinone, introduced by Phillips w5x in the conventional bleaches, and obtained good results. In conventional bleaching techniques the developed hologram is treated with a fixing bath to remove the unexposed silver-halide grains from the emulsion w6x. Then, during rehalogenating bleaching the developed silver is converted back into silver halide. Therefore, a phase hologram is formed as a result of the refractive index modulation created between the exposed and the non-exposed zones. A disadvantage of conventional bleaching techniques is that the emulsion shrinks after the procedure, due to the removal of material from the emulsion. This distorts the interference fringes, created during exposure. The fixation-free method, which was improved
0030-4018r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 0 0 . 0 0 8 1 4 - 2
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C. Neipp et al.r Optics Communications 182 (2000) 107–114
by Crespo et al. w7x and Kostuk w8x, avoids these problems, because the shrinkage of the emulsion is minimum. In this method the hologram is bleached directly after development, without fixing, leaving the unexposed silver-halide crystals in the emulsion. This technique is particularly interesting for reflection holograms where the geometry of the fringes is highly sensitive to any thickness variation. Hariharan and Chidley w9x studied the fixation-free method using rehalogenating bleaches and showed that the diffusion of material during the bleach bath is the main mechanism involved in the process. During the bleach bath there is a transference of material from the exposed to the unexposed zones and as a result the silver-halide grains in the non-exposed zones increase in size. The refractive index modulation is a result of the difference in size of the silver halide grains in the exposed and non-exposed zones. This diffusion process is particularly influenced by the halide concentration in the bleach solution w10x. The concentration of the halide must be optimized in order to obtain high values of diffraction efficiency and low values of scattering. Over the last decades the emulsions most commonly used to obtain high-quality bleached holograms were supplied by the Agfa company: the series 8E75 HD and 8E56 HD. Nevertheless Agfa ceased production of holographic material in 1997 w11x. This represents a serious drawback for people who have been using the Agfa material in their work and implies that it is necessary to develop and study alternative silver-halide materials – in particular, the currently available silver halide materials such as those of Slavich ŽRussia., Royal Holographic Art Gallery ŽGreat Britain. and the BB series from Holographic Recording Technologies ŽGermany. – which could fill the gap created by withdrawal of the Agfa material from the market. In this communication, we study and optimize an R-10 type bleach applied to the novel BB-640 emulsion to be used for recording reflection holograms. The bleach is composed of potassium dichromate, potassium bromide and sulfuric acid. There are some differences between the BB-640 emulsion and the Agfa 8E75 HD emulsion, therefore the optimum concentrations of the different components of the bleach solution must been found in order to obtain high-quality holograms. In particular, we adjusted
the concentration of the potassium bromide contained in the bleach solution. 2. Previous considerations One of the most important factors to be taken into account in the optimization of the bleach is the size of the silver halide grains suspended in the gelatin of the emulsion. The size of the silver-halide grains of BB-640 emulsion is, according to manufacturer, between 20 and 25 nm, whereas the size of the grains in the case of Agfa 8E75 HD is 44 nm w11x. The size of the silver halide grains influence the scattering at the recording, as was analyzed in a previous article w12x. In this respect the BB-640 plates gave better results than the Agfa plates. The smaller size of the silver halide grains of the BB-640 plates also makes it possible to obtain holograms with less scattering than those recorded on Agfa 8E75 HD plates. The response of the diffraction efficiency as a function of the spatial frequency is also improved, and consequently reflection holograms of higher quality may be obtained using the BB-640 silver halide emulsion. The size and concentration of the silver halide grains also influence the optimum concentration of potassium bromide in the bleach bath. We varied the potassium bromide concentration to find the optimum concentration leading to high values of diffraction efficiency. 3. Experimental designr r set-up The experiments were carried out on red sensitive BB-640 silver halide photographic plates. Holographic reflection gratings were recorded using two collimated beams from a 15 mW He–Ne laser Ž633 nm., with the polarization vector perpendicular to the plane of incidence. The K ratio, defined as the ratio of the reference beam intensity to the object beam intensity, was taken to be 1 inside the emulsion. One beam impinged perpendicular to the emulsion, whereas the other beam fell on the back side of the plate forming an angle in air of 308 with the perpendicular to the surface of the emulsion. With the geometry described, the spatial frequency of the gratings was calculated as ; 5100 linesrmm.
