Making Holographic Recording Material

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A Simple Procedure for the Generation of Holographic Emulsions

Jeff Blyth*, Roger B. Millington, Andrew G. Mayes, and Christopher R. Lowe

Institute of Biotechnology, University of Cambridge, Tennis Court Road, Cambridge, CB2 1QT, UK.

*To whom correspondence should be sent . (Email : jb10002@biotech.cam.ac.uk)

(After an editorial "thirdpersonalization" process the complete paper was published in The Imaging Science Journal Vol 47 pp87-91 1999)

Introduction - Experimental - Results -

ABSTRACT

A simple diffusion method for the distribution of ultra-fine AgBr grains in pre-coated gelatin and customised polymeric films for holographic recording has been developed. The method involves two steps: immersion of the pre-formed film in a solution of a silver salt, followed by agitation of the film in a solution of a bromide salt containing sensitizing dye. Repetition of the operation on the same film with different sensitizing dyes in the bromide bath, enables the production of a film with panchromatic response. Transmission electron microscopy reveals a grain structure very suitable for use in holography. A reflection hologram made by this method has very similar brightness under the same exposure and processing conditions to one made from a proprietary hologram recording material currently available.

Introduction

The original method conceived by Gabriel Lippmann over a century ago to produce the first filter-less colour photographs used the constructive interference of each wavelength in white light when it was returned from a reflective surface in intimate contact with the emulsion. The system gained a somewhat notorious reputation for making difficult demands on makers of silver halide emulsions. The emulsions needed to be both exclusively of ultra-fine grain (<25nm) and panchromatic. 1-3.

Such fine grained emulsions have seen a renaissance in the last 30 years with the development of holography, since the grain sizes for both techniques are comparable. However, commercially available silver halide materials intended for holography have been restricted to a relatively narrow-band photosensitivity and have been generally unsuitable for the much more stringent demands of Lippmann's photographic method2. The long held impression that the fabrication of do-it-yourself Lippmann emulsions is not for the faint hearted may still linger - 4, 5 Consequently, there is a need for the introduction of a simple technique for making ultra-fine grain emulsions . This paper describes a diffusion method for producing a holographic recording material which avoids the traditional laborious emulsion-forming methods for producing AgBr grains in gelatin films. Light sensitive ultra-fine AgBr grains can be introduced into thin films of a number of natural or synthetic polymers even if they are somewhat hydrophobic. These polymers may be pre-formed or polymerised in situ by UV irradiation on a suitable substrate6. It has recently been pointed out to us by Hans Bjelkhagen that this diffusion method is a variation of one used in Germany some 85 years ago and claimed to give "extraordinarily fine grain" for the Lippmann photographic process - 7. It was investigated further by Liesegang - 8 in the same year.

Experimental

1 Materials

Gelatin (~ 300 bloom, swine skin), silver nitrate (1M volumetric standard), Lithium bromide(99%), Chromium (III) acetate hydroxide, 3-aminopropyltriethoxysilane, ascorbic acid and the dyes, pinacyanol chloride (Quinaldine Blue;) and 1,1 -diethyl-2,2 -cyanine iodide (97%) , were purchased from Aldrich Chemical Co (Gillingham, UK). Cellulose acetate sheets (250 m; 500 m) were obtained from Courtaulds Speciality Plastics (Derby, UK).

2 Stock solutions

Stock solutions comprised silver nitrate (0.3M), hardener (1% (w/v) chromium acetate), sensitizer (1% (w/v) ascorbic acid in water adjusted to pH 5.0 with NaOH), dyes (0.1% (w/v) in methanol) and LiBr (3% (w/v) solution in 60:40 (v/v) methanol:water).

Each 100ml LiBr solution contained 0.4ml of the ascorbic acid solution and either 2.5ml pinacyanol solution to generate photosensitivity to 633nm (HeNe laser) or 5ml diethylcyanine to obtain sensitivity to 532nm from a frequency doubled YAG laser.

(Both dyes were not used together in the same bath as this was not a satisfactory route to obtaining panchromatic sensitization).

3 Methods

Pre-subbing

Glass sheets were pre-subbed as follows: immersion in 10% (w/v) NaOH solution overnight, rinsing in deionized water, drying, and rubbing a 1% (w/v) freshly prepared acetone solution of 3-aminopropyltriethoxysilane over one or both sides with a tissue for about 1 min until the solvent had evaporated. The glass sheet was allowed to stand for several hours to ensure reaction of the silane, then the surfaces were rinsed with methanol. Cellulose acetate sheets of thickness 250 and 500 m were pre-subbed by immersion for 100s in a bath of 12% (w/v) NaOH followed by extensive rinsing in deionized water.

Coating:

Substrates were coated with gelatin solution: 15g. (w/v) granules were dissolved in 100ml well stirred cold water, heated to 80C on a water bath and filtered the solution through a fine nylon mesh into a pre-warmed beaker.

Two coating methods were used: spreading the solution with a Meyer bar (7 windings.cm-1) or by "curtain coating" achieved by inclining the warmed glass sheet at about 30 degrees to the vertical and pouring the hot gelatin solution across the top to uniformly cascade down the glass. The coating was allowed to stand for 10 minutes to form a firm gel. The film was then dried in a current of cold air and hardened by immersion in a bath of cold 1% (w/v) chromium acetate for 1 min. Then taken out, shaken and any surface droplets removed by very gentle blotting with a soft tissue paper. The film was left in a cool airflow until it was dry to the touch and then incubated overnight at ~ 60C. The hardened film was freed from excess chromium salt by rinsing with deionized water and dried.

