Investigation of AGFA-8E56HD photographicemulsion and relief hologram structures by atomicforce microscopy

Fevzi Necati Ecevit, Ali Alacakir, and Ramazan Aydin

Relief holograms are obtained on Agfa 8E56HD holographic emulsions by a Russian chemical-processingtechnique that is developed for their PE-2 holographic emulsion. We have shown that the three-dimensional surface profiles can easily be visualized by applying atomic forcemicroscopy tomeasurementof the relief depth and relief spacing on holographic emulsions. The relief depth and thus diffractionefficiency decreases with increasing exposure time. © 1996 Optical Society of America

Key words: Photographic emulsion, relief hologram, atomic force microscopy.

1. Introduction

We use surface-relief holograms in the mass produc-tion of holographic optical elements and display ho-lograms by applying an embossing technique for themass replication of their surface relief. The mostwidely used recording medium for obtaining a reliefhologram is the photoresist material. But its sensi-tivity is rather low and sensitive only to UV and bluelight, and it generally requires coating facilities. Onthe other hand, silver-halide films, being quite sen-sitive to visible light and available in large dimen-sions, can be used to obtain relief holograms wheninexpensive low-power lasers with suitable chemicalprocessing are used.To obtain pure relief holograms, scientists have

used Kodak 649F emulsion by applying Kodak HRPdeveloper and Kodak R-9 bleach in which the reliefdepth was varied from 30 to 140 nm at 65 linesymmis with increasing exposure, and they measured thesurface profile of emulsion by using Taylor HobsonTaylsurf.1–3 Kodak 6451 and 649GH silver-halidematerials were utilized to record the spatial frequen-cies to as many as 200 linesymm.4 In the literatureon former USSR ultrahigh grained PE-2 silver-halide

F. N. Ecevit and R. Aydin are with the Department of Physics,Middle East Technical University, 06531, Ankara, Turkey. A.Alacakir is with the Ankara Nuclear Research and Training Cen-ter, 06105 Saray, Ankara, Turkey.Received 7 August 1995; revised manuscript received 14 Febru-

ary 1996.0003-6935y96y316227-04$10.00y0© 1996 Optical Society of America

emulsion5–8 the effect of different chemical developersand the thickness of emulsion on the relief height arereported. Bjelkhagen9 also tested the technique ofobtaining a relief structure on western Agfa 8E56HDemulsion by recording a diffraction grating with afringe spacing of 1 mm, using the Russian chemical-processing technique.6 The grating obtained was in-vestigated with a scanning electron microscope~SEM!. But detailed data about the relief depth andits dependence on exposure are not available. Othertechniques for investigating amplitude and phase ho-lograms, such as SEM, phase-contrast microscopy,and x-ray fluorescence analysis, are also reported inthe literature.9–12 Phase-contrast microscopy, forexample, does not provide direct information on reliefdepth and provides only two-dimensional images.The SEM technique, however, requires a metalliccoating of a sample and high-vacuum enclosure thatmay deform the sample surface.Following its invention and commercialization,

atomic forcemicroscopy ~AFM!, has become one of theuseful tools for visualizing and investigating opticalsurfaces.13 The use of AFM to investigate holo-graphic emulsion has several advantages. It pro-vides three-dimensional images. It can be used toinvestigate conducting and nonconducting sampleswithout requiring special processing and coating asin the SEM technique. High- or low-resolution sur-face images of the sample can be obtained by chang-ing only the tip of the AFM instrument.The goal of this research is to introduce the AFM

into the investigation of the surface morphology ofholographic emulsions, which we believe has not beenused before in this type of research. We have mea-

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sured directly the variation in the spacing and thedepths of the holographic relief structures as a func-tion of exposure and spatial frequency of the reliefs byutilizing the AFM technique. To obtain the holo-grams and to show the effect of exposure and spatialfrequency on the relief depth, we have applied theAFM technique and the Russian chemical-processingmethod for Agfa 8E56HD emulsion previously usedfor PE-2 emulsion.

2. Experimental

The experimental setup for recording the holographicgratings, consisting of an argon-ion laser, a spatialfilter and a collimator, a beam splitter, a mirror, anda holographic plate, is shown in Fig. 1. The essen-tial idea here is to record on a holographic plate theinterference fringes of two split plane waves. Thetotal beam power on the holographic plate was 60mWycm2 with a beam power ratio of approximately1:1. The wavelength of the argon-ion laser was514.5 nm. Two sets of gratings ~A and B! were re-corded on an Agfa 8E56HD holographic plate bychanging the interbeam angle ~2u 5 10°, 22°! at fivedifferent exposure times for each set.The exposed plates were developed with a relief

developer having a composition similar to that usedfor PE-2 emulsion.6 More specifically the developercontains 4 g of metol, 8 g of hydroquinone, 26 g ofascorbic acid, 40 g of sodium carbonate, 4 g of potas-sium thiocyanate, 2 g of ammonium bromide, andwater added to make a 1-L solution. The surface-relief processing steps were as follows: expose theplate, develop it in the relief developer for ;1 min,and wash and bleach the plate in Kodak R-10 until itis clear, soak it in 5% ammonium dichromate solutionfor 5 min, dip it in a sulfuric acid solution of pH 3,wash it in water at 20 °C, and dry it.After chemically processing the relief holograms,

we measured the diffraction efficiencies, h, of the ho-lographic gratings, which are defined as

h 5 I1yI0, (1)

where I1 is the power of the first-order diffractedbeam and I0 is the power of the incident beam cor-rected because of the reflections on the two surfaces.We used the TopoMetrix 2000 Explorer operating

in contact mode to investigate the three-dimensionalsurface profile of the holographic relief gratings. We

Fig. 1. Basic setup for the recording diffraction grating: SF, spa-tial filter; CL, collimator; BS, beam splitter; M, plane mirror; HP,holographic plate; 2u, interbeam angle.

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measured the relief spacing and the height of the re-corded gratings on these emulsions with the tip radiusof ;100 nm and the pyramidal tip having an aspectratio of 1:1. Acquisition parameters were exactly thesame for each plate. The force between the surfaceand pyramidal tip has been adjusted in order not tocreate extra surface features in consecutive measure-ments. Ten holographic gratings ~five for each set, Aand B! were scanned twice to discard surface and tipartifacts. Two images collected for each relief profilehave been compared to determine the final real image.Finally, to confirm the reproducibility of the results, werepeated the observation and the measurements sev-eral times by changing the scanning direction.

3. Discussion and Conclusion

For a holographic relief grating, the intensity of theradiation emitted in a given diffraction order is pro-portional to6

DE < Jq2~f!, (2)

where Jq is the Bessel function of order q and f is thephase difference between the maximum and the min-imum of the relief depth. The value of f is related tothe relief depth as follows:

f 52pL~n 2 no!

l, (3)

where L is the relief depth, n is the refractive index ofthe grating material, no is the refractive index of thesurrounding medium, and l is the radiation wave-length.As seen from Eq. ~3!, one of the key parameters

controlling the diffraction efficiency is the reliefdepth. Therefore it is possible to associate the be-havior of the relief depth and diffraction efficiencywith exposure. The variations in the diffraction ef-ficiency and the relief depth are plotted in Figs. 2 and3, respectively. It can be shown that diffraction ef-ficiency versus relief depth can be expressed as aBessel function of the order of 1 for set A. For a

Fig. 2. Diffraction efficiencies of gratings as a function of expo-sure.

thin-phase transmission grating the theoretical max-imum diffraction efficiency is ;34%. For our diffrac-tion gratings the measured diffraction efficiency is;10–20%.The surface images of set A, exposed for 1 and 5 s,

are shown in Figs. 4~a! and 4~b!, respectively. It isclearly seen that the relief shape deviates from thesinusoidal nature with increasing exposure, whichdegrades the diffraction efficiency as expected. Forset B the same behavior can be observed ~see Fig. 5!.The variation in the arbitrarily chosen single line onthese images is at the bottoms of Figs. 4 and 5 to showthe single line profile of the reliefs. The profile curveand the tip dimension are shown in the same scale inFig. 6 to illustrate the geometric tip effects. Note

Fig. 4. ~a! Surface image of the holographic grating for set Aexposed for 1 s. ~b! Surface image of the holographic grating forset A exposed for 5 s.

Fig. 3. Change in the relief depths of gratings with exposure time.

that the relief depths become smaller with increasingexposure time for each set ~Fig. 3!. The fringe spac-ing was measured as 715 and 1430 nm for sets A andB, respectively.Holograms can be classified as thin ~plane! or thick

~volume! according to the ratio of the thickness of therecording medium and recorded interference fringespacing within the recording medium. The Q pa-rameter is used to distinguish between the two types,as defined by9

Q 52pldnL2 , (4)

where l is the wavelength of the illuminating light, dis the thickness of the emulsion layer, n is the refrac-tive index of the emulsion, and L is the spacing be-tween the recorded fringes. A hologram is consideredthick if Q $ 10 and thin if Q # 1. Holograms with Q

Fig. 5. ~a! Surface image of the holographic grating for set Bexposed for 1 s. ~b! Surface image of the holographic grating forset B exposed for 5 s.

Fig. 6. Tip and profile dimensions in the same scale.

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values between 1 and 10 are sometimes consideredthin and at other times thick. For set B, although thefringe depth is small, its high value of diffraction effi-ciency may be attributed to the refractive-index mod-ulation in the emulsion, which is in the thick hologramregime, Q ' 28, whereas for set A, Q ' 6, which is inthe thin hologram regime.Themechanism of the creation of the relief structure

has been explained elsewhere.3,7,8 The mechanismbehind the rehalogenating bleaching process is be-lieved to be a diffusion process.14 After development,without fixing, the emulsion contains both the unex-posed silver bromide and the silver. Part of the dis-solved silver ions diffuse away to create silver bromidein the unexposed silver bromide grains, and the re-maining silver ions are deposited on the exposed andthe developed grains. These newly created silver bro-mide grains in the emulsion deform the relief shape forcreating extra surface tensions on the gelatin emul-sion. This behavior can be seen from the surface im-ages at high exposures in Figs. 4~b! and 5~b!.AFM is a very useful and the easiest way of provid-

ing high-resolution images of gratings obtained bylaser-exposed photoemulsion surfaces. Therefore it ispossible for us to understand the effect of the chemicalprocess and optical radiation on the holographic emul-sion by using this powerful instrument.

The authors are grateful to Husnu Ozkan for care-fully reading and correcting the manuscript. Thisresearch was partially supported by the ResearchFund of the Middle East Technical University. R.Aydin thanks the Alexander von Humboldt Founda-tion of Germany for the scientific equipment.

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