C. Neipp et al.r Optics Communications 182 (2000) 107–114 Table 1 Processing schedule and developer composition Processing schedule 1. Develop in D-19 for 5 min 2. Rinse in running water for 1 min 3. Bleach for 1 min after the plate has cleared 4. Rinse in running water for 5 min 5. Dry at room temperature for 24 h All solutions are at 208C except the bleaching step D-19 deÕeloper Metol Sodium sulfite Žanhydrous. Hydroquinone Sodium carbonate Žanhydrous. Potassium bromide Distilled water
2g 45 g 8g 50 g 5g 1 liter
109
beam intensity to the incident collimated probe-beam intensity of the He–Ne laser. In order to take into account Fresnel losses and absorption due to the glass substrate, this expression was corrected by multiplying by an appropriate factor. As explained by Kostuk w8x, noise gratings are very efficient at Bragg angle. Therefore the plates were rotated until maximum diffraction efficiency was reached. The efficiency of the zero-order or transmission t was similarly calculated as the ratio of the directly transmitted beam intensity to the incident power and was corrected using the same factor. 4. Results and discussion 4.1. Optimization of the B r A ratio
In previous experiments w13x we found that the gelatin of the BB-640 plates is hardened to a high degree, so that the products of the developer do not penetrate easily into the emulsion. Therefore, the plates were hypersensitized by immersing them in a solution of distilled water with a sodium sulfite concentration of 1% and urea concentration of 5% Žby weight. for 10 min at 208C. After rinsing in running water for 1 min at 208C the plates were dried for 24 h at 208C and 60% RH. The urea of this solution softens the gelatin, and the sensitivity of the emulsion is also increased due to the sodium sulfite in the solution. In the bleach bath process, diffusion is the main mechanism involved in the refractive index modulation. We decided to use D-19, which is a tanning developer. D-19 causes the silver halide grains to increase in size after the bleach bath, due to oxidation products. Nevertheless oxidation products of the D-19 also create a differential degree of hardening of the gelatin during the bleach bath. This differential hardening promotes the diffusion process, thus favouring the refractive index modulation created by the transference of material between the exposed and the unexposed zones. After exposure, the plates underwent the schedule procedure illustrated in Table 1, so that phase reflection holograms were finally obtained. The diffraction efficiency h of the recorded phase holograms was calculated as the ratio of the diffracted
The R-10 bleach has been used with Agfa 8E75 HD plates with good results. Table 2 shows the composition of a modified version of this bleach Žmod R-10.. The bleach bath solution is composed of two different solutions: A and B. The oxidizer Žpotassium dichromate. is in solution A, whereas the rehalogenating agent Žpotassium bromide. is in solution B. To obtain the bleach solution, 1 part of A is mixed with 10 parts of distilled water and X parts of B. The ratio X s BrA indicates the relation between the potassium bromide concentration and the oxidizer. For Agfa 8E75 HD plates, an optimized R-10 type bleach is the one used by Crespo et al. w7x. Nevertheless the differences in the degree of hardening of the gelatin, and the differences in the size and
Table 2 Bleach bath formula Bleach bath composition Žmodified version of R-10. Solution A Potassium dichromate Sulfuric acid Distilled water
20 g 15 ml 1 liter
Solution B Potassium bromide Distilled water
100 g 1 liter
Just before use, mix 1 part of A with 10 parts of distilled water and add X parts of B, Br As X.
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concentration of the silver halide grains in the Agfa 8E75 HD and BB-640 plates, make it necessary to find a new optimum BrA ratio for BB-640 plates. In our experiments the BrA ratios took the values: 2, 4, 8, 15, 30, 60, 120. Fig. 1 shows diffraction efficiency as a function of the BrA ratio. It can be seen from this figure that the best results were obtained for a BrA ratio of 15. A high peak diffraction efficiency of 72% Ž67% if Fresnel losses are not taken into account. was obtained with this BrA ratio. This result is comparable to the best results obtained by recording bleached reflection gratings on Agfa 8E75 HD emulsion. Crespo et al. w7x obtained a peak diffraction efficiency of ; 70%. They developed the plates with AAC and bleached them with the mod R-10, which was made up with 2-grliter potassium dichromate, 35-grliter potassium bromide and 10-cm3rliter sulfuric acid. Nevertheless, for D-19 the values of the diffraction efficiency which they obtained were lower than 50%. Based on fixation-free bleaching techniques, Kostuk w8x also obtained high values of diffraction efficiency Ž65% without considering Fresnel reflections. for reflection holograms recorded on Agfa 8E75 HD emulsion. The plates were developed with a variant of CW-C1 mixture, and bleached with a modified version of the R-10 bleach. In order to optimize the bleach bath he varied the concentrations of the potas-
Fig. 1. Diffraction efficiencies as a function of the BrA ratio for reflection gratings recorded on BB-640 plates bleached with an R-10 type bleach Žat 508C. with seven different BrA ratios: 2, 4, 8, 15, 30, 60 and 120.
sium dichromate ŽKD., the potassium bromide ŽKBr. and the sulfuric acid ŽSA.. The optimized values were 36 grliter of KBr, 0.5 grliter of KD, and 2 mliterrliter of SA. For Agfa 8E75 HD, Kostuk indicated that the concentration of KBr should be high to maximize efficiency, the concentrations of KD should be low to minimize noise gratings and reduce emulsion swelling and the concentration of SA should also be minimized in order to reduce emulsion shrinkage. It is interesting to notice that the concentrations of our optimized bleach solution ŽBrA s 15. are 0.76 grliter of KD, 57 grliter of KBr and 0.57 cm3rliter of SA, which agree with Kostuk statements. The values of diffraction efficiency obtained by Cooke and Ward w4x were also very high, 75% Žafter considering Fresnel reflections., they developed the plates with CW-C1 and bleached them in PBQ, which has the inconvenient of being hazardous, the temperature of the baths Žincluding the bleach. was maintained at 208C. This difference in the bleach bath temperature with respect to our process is due to the different degree of hardening of the gelatin of Agfa 8E75 HD and BB-640 plates. It can also be seen that the peak diffraction efficiency is highly sensitive to the BrA ratio. The presence of potassium bromide in the bleach solution favours the diffusion process, because the solubility of the silver ion is increased. As the concentration of potassium bromide increases, so does the solubility of the silver ion, which will cause the silver bromide grains in the non-exposed zones to increase in size. A high concentration of potassium bromide also favours coalescence of separated crystals w9x. The amount of light scattered by a small particle is proportional to the sixth power of its radius. Therefore an increase in the potassium bromide concentration implies an increase in the scattered light intensity and a corresponding decrease in the maximum diffraction efficiency achievable. For transmission holograms Žspatial frequency ; 1200 linesrmm. the small size of the silver halide grains in the non-exposed zones permits high diffraction efficiencies to be obtained with a large range of potassium bromide concentrations w10x. For reflection holograms the path along which the silver ion travels between the exposed and the non-exposed zones is shorter, thus the silver halide grains in the non-exposed zones in-
C. Neipp et al.r Optics Communications 182 (2000) 107–114
crease in size more rapidly as the BrA ratio increases. 4.2. Influence of the B r A ratio on the final thickness of the emulsion During the rehalogenating bleaching procedure, the gelatin of the emulsion varies in thickness. The maximum changes in thickness in the emulsion occur during the drying process w7x. The extent to which the emulsion varies in thickness depends on the degree of hardening of the gelatin after the bleaching process. Processed hardened layers when dry are thicker than unhardened ones because of a significant loss of elasticity in the layer. The change in the thickness of the emulsion after the procedure alters the fringe spacing, which also affects the wavelength at reconstruction. The relation between the wavelength at reconstruction and the wavelength at the recording can be expressed by the following relation:
lX
LX s
l
L
Ž 1.
where L, LX are the grating periods at the recording and reconstruction, respectively, and l, lX are the wavelengths at the recording and reconstruction, respectively. The changes in thickness of the emulsion are mainly due to the following factors: Factors which contribute to the swelling of the gelatin: 1. The hypersensitization process causes the gelatin of the emulsion to swell, due to the urea present in the solution. 2. During the development step the immersion of the emulsion in an alkaline solution could contribute to the emulsion swelling w14x. Factors which contribute to the hardening of the gelatin: The hardening action is promoted by potassium dichromate. In the oxidation process the Crq6 ion is chemically reduced to Crq3 . The hardening of the gelatin is a consequence of the cross-links created by Crq3 ions between the gelatin chains. This action takes place in the vicinity of the oxidized silver grains.
111
Fig. 2 shows the wavelength at reconstruction as a function of the exposure for BB-640 plates bleached using the different BrA ratios. It can be seen that in all cases there is an increase in the wavelength of reconstruction as the exposure increases. The concentration of the developed silver grains in the emulsion increases with exposure, as does the presence of the Crq3 ion in the gelatin and the subsequent degree of hardening of the gelatin. This hardening action inhibits the shrinkage of the emulsion as the emulsion dries, which explains why the wavelength at reconstruction for maximum diffraction efficiencies increases with exposure. The influence of the BrA ratios on the wavelength at reconstruction is clear from Fig. 2. It can be seen that much care must be taken to minimize the wavelength shift. For instance, when the BB-640 plates are bleached using the BrA ratio which yields maximum diffraction efficiency ŽBrA s 15., the wavelength shift is less than 1% within a range of exposures from 900 to 5000 mJrcm2 . However, for a BrA ratio of 4, the emulsion shrank after the procedure, so that the wavelength at reconstruction remained below 623 nm. The dependence of the wavelength at reconstruction on the BrA ratio and on exposure could give a method to control the wavelength at reconstruction by adjusting the BrA ratio and exposure appropriately.
4.3. Angular response of the diffraction efficiency The efficiency of the diffracted and the transmitted beams, h and t respectively, together with the absorption of the material a d Žwhere a is the absorption coefficient and d is the thickness of the photographic emulsion after processing. are three parameters that may be used to characterize holographic reflection gratings recorded on the silver halide sensitized gelatin of BB-640 plates. By means of Kogelnik’s coupled wave theory w15x, it is possible to obtain an analytical expression for the diffraction efficiency of volume holograms. As Kogelnik’s theory is limited to the description of two diffracted orders, then
h q t s exp Ž ya drcos u X .
Ž 2.
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and t can be obtained as follows:
t s exp Ž ya drcos u X . y h
Ž 3.
X
t s exp Ž ya drcos u .
= 1y
Ž 1 y j 2rn 2 . 1q 1r2 sinh2 Ž n 2 y j 2 .
y1
0
Ž 4.
In Eqs. Ž2. – Ž4., a takes into account the absorption Žand also the scattering; we have no means of differentiating between the two at this point.; d is the thickness of the final hologram and u X is the angle of reconstruction in the medium, which is related to the angle of reconstruction in air u by Snell’s law. The rest of parameters of Eq. Ž4. are the usual used in Kogelnik’s Theory and are defined in Ref. w15x. We tested the holograms which yielded peak diffraction efficiency by rotating them, and the variation in transmission with the angle of incidence t in air was measured. The values of transmission were corrected to take into account Fresnel’s reflections and the absorption of the glass substrate. Fig. 3 shows the angular dependence of the transmission for the holograms which led to the best agreement between theory and experiment ŽBB-640 bleached using a BrA ratio of 8.. The parameters obtained by fitting the theoretical function to the experimental data where: refractive index modulation, D n s 0.034, thickness, d s 8.1 mm, and absorption coefficient, a s 0.0288 mmy1 .
Fig. 2. Reconstruction wavelength for Bragg’s condition as a function of exposure for reflection gratings recorded on BB-640 plates bleached with an R-10 type bleach Žat 508C. with seven different BrA ratios: 2, 4, 8, 15, 30, 60 and 120.
Fig. 3. Experimental measurements of transmittance and theoretical fit Žsolid curve. as a function of the reconstruction angle in air and for reflection gratings recorded on BB-640 plates bleached with an R-10 type bleach Žat 508C. with a BrA ratio of 8.
C. Neipp et al.r Optics Communications 182 (2000) 107–114
113
4.4. Response of the diffraction efficiency as a function of the waÕelength 4.4.1. Consideration of noise gratings and undesired reflections A small amount of light reflects from the rear part of the plate during the recording. Consequently, reflection gratings of low modulation are created, due to the interference of the incident light and the reflected light. In addition the self interference of the incident light and light scattered at the recording also create noise gratings which lower the maximum achievable diffraction efficiency. To study the effect of these gratings, BB-640 plates were exposed to a single beam and the exposed plates were bleached using BrA ratios of 8
Fig. 5. Experimental measurements of transmittance and theoretical fit Žsolid curve. as a function of the reconstruction wavelength for reflection gratings recorded on BB-640 plates bleached with an R-10 type bleach Žat 508C. with a BrA ratio of 8. The reconstruction of the gratings was made with the illuminating beam entering normal to the surface of the plate.
and 15. The dependence of the transmission on the wavelength was measured using a spectrophotometer. Fig. 4 shows the transmittance of the BB-640 plates bleached with two different bleaches ŽBrA s 8 and BrA s 15. versus the wavelength for three different exposures. It can be seen that maximum efficiency for noise gratings was obtained for a replay at a wavelength of ; 650 nm.
Fig. 4. Transmittance as a function of the reconstruction wavelength under Bragg’s condition for noise gratings and undesired reflection gratings recorded on BB-640 plates bleached with an R-10 type bleach Žat 508C. and with BrA ratios of 8 and 15.
4.4.2. Response of the diffraction gratings as a function of the waÕelength The theoretical response of the diffraction efficiency as a function of the wavelength can also be derived from Kogelnik’s theory. The angle of incidence at reconstruction was fixed at 08, whereas the wavelength was now the free parameter. The response of the transmission efficiency as a function of the wavelength for reflection holograms recorded on BB-640 plates was also determined experimentally using a spectrophotometer. In order to obtain the theoretical function, the absorption of the gelatin was fitted to a polynomial function of the third order. To obtain the theoretical function we used the values of D n, a and d obtained in the previous section. Fig. 5 shows the measured and theoretical values of transmittance for reflection holograms recorded
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on the BB-640 plates bleached using a BrA ratio of 8. The reconstruction of the gratings was made with the illuminating beam entering normal to the surface of the plates. As mentioned, the presence of a noise grating at a wavelength of ; 650 nm causes deviations of the theoretical function from the experimental data, so that the theoretical curve does not reproduce exactly the behaviour of the transmittance from the right part of the principal lobe. Nevertheless good reproduction of the experimental data was obtained. 5. Conclusions Fixation free rehalogenating techniques to obtain phase reflection holograms on the novel BB-640 plates have been analyzed. The smaller grain size of the BB-640 emulsion makes it possible to obtain reflection holograms with high diffraction efficiencies compared with those obtained using Agfa 8E75 HD plates. The bleach used was a version of the R-10 bleach, and the concentrations of solution B, containing the rehalogenating agent, and solution A, containing the oxidizer, were adjusted to obtain the highest values of diffraction efficiency. By bleaching the plates using a BrA ratio of 15 a high peak diffraction efficiency of 72% Ž67% if Fresnel reflections are not taken into account. was achieved. It was also demonstrated that the diffraction efficiency of the final holograms is highly sensitive to the BrA ratio. Nevertheless high diffraction efficiencies were achieved over a wide range of BrA ratios Žfrom 4 to 60 the values remained over 52%.. For reflection gratings recorded in Agfa emulsions – and for D-19 developer – a lower value of diffraction efficiency
Ž- 50%. was obtained by other authors w7x. Finally, the results presented show that reflection holograms of high quality can be obtained using the novel BB-640 plates. This implies that BB-640 plates are a good substitute for Agfa 8E75 HD plates with respect to the recording of bleached reflection holograms.
Acknowledgements This work was partially financed by the CICYT ŽComision ´ Interministerial de Ciencia y Tecnologıa, ´ Spain. under project No. MAT97-0705-C02-02.
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x
W.T. Cathey, J. Opt. Soc. Am. 55 Ž1965. 457. J. Upatnieks, C. Leonard, Appl. Opt. 8 Ž1969. 85. D.H. McMahon, A.R. Franklin, Appl. Opt. 8 Ž1969. 1927. D.J. Cooke, A.A. Ward, Appl. Opt. 23 Ž1984. 993. N.J. Phillips, A.A. Ward, R. Cullen, D. Porter, Phot. Sci. Eng. 24 Ž1984. 120. H.I. Bjelkhagen, Silver Halide Recording Materials for Holography and Their Processing, Springer, Berlin, 1993. J. Crespo, A. Fimia, J.A. Quintana, Appl. Opt. 25 Ž1986. 1642. R. Kostuk, Appl. Opt. 30 Ž1991. 1611–1616. P. Hariharan, Appl. Opt. 29 Ž1990. 2983–2985. P. Hariharan, C.M. Chidley, Appl. Opt. 26 Ž1987. 3895– 3898. N.J. Phillips, Hologr. Newsl. 8 Ž1. Ž1997. 7. C. Neipp, I. Pascual, A. Belendez, Appl. Opt. 38 Ž1999. ´ 1348–1356. A. Belendez, C. Neipp, M. Flores, I. Pascual, J. Mod. Opt. ´ 45 Ž1998. 1985–1992. P. Hariharan, C.M. Chidley, Appl. Opt. 28 Ž1999. 422–424. H. Kogelnik, Bell. Syst. Tech. J. 48 Ž1969. 1909–2947.
Optics Communications 182 Ž2000. 107–114 www.elsevier.comrlocateroptcom
Fixation-free rehalogenating bleached reflection holograms recorded on BB-640 plates C. Neipp a , I. Pascual b, A. Belendez ´ a
a,)
Departamento de Fısica, Ingenierıa ´ ´ de Sistemas y Teorıa ´ de la Senal, ˜ UniÕersidad de Alicante, Apartado 99, E-03080 Alicante, Spain b ´ Departamento InteruniÕersitario de Optica, UniÕersidad de Alicante, Apartado 99, E-03080 Alicante, Spain Received 30 March 2000; received in revised form 12 June 2000; accepted 14 June 2000
Abstract Fixation-free rehalogenating bleaching is an interesting process for the production of phase reflection holograms. The shrinkage of the emulsion is reduced in comparison with other bleaching methods Žreversal bleaching or rehalogenating bleaching with a fixation step., which is particularly interesting in the case of reflection holograms. In this communication, we present experimental results for fixation-free rehalogenating bleached holograms derived from the novel BB-640 emulsion, a red-sensitive ultra-fine grain emulsion from Holographic Recording Technologies using an R-10 type bleach bath. The influence of the potassium bromide concentrations in the bleach solution on the final quality of the holograms is also studied. The concentrations of the different components of the bleach solution are adjusted to obtain the highest diffraction efficiencies. A high diffraction efficiency of 72% is obtained. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Holography; Holographic recording materials; Photographic emulsions; Photosensitive processing
1. Introduction Bleaching is one of the most popular techniques used to obtain phase holograms with high diffraction efficiencies from silver halide emulsions. This technique was first developed for laser-recorded holograms by Cathey w1x and subsequently several bleaching processes were studied. Upatnieks w2x introduced ferricyanide and copper bleaches, whereas the popular Kodak R-10 bleach was improved by McMahon and Franklin w3x. Concerning phase reflection holograms, Cooke and Ward w4x studied a bleach
) Corresponding author. Tel.: q34-96-590-3651; fax: q34-96590-3464; e-mail: [email protected]
containing an organic oxidizing agent, quinone, introduced by Phillips w5x in the conventional bleaches, and obtained good results. In conventional bleaching techniques the developed hologram is treated with a fixing bath to remove the unexposed silver-halide grains from the emulsion w6x. Then, during rehalogenating bleaching the developed silver is converted back into silver halide. Therefore, a phase hologram is formed as a result of the refractive index modulation created between the exposed and the non-exposed zones. A disadvantage of conventional bleaching techniques is that the emulsion shrinks after the procedure, due to the removal of material from the emulsion. This distorts the interference fringes, created during exposure. The fixation-free method, which was improved
0030-4018r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 Ž 0 0 . 0 0 8 1 4 - 2
108
C. Neipp et al.r Optics Communications 182 (2000) 107–114
by Crespo et al. w7x and Kostuk w8x, avoids these problems, because the shrinkage of the emulsion is minimum. In this method the hologram is bleached directly after development, without fixing, leaving the unexposed silver-halide crystals in the emulsion. This technique is particularly interesting for reflection holograms where the geometry of the fringes is highly sensitive to any thickness variation. Hariharan and Chidley w9x studied the fixation-free method using rehalogenating bleaches and showed that the diffusion of material during the bleach bath is the main mechanism involved in the process. During the bleach bath there is a transference of material from the exposed to the unexposed zones and as a result the silver-halide grains in the non-exposed zones increase in size. The refractive index modulation is a result of the difference in size of the silver halide grains in the exposed and non-exposed zones. This diffusion process is particularly influenced by the halide concentration in the bleach solution w10x. The concentration of the halide must be optimized in order to obtain high values of diffraction efficiency and low values of scattering. Over the last decades the emulsions most commonly used to obtain high-quality bleached holograms were supplied by the Agfa company: the series 8E75 HD and 8E56 HD. Nevertheless Agfa ceased production of holographic material in 1997 w11x. This represents a serious drawback for people who have been using the Agfa material in their work and implies that it is necessary to develop and study alternative silver-halide materials – in particular, the currently available silver halide materials such as those of Slavich ŽRussia., Royal Holographic Art Gallery ŽGreat Britain. and the BB series from Holographic Recording Technologies ŽGermany. – which could fill the gap created by withdrawal of the Agfa material from the market. In this communication, we study and optimize an R-10 type bleach applied to the novel BB-640 emulsion to be used for recording reflection holograms. The bleach is composed of potassium dichromate, potassium bromide and sulfuric acid. There are some differences between the BB-640 emulsion and the Agfa 8E75 HD emulsion, therefore the optimum concentrations of the different components of the bleach solution must been found in order to obtain high-quality holograms. In particular, we adjusted
the concentration of the potassium bromide contained in the bleach solution. 2. Previous considerations One of the most important factors to be taken into account in the optimization of the bleach is the size of the silver halide grains suspended in the gelatin of the emulsion. The size of the silver-halide grains of BB-640 emulsion is, according to manufacturer, between 20 and 25 nm, whereas the size of the grains in the case of Agfa 8E75 HD is 44 nm w11x. The size of the silver halide grains influence the scattering at the recording, as was analyzed in a previous article w12x. In this respect the BB-640 plates gave better results than the Agfa plates. The smaller size of the silver halide grains of the BB-640 plates also makes it possible to obtain holograms with less scattering than those recorded on Agfa 8E75 HD plates. The response of the diffraction efficiency as a function of the spatial frequency is also improved, and consequently reflection holograms of higher quality may be obtained using the BB-640 silver halide emulsion. The size and concentration of the silver halide grains also influence the optimum concentration of potassium bromide in the bleach bath. We varied the potassium bromide concentration to find the optimum concentration leading to high values of diffraction efficiency. 3. Experimental designr r set-up The experiments were carried out on red sensitive BB-640 silver halide photographic plates. Holographic reflection gratings were recorded using two collimated beams from a 15 mW He–Ne laser Ž633 nm., with the polarization vector perpendicular to the plane of incidence. The K ratio, defined as the ratio of the reference beam intensity to the object beam intensity, was taken to be 1 inside the emulsion. One beam impinged perpendicular to the emulsion, whereas the other beam fell on the back side of the plate forming an angle in air of 308 with the perpendicular to the surface of the emulsion. With the geometry described, the spatial frequency of the gratings was calculated as ; 5100 linesrmm.