A 13cm x 10cm (5x4 inch) hardened-gelatin-coated glass plate was laid on a flat surface under subdued white lighting, 0.3M AgNO3 solution (2ml) was pipetted onto its centre and immediately a thick (250 m) transparent polyester plastic sheet carefully laid on top and trapped air bubbles gently squeezed out. After incubation for about 3 min, the cover slip was carefully removed, the gelatin film briefly blotted with filter paper to remove surface droplets of AgNO3 solution and the coated plate blown dry with warm (40C )air.

Under moderate safelighting, the plate was plunged into a well stirred ( 2 cm deep ) bath of the bromide/ dye solution, agitated continuously for 4 min. and then washed under running tap water. Plates were finally placed in a sensitizing bath of 1% (w/v) ascorbic acid pH 5.0 for 1 min followed by a brief rinse in deionized water.

It was often required to have a shorter replay wavelength in the finished hologram, in which case the ascorbate sensitizing bath was replaced by one with triethanolamine of suitable concentration. 9, 10.

Plates were exposed and processed as per conventional holographic emulsions 11, 12..

Spectral measurements:

The reflected spectra from two sample holograms were analysed using an LOT-ORIEL MS127i Model 77480 imaging spectrograph in single-channel mode with a 256X1024 pixel InstaSpec IV CCD detector and processing software. The light source was a World Precision Instruments F-O-LITEH halogen lamp delivered by an FCB-UV600-2 bifurcated optical fibre which allowed reflected light to be delivered in turn to the spectrograph.

4 Panchromatic sensitization

Sensitivity of the emulsions to both 532nm and 633nm was obtained by first producing grains sensitized for 532nm as described above but without using the final ascorbate sensitizing bath, and then thoroughly rising the film in deionized water to free it from all traces of soluble halide ions. The plate was rinsed for several minutes in 4:1 (v/v) methanol: water to remove uncomplexed dye, dried under a stream of cool air and treated again with AgNO3 solution under safelighting. The plate was then sensitized to 633nm by immersion for 1 min in an agitated bath of LiBr solution containing the pinacyanol dye as detailed already , agitated in a 4:1 (v/v) methanol:water for 5 min to remove excess dye and finally rinsed in 1% (w/v) ascorbic acid pH 5.0.

Results and Discussion

The grain structure resulting from the diffusion method can be seen in the electron micrograph of Fig.1. The good rounded structure compares well with that of the grains in the proprietary holographic film shown in Fig. 2.

The same materials on glass plates produced the two simple Denisyuk holograms shown in Fig.3. Both plates were given the same exposure time to a HeNe laser (633nm) in the same optical set-up. A spectral analysis of each hologram produced the data in Table 1. The three measured parameters of peak wavelength (with respect to the spectral response of the human eye), the bandwidth (expressed as full width half maximum, fwhm) and film thickness (taken as proportional to the number of contributing fringes) were combined crudely to give a figure of merit (fom) for brightness as:

brightness fom == fwhm*thickness2/(peak wavelength)

where fom(diffusion method) = 1.8 and fom(Slavich) = 1.6. Within the tolerance of this crude calculation, not accounting for in-film transmission loss and the true functional form of human eye response, the brightnesses can be expected to be similar, as was observed.

We had no knowledge of the old German work7,8 as we developed this process. Their work was based essentially on operating the system with the bath order reversed compared to our system. They first treated the gelatin layer with potassium bromide solution and then used the silver nitrate bath.. Liesegang8 explained that the relative concentrations should be in matched ionic proportion . We also through trial and error found that the ionic concentrations of Ag+ as nitrate solution seemed to require it to approximately match that of the Br- in the bromide bath. Liesegang also concluded that the optimum concentration of silver nitrate should be slightly greater than 5% w/v. This surprisingly corresponds with our figure which works out at 6%.

Liesegang emphasised particularly the importance of not allowing the layer to dry out before using the second bath (the Ag+ bath in his case). This is an interesting statement since it is the exact opposite of the procedure we found necessary with our second bath (Br - ). His argument was that through drying out, the effective concentration of bromide ions per unit volume of film was increased which then consequently caused a mismatch with the concentration in the Ag+ bath leading to a thick opaque layer depositing on the surface of the gelatin. This useless layer was described as "mit dem Finger abwischbar". Whereas for reasons we do not fully understand, we found that unless we dried our Ag+ laden layer first , most or all Ag+ ions apparently migrated faster out of the layer than the Br- ions could migrate in. Thus like Liesegang we also obtained a useless thick layer of AgBr which could be wiped off with the finger. When operating optimally our coatings come out of the bromide bath impressively clear without any surface deposit.

From the principles needed to obtain photosensitivity in AgBr films, it might be supposed that the bath order used by Liesegang should be preferable since the second bath would leave the AgBr grain in a rich excess of Ag+ ions and this is well known to enhance photosensitivity whereas in an environment which leaves the grain surrounded by an excess of Br- ions it would be expected to depress sensitivity13. However we seem to have obtained substantial photosensitivity by adding dye to our well agitated Br- bath. (We have had to have a high proportion of methanol in the Br- bath in order to have the dye in solution). If we instead put our plate in a separate dye bath at the end , the resulting photosensitivity is considerably less with the additional disadvantage that simple staining of the gelatin by the dye can occur strongly which causes needless absorption of laser light . In our method the dye is available at the start of the birth of the grain to complex with the Ag+ ions as opposed to having to activate all Ag+ ions in the completed grain.

We have found that it certainly worked to reverse the bath order and to initially saturate the gelatin with bromide ion plus dye, and then to immerse it in a silver nitrate bath. However, we were not able to gain any additional photosensitivity and that fact plus the inconvenience and expence of using a larger bath of highly spoilable silver nitrate has made us decide that our bath order was preferable for our needs. Liesegang s paper does mention briefly that reversing his bath order also worked.

We have substituted the silver nitrate with silver perchlorate dissolved in organic liquids in order to produce gratings in hydrophