Improving the holographic sensitivity ofdichromated gelatin in the blue–green part of thespectrum by sensitization with xanthene dyes

Dejan Pantelic and Branka Muric

The holographic properties of dichromated gelatin ~DCG! sensitized with various xanthene dyes werestudied, and results are reported. The sensitivity of dyed DCG in the green part of the spectrumcompared with that of pure DCG ~215 mJycm2! was significantly improved by addition of Rhodamine 6G~140 mJycm2! or Erythrosin B ~90 mJycm2!. Diffraction gratings were recorded with a He–Ne laser at543.5 nm. The maximum diffraction efficiency was found to be 32% at normal incidence of the readoutbeam; it was 80% at the Bragg angle. © 2001 Optical Society of America

OCIS codes: 090.2900, 050.1950.

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1. Introduction

Gelatin sensitized with ammonium dichromate@dichromated gelatin ~DCG!# is one of the best holo-graphic recording materials because of its well-known properties: high diffraction efficiency, largecapacity for refractive-index modulation, high reso-lution ~as much as 5000 linesymm!, high signal-to-noise ratio, and good environmental stability ~with acover plate or a thin protective coating!.1,2 A majorshortcoming of DCG is that its spectral sensitivity islimited to the UVyblue–green part of the spectrum.1,3

The majority of reported papers concentrate on in-creasing the spectral sensitivity of DCG to the redpart of spectrum by addition of appropriate dyes ~forexample, Methylene Blue4! to the DCG solution ~asensitivity of 150–400 mJycm2 at 632.8 nm was re-ported!.5 Only a few papers have been concernedwith the increase of sensitivity in the green part ofthe spectrum.6,7

Although the sensitivity in the blue–green part ofthe spectrum is much higher than that in the redpart, it is still low compared with that of materialssuch as silver halogenide. That is, DCG is approxi-mately 102–105 times slower photographically than

D. Pantelic ~e-mail: pantelic@phy.bg.ac.yu! and B. Muric ~e-mail:muric@phy.bg.ac.yu! are with the Institute of Physics, Pregrevica,118, 11080 Zemun, Belgrade, Yugoslavia.

Received 26 October 2000; revised manuscript received 26 Feb-ruary 2001.

0003-6935y01y172871-05$15.00y0© 2001 Optical Society of America

holographic silver halide emulsions. Therefore re-cording a hologram requires a powerful and expen-sive laser such as Ar-ion or He–Cd and large exposuretimes.

It was our intention to improve the sensitivity ofDCG in the green part of the spectrum by adding adye such that a cheaper and simpler green He–Nelaser could be used. In this paper we present resultsof using various xanthene dyes ~Rhodamine 6G and

rythrosin B! and describe their influence on theolographic properties of DCG.

2. Experiment

Chemically, gelatin is a polymer, which is composedof a number of a-amino acids and has a complexmolecular structure. The molecular weight of gela-tin ranges from 768 to 96,000, with an average valueof approximately 27,000.9 DCG possesses good holo-graphic characteristics. The mechanism of holo-gram formation in DCG has been described in theliterature.1,10,11

In our experiments, a 2–20% by weight aqueousgelatin suspension was prepared by the procedureoutlined in Table 1. Various concentrations ~from 5o 50% by weight of dry gelatin! of ammonium dichro-ate were tested. The DCG film was prepared by

he gravity settling method12; nonphotographic qual-ity gelatin ~manufacturer trademark Gelatin-ALBA-Ph. Jug IV! was used. A DCG solution of constantvolume was poured onto a precisely leveled and well-cleaned microscope glass substrate. Thus we wereable to make DCG films with thicknesses that variedfrom 15 to 60 mm ~thickness of dried films!.

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Table 1. Procedure for Preparing Dyed DCG Film Table 2. Procedure for Developing a Dyed DCG Platea

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The DCG plates were dried in the dark, for 24 h, inrelatively stable environmental conditions ~the tem-perature fluctuated in the 25–27 °C range, and therelative humidity was 50–60%!. The best resultswere obtained with a film made from a 5% aqueoussolution of gelatin, with 10% ammonium dichromate~by weight of dry gelatin! and a thickness of 30 mm.

One of the dyes used for DCG sensitization washodamine 6G, which belongs to the group of xan-

hene dyes ~absorption range, 500–700 nm!.13,14

The addition of the dye can be accomplished in any ofa number of ways.15 In our case, various quantitiesf a 0.1% aqueous solution of Rhodamine 6G werentroduced into a DCG solution. We found that theest results were obtained with a dye concentration of.5 mgyl of DCG solution.Using the same experimental conditions ~thickness

of the DCG film, dichromate and dye concentrationsunchanged!, we investigated the influence ofErythrosin B ~another xanthene dye! on the holo-graphic properties of DCG.

The absorption spectra of pure DCG and dyed DCGfilms ~Fig. 1! were measured with a UV–visible spec-rophotometer. The three spectra have approxi-ately the same shape. As expected, at 543.5 nm

he absorption values of dyed DCG films increasedignificantly compared with those of pure DCG film.Transmission diffraction gratings, as the simplest

Fig. 1. Absorption spectra of pure DCG film and dyed DCG films.

Step Description

1. Swell gelatin in deionized water for 60 min.2. Heat the suspension in a thermostated water

bath at 50 °C, stirring for 10 min.3. Add an adequate amount ~see text! of ammonium

dichromate, by weight of dry gelatin, and heatin a thermostated water bath at 50 °C, stirringfor 10 min.

4. Add an appropriate amount of xanthene dye ~seeFig. 3 below for the optimum value! to theDCG solution.

5. Pipette out a known volume of dyed DCG solu-tion onto a leveled glass microscope plate.

6. Dry the plate overnight.

872 APPLIED OPTICS y Vol. 40, No. 17 y 10 June 2001

type of hologram, were recorded by interference oftwo expanded divergent laser beams that had equalintensities and optical path lengths. A green He–Nelaser operating at l 5 543.5 nm ~with an output

ower of 1 mW! was used as a light source. The totalenergy density of the recording light beams was 0.3mJycm2. The angle between beams was approxi-

ately 35° and, consequently, the spatial frequencyf the transmission gratings was ;1000 linesymm.he hologram area was 0.8 cm2. A readout beam at

632.8 nm was used to measure the diffraction effi-ciency.

During development, an exposed plate was washedimmediately in deionized water and dehydrated inseveral baths of isopropyl alcohol with increasingconcentrations ~see Table 2!.

Also, we measured the diffraction efficiency in realime as a function of exposure. A schematic dia-ram of the real-time holographic setup is shown inig. 2. As can be seen, the measurements were noterformed at the Bragg angle. Instead, reconstruc-ion was done at normal incidence of a 632.8-nmeam. We found that this configuration is easier toosition accurately and gives better reproducible re-ults ~a slight deviation from the Bragg angle signif-cantly influenced our measurements!.

Fig. 2. Real-time holographic experimental setup: B.S., beamsplitter; MS, mirrors; Ls, lenses.

Step Description

1. Wash in water for 5 min.2. Dehydrate in a 50:50% isopropyl alcohol and water

solution, stirring for 3 min.3. Dehydrate in a 70:30% isopropyl alcohol and water

solution, stirring for 3 min.4. Dehydrate in 100% isopropyl alcohol for 1.5 min.5. Dry plate with flowing hot air for 5 min and then

in a chamber containing a desiccant.

aDeionized water was used. All steps were at a temperature of25 °C.

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3. Results and Discussion

Diffraction efficiency, as one of the most important holo-graphic parameters for judging the performance of DCGholograms, has been measured. Diffraction efficiency isdefined as the ratio between the intensities of the first-order diffracted light and the incident light.16

Our measurements were made at normal incidenceof the laser beam, both in real time and after devel-opment. Again, we found this configuration favor-able with respect to ease of positioning andreproducibility. Besides, the purpose of our mea-surements was to determine holographic sensitivityof DCG ~DCG 1 sensitizer!, which depends on chem-cal composition and processing and not on the anglef the readout beam. For the sake of completeness,e include here our measurement of the maximumiffraction efficiency that can be obtained with ourechnique at the Bragg angle ~at wavelengths of both43.5 and 632.8 nm!.The dependence of diffraction efficiency on the con-

entration of Rhodamine 6G is shown in Fig. 3. Ashe concentration of Rhodamine 6G increases, theiffraction efficiency increases; it reaches a maximumhen the dye concentration is 7.5 mgyl. After theaximum concentration is attained, the diffraction

fficiency decreases because Rhodamine 6G precipi-ates at higher concentrations in the presence of theichromate ion.The dependence of diffraction efficiency on expo-

ure for pure DCG and for DCG with Rhodamine 6Gs shown in Fig. 4. It is obvious that holographicensitivity increases in the presence of RhodamineG. The maximum diffraction efficiency was foundo be 32% for both DCG and DCG with RhodamineG, but the energy densities required for maximumfficiency were 215 and 140 mJycm2, respectively.An increase in sensitivity was also observed after

we added Erythrosin B to the DCG solution ~Fig. 5!.We can see that the maximum value of the diffractionefficiency for DCG sensitized with Erythrosin B was32%, too. Moreover, in the presence of Erythrosin B,maximum efficiency was obtained at an energy den-sity of 90 mJycm2, which is less than for both DCGsensitized with Rhodamine 6G and pure DCG.

The increase in holographic sensitivity dependsonly on properties of the chemical composition andprocessing of DCG films. Although previous mea-surements were made at normal incidence, it is im-portant to determine the maximum diffractionefficiency at the Bragg angle. We did that for themost favorable circumstances for chemical process-ing, as previously established ~at normal incidence!.

e found a maximum of 80% diffraction efficiency.We found it hard to determine exactly whether our

ratings are of a thin, a thick, or an intermediateype.17,18 Because such a determination is notithin the scope of our research, we give here only arief and incomplete analysis.Strictly speaking, one can determine the type of

rating by calculating the so-called Q9 and g factorsdefined in Refs. 19 and 20. Along with other param-eters ~DCG emulsion thickness, wavelength, gratingperiod, angle of incidence of the readout beam!, Q9depends on the refractive index of gelatin, which wedid not measure. Therefore we were able only toestimate that the parameter value is 77 by assumingthat the refractive index of gelatin is 1.55.21 Like-wise, the g factor depends on film thickness, wave-ength, angle of incidence, and amplitude of the

Fig. 4. Relation of diffraction efficiency to exposure for a 30-mm-thick film with a constant concentration of Rhodamine 6G of 7.5mgyL.

Fig. 5. Relation of diffraction efficiency to exposure for a 30-mm-thick film with a constant concentration of Erythrosin B of 7.5mgyl.

Fig. 3. Relation of diffraction efficiency to concentration of Rho-damine 6G for a 30-mm-thick film with a constant exposure densityf 140 mJycm2.

10 June 2001 y Vol. 40, No. 17 y APPLIED OPTICS 2873

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refractive-index modulation. The last parameterwas not measured; instead we took the value Dn 50.00135 from Ref. 21 and found that g 5 0.2.

Because of the unpredictable properties of gelatinand of processing peculiarities, Dn can have variousvalues.22,23 That is, the refractive index of DCG andits variation depend on many parameters, such ashumidity, temperature, concentration of ammonimdichromate, polarization, birefringence of gelatin,nonuniformity, and exposure level. Therefore thevalues that we obtained should be taken with a de-gree of caution.

At any rate, if the Q9 and g values are correct,according to Ref. 19 our gratings will be on the borderbetween the Bragg and the Raman–Nath regimes.To support this conclusion we illustrate the distribu-tion of energy among various diffraction orders atnormal incidence and at the Bragg angle ~Fig. 6!. Ascan be seen, there is a significant amount of energydistributed outside 0th and 21st orders at the Braggangle. This means that the grating is not purelyBragg. Also, the grating is not Raman–Nath be-cause the 0th order should have a higher intensitythan other orders ~for g 5 0.2, as in our case!. Ad-ditionally, the diffraction efficiency should not de-pend significantly on the angle of incidence in anangular range of 620° ~whereas in our case the dif-fraction efficiency was 32% at normal incidence,reaching a maximum of 80% at an angle of ;10°!.The total diffracted energy ~all orders! does not add to

874 APPLIED OPTICS y Vol. 40, No. 17 y 10 June 2001

100% because of scattering, absorption, and Fresnellosses.

We measured diffraction efficiency at 543.5 nm andfound only slight differences from that at 632.8 nm~apart from a change of Bragg angle!.

It is well known that pure DCG is sensitive toUVyblue–green light and that this sensitivity isgreater at shorter wavelengths ~the sensitivity ratioat 488 and 514.5 nm is approximately 5:1!.16,24 To ourknowledge, sensitivity measurements at 543.5 nmwere not performed, so we measured this value andfound it to be 215 mJycm2.

We wanted to correlate our results with the pre-viously published measurements of DCG sensi-tivity.24–26 Therefore in Fig. 7 we illustrate thesensitivity of DCG ~defined as the energy densityequired for the maximum diffraction efficiency! asfunction of wavelength. It is obvious that sensi-

ization of DCG results in significant improvementf sensitivity at 543.5 nm.We also found that the sensitivity of DCG at 543.5

m ~He–Ne laser! in the presence of xanthene dyes—hodamine 6G and Erythrosin B—is approximately

he same as the sensitivity of pure DCG at 514.5 nmAr-ion laser!.

In Fig. 8, we show the real-time diffraction effi-

Fig. 7. Sensitivity ~filled squares, reported values24–26; opensquare, circle, and triangle, our results! of DCG as a function ofwavelength.

Fig. 8. Real-time diffraction efficiency as a function of exposure.

Fig. 6. Energy distribution among various diffraction orders atnormal incidence and at the Bragg angle. Distribution was mea-sured on the same DCG film, optimally exposed and processed.

7. G. Manivannan, R. Changkakoti, R. A. Lessard, G. Maihot,

ciency as a function of exposure for pure DCG and fordyed DCG. The real-time effects confirm the im-provement in sensitivity of dyed DCG films. Also,we can see that presence of Rhodamine 6G orErythrosin B in DCG, besides increasing the sensi-tivity of DCG, enhances the real-time diffraction ef-ficiency. Maxima of 0.004% for pure DCG, 0.008%for DCG with Erythrosin B, and 0.012% for DCG withRhodamine 6G were obtained.

4. Conclusions

The aim of our research was to improve the holo-graphic sensitivity of DCG in the green part of thespectrum by sensitization with various xanthenedyes. Although we used a rather simple method forpreparation and development of pure DCG, our re-sults are in good agreement with those of previousresearch. The sensitivity of DCG was significantlyimproved by addition of Rhodamine 6G andErythrosin B. The improvement in the sensitivity ofdyed DCG was confirmed by real-time measure-ments.

The photochemical reaction processes of DCG sen-sitized with xanthene dyes are complex and not yetclear. Therefore additional research should be donein order to elucidate the chemical mechanisms inmore detail.

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