C. Neipp et al.r Optics Communications 182 (2000) 107–114 Table 1 Processing schedule and developer composition Processing schedule 1. Develop in D-19 for 5 min 2. Rinse in running water for 1 min 3. Bleach for 1 min after the plate has cleared 4. Rinse in running water for 5 min 5. Dry at room temperature for 24 h All solutions are at 208C except the bleaching step D-19 deÕeloper Metol Sodium sulfite Žanhydrous. Hydroquinone Sodium carbonate Žanhydrous. Potassium bromide Distilled water
2g 45 g 8g 50 g 5g 1 liter
109
beam intensity to the incident collimated probe-beam intensity of the He–Ne laser. In order to take into account Fresnel losses and absorption due to the glass substrate, this expression was corrected by multiplying by an appropriate factor. As explained by Kostuk w8x, noise gratings are very efficient at Bragg angle. Therefore the plates were rotated until maximum diffraction efficiency was reached. The efficiency of the zero-order or transmission t was similarly calculated as the ratio of the directly transmitted beam intensity to the incident power and was corrected using the same factor. 4. Results and discussion 4.1. Optimization of the B r A ratio
In previous experiments w13x we found that the gelatin of the BB-640 plates is hardened to a high degree, so that the products of the developer do not penetrate easily into the emulsion. Therefore, the plates were hypersensitized by immersing them in a solution of distilled water with a sodium sulfite concentration of 1% and urea concentration of 5% Žby weight. for 10 min at 208C. After rinsing in running water for 1 min at 208C the plates were dried for 24 h at 208C and 60% RH. The urea of this solution softens the gelatin, and the sensitivity of the emulsion is also increased due to the sodium sulfite in the solution. In the bleach bath process, diffusion is the main mechanism involved in the refractive index modulation. We decided to use D-19, which is a tanning developer. D-19 causes the silver halide grains to increase in size after the bleach bath, due to oxidation products. Nevertheless oxidation products of the D-19 also create a differential degree of hardening of the gelatin during the bleach bath. This differential hardening promotes the diffusion process, thus favouring the refractive index modulation created by the transference of material between the exposed and the unexposed zones. After exposure, the plates underwent the schedule procedure illustrated in Table 1, so that phase reflection holograms were finally obtained. The diffraction efficiency h of the recorded phase holograms was calculated as the ratio of the diffracted
The R-10 bleach has been used with Agfa 8E75 HD plates with good results. Table 2 shows the composition of a modified version of this bleach Žmod R-10.. The bleach bath solution is composed of two different solutions: A and B. The oxidizer Žpotassium dichromate. is in solution A, whereas the rehalogenating agent Žpotassium bromide. is in solution B. To obtain the bleach solution, 1 part of A is mixed with 10 parts of distilled water and X parts of B. The ratio X s BrA indicates the relation between the potassium bromide concentration and the oxidizer. For Agfa 8E75 HD plates, an optimized R-10 type bleach is the one used by Crespo et al. w7x. Nevertheless the differences in the degree of hardening of the gelatin, and the differences in the size and
Table 2 Bleach bath formula Bleach bath composition Žmodified version of R-10. Solution A Potassium dichromate Sulfuric acid Distilled water
20 g 15 ml 1 liter
Solution B Potassium bromide Distilled water
100 g 1 liter
Just before use, mix 1 part of A with 10 parts of distilled water and add X parts of B, Br As X.
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concentration of the silver halide grains in the Agfa 8E75 HD and BB-640 plates, make it necessary to find a new optimum BrA ratio for BB-640 plates. In our experiments the BrA ratios took the values: 2, 4, 8, 15, 30, 60, 120. Fig. 1 shows diffraction efficiency as a function of the BrA ratio. It can be seen from this figure that the best results were obtained for a BrA ratio of 15. A high peak diffraction efficiency of 72% Ž67% if Fresnel losses are not taken into account. was obtained with this BrA ratio. This result is comparable to the best results obtained by recording bleached reflection gratings on Agfa 8E75 HD emulsion. Crespo et al. w7x obtained a peak diffraction efficiency of ; 70%. They developed the plates with AAC and bleached them with the mod R-10, which was made up with 2-grliter potassium dichromate, 35-grliter potassium bromide and 10-cm3rliter sulfuric acid. Nevertheless, for D-19 the values of the diffraction efficiency which they obtained were lower than 50%. Based on fixation-free bleaching techniques, Kostuk w8x also obtained high values of diffraction efficiency Ž65% without considering Fresnel reflections. for reflection holograms recorded on Agfa 8E75 HD emulsion. The plates were developed with a variant of CW-C1 mixture, and bleached with a modified version of the R-10 bleach. In order to optimize the bleach bath he varied the concentrations of the potas-
Fig. 1. Diffraction efficiencies as a function of the BrA ratio for reflection gratings recorded on BB-640 plates bleached with an R-10 type bleach Žat 508C. with seven different BrA ratios: 2, 4, 8, 15, 30, 60 and 120.
sium dichromate ŽKD., the potassium bromide ŽKBr. and the sulfuric acid ŽSA.. The optimized values were 36 grliter of KBr, 0.5 grliter of KD, and 2 mliterrliter of SA. For Agfa 8E75 HD, Kostuk indicated that the concentration of KBr should be high to maximize efficiency, the concentrations of KD should be low to minimize noise gratings and reduce emulsion swelling and the concentration of SA should also be minimized in order to reduce emulsion shrinkage. It is interesting to notice that the concentrations of our optimized bleach solution ŽBrA s 15. are 0.76 grliter of KD, 57 grliter of KBr and 0.57 cm3rliter of SA, which agree with Kostuk statements. The values of diffraction efficiency obtained by Cooke and Ward w4x were also very high, 75% Žafter considering Fresnel reflections., they developed the plates with CW-C1 and bleached them in PBQ, which has the inconvenient of being hazardous, the temperature of the baths Žincluding the bleach. was maintained at 208C. This difference in the bleach bath temperature with respect to our process is due to the different degree of hardening of the gelatin of Agfa 8E75 HD and BB-640 plates. It can also be seen that the peak diffraction efficiency is highly sensitive to the BrA ratio. The presence of potassium bromide in the bleach solution favours the diffusion process, because the solubility of the silver ion is increased. As the concentration of potassium bromide increases, so does the solubility of the silver ion, which will cause the silver bromide grains in the non-exposed zones to increase in size. A high concentration of potassium bromide also favours coalescence of separated crystals w9x. The amount of light scattered by a small particle is proportional to the sixth power of its radius. Therefore an increase in the potassium bromide concentration implies an increase in the scattered light intensity and a corresponding decrease in the maximum diffraction efficiency achievable. For transmission holograms Žspatial frequency ; 1200 linesrmm. the small size of the silver halide grains in the non-exposed zones permits high diffraction efficiencies to be obtained with a large range of potassium bromide concentrations w10x. For reflection holograms the path along which the silver ion travels between the exposed and the non-exposed zones is shorter, thus the silver halide grains in the non-exposed zones in-
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crease in size more rapidly as the BrA ratio increases. 4.2. Influence of the B r A ratio on the final thickness of the emulsion During the rehalogenating bleaching procedure, the gelatin of the emulsion varies in thickness. The maximum changes in thickness in the emulsion occur during the drying process w7x. The extent to which the emulsion varies in thickness depends on the degree of hardening of the gelatin after the bleaching process. Processed hardened layers when dry are thicker than unhardened ones because of a significant loss of elasticity in the layer. The change in the thickness of the emulsion after the procedure alters the fringe spacing, which also affects the wavelength at reconstruction. The relation between the wavelength at reconstruction and the wavelength at the recording can be expressed by the following relation:
lX
LX s
l
L
Ž 1.
where L, LX are the grating periods at the recording and reconstruction, respectively, and l, lX are the wavelengths at the recording and reconstruction, respectively. The changes in thickness of the emulsion are mainly due to the following factors: Factors which contribute to the swelling of the gelatin: 1. The hypersensitization process causes the gelatin of the emulsion to swell, due to the urea present in the solution. 2. During the development step the immersion of the emulsion in an alkaline solution could contribute to the emulsion swelling w14x. Factors which contribute to the hardening of the gelatin: The hardening action is promoted by potassium dichromate. In the oxidation process the Crq6 ion is chemically reduced to Crq3 . The hardening of the gelatin is a consequence of the cross-links created by Crq3 ions between the gelatin chains. This action takes place in the vicinity of the oxidized silver grains.
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Fig. 2 shows the wavelength at reconstruction as a function of the exposure for BB-640 plates bleached using the different BrA ratios. It can be seen that in all cases there is an increase in the wavelength of reconstruction as the exposure increases. The concentration of the developed silver grains in the emulsion increases with exposure, as does the presence of the Crq3 ion in the gelatin and the subsequent degree of hardening of the gelatin. This hardening action inhibits the shrinkage of the emulsion as the emulsion dries, which explains why the wavelength at reconstruction for maximum diffraction efficiencies increases with exposure. The influence of the BrA ratios on the wavelength at reconstruction is clear from Fig. 2. It can be seen that much care must be taken to minimize the wavelength shift. For instance, when the BB-640 plates are bleached using the BrA ratio which yields maximum diffraction efficiency ŽBrA s 15., the wavelength shift is less than 1% within a range of exposures from 900 to 5000 mJrcm2 . However, for a BrA ratio of 4, the emulsion shrank after the procedure, so that the wavelength at reconstruction remained below 623 nm. The dependence of the wavelength at reconstruction on the BrA ratio and on exposure could give a method to control the wavelength at reconstruction by adjusting the BrA ratio and exposure appropriately.
4.3. Angular response of the diffraction efficiency The efficiency of the diffracted and the transmitted beams, h and t respectively, together with the absorption of the material a d Žwhere a is the absorption coefficient and d is the thickness of the photographic emulsion after processing. are three parameters that may be used to characterize holographic reflection gratings recorded on the silver halide sensitized gelatin of BB-640 plates. By means of Kogelnik’s coupled wave theory w15x, it is possible to obtain an analytical expression for the diffraction efficiency of volume holograms. As Kogelnik’s theory is limited to the description of two diffracted orders, then
h q t s exp Ž ya drcos u X .
Ž 2.
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and t can be obtained as follows:
t s exp Ž ya drcos u X . y h
Ž 3.
X
t s exp Ž ya drcos u .
= 1y
Ž 1 y j 2rn 2 . 1q 1r2 sinh2 Ž n 2 y j 2 .
y1
0
Ž 4.
In Eqs. Ž2. – Ž4., a takes into account the absorption Žand also the scattering; we have no means of differentiating between the two at this point.; d is the thickness of the final hologram and u X is the angle of reconstruction in the medium, which is related to the angle of reconstruction in air u by Snell’s law. The rest of parameters of Eq. Ž4. are the usual used in Kogelnik’s Theory and are defined in Ref. w15x. We tested the holograms which yielded peak diffraction efficiency by rotating them, and the variation in transmission with the angle of incidence t in air was measured. The values of transmission were corrected to take into account Fresnel’s reflections and the absorption of the glass substrate. Fig. 3 shows the angular dependence of the transmission for the holograms which led to the best agreement between theory and experiment ŽBB-640 bleached using a BrA ratio of 8.. The parameters obtained by fitting the theoretical function to the experimental data where: refractive index modulation, D n s 0.034, thickness, d s 8.1 mm, and absorption coefficient, a s 0.0288 mmy1 .
Fig. 2. Reconstruction wavelength for Bragg’s condition as a function of exposure for reflection gratings recorded on BB-640 plates bleached with an R-10 type bleach Žat 508C. with seven different BrA ratios: 2, 4, 8, 15, 30, 60 and 120.
Fig. 3. Experimental measurements of transmittance and theoretical fit Žsolid curve. as a function of the reconstruction angle in air and for reflection gratings recorded on BB-640 plates bleached with an R-10 type bleach Žat 508C. with a BrA ratio of 8.
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4.4. Response of the diffraction efficiency as a function of the waÕelength 4.4.1. Consideration of noise gratings and undesired reflections A small amount of light reflects from the rear part of the plate during the recording. Consequently, reflection gratings of low modulation are created, due to the interference of the incident light and the reflected light. In addition the self interference of the incident light and light scattered at the recording also create noise gratings which lower the maximum achievable diffraction efficiency. To study the effect of these gratings, BB-640 plates were exposed to a single beam and the exposed plates were bleached using BrA ratios of 8
Fig. 5. Experimental measurements of transmittance and theoretical fit Žsolid curve. as a function of the reconstruction wavelength for reflection gratings recorded on BB-640 plates bleached with an R-10 type bleach Žat 508C. with a BrA ratio of 8. The reconstruction of the gratings was made with the illuminating beam entering normal to the surface of the plate.
and 15. The dependence of the transmission on the wavelength was measured using a spectrophotometer. Fig. 4 shows the transmittance of the BB-640 plates bleached with two different bleaches ŽBrA s 8 and BrA s 15. versus the wavelength for three different exposures. It can be seen that maximum efficiency for noise gratings was obtained for a replay at a wavelength of ; 650 nm.
Fig. 4. Transmittance as a function of the reconstruction wavelength under Bragg’s condition for noise gratings and undesired reflection gratings recorded on BB-640 plates bleached with an R-10 type bleach Žat 508C. and with BrA ratios of 8 and 15.
4.4.2. Response of the diffraction gratings as a function of the waÕelength The theoretical response of the diffraction efficiency as a function of the wavelength can also be derived from Kogelnik’s theory. The angle of incidence at reconstruction was fixed at 08, whereas the wavelength was now the free parameter. The response of the transmission efficiency as a function of the wavelength for reflection holograms recorded on BB-640 plates was also determined experimentally using a spectrophotometer. In order to obtain the theoretical function, the absorption of the gelatin was fitted to a polynomial function of the third order. To obtain the theoretical function we used the values of D n, a and d obtained in the previous section. Fig. 5 shows the measured and theoretical values of transmittance for reflection holograms recorded
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on the BB-640 plates bleached using a BrA ratio of 8. The reconstruction of the gratings was made with the illuminating beam entering normal to the surface of the plates. As mentioned, the presence of a noise grating at a wavelength of ; 650 nm causes deviations of the theoretical function from the experimental data, so that the theoretical curve does not reproduce exactly the behaviour of the transmittance from the right part of the principal lobe. Nevertheless good reproduction of the experimental data was obtained. 5. Conclusions Fixation free rehalogenating techniques to obtain phase reflection holograms on the novel BB-640 plates have been analyzed. The smaller grain size of the BB-640 emulsion makes it possible to obtain reflection holograms with high diffraction efficiencies compared with those obtained using Agfa 8E75 HD plates. The bleach used was a version of the R-10 bleach, and the concentrations of solution B, containing the rehalogenating agent, and solution A, containing the oxidizer, were adjusted to obtain the highest values of diffraction efficiency. By bleaching the plates using a BrA ratio of 15 a high peak diffraction efficiency of 72% Ž67% if Fresnel reflections are not taken into account. was achieved. It was also demonstrated that the diffraction efficiency of the final holograms is highly sensitive to the BrA ratio. Nevertheless high diffraction efficiencies were achieved over a wide range of BrA ratios Žfrom 4 to 60 the values remained over 52%.. For reflection gratings recorded in Agfa emulsions – and for D-19 developer – a lower value of diffraction efficiency
Ž- 50%. was obtained by other authors w7x. Finally, the results presented show that reflection holograms of high quality can be obtained using the novel BB-640 plates. This implies that BB-640 plates are a good substitute for Agfa 8E75 HD plates with respect to the recording of bleached reflection holograms.
Acknowledgements This work was partially financed by the CICYT ŽComision ´ Interministerial de Ciencia y Tecnologıa, ´ Spain. under project No. MAT97-0705-C02-02.
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x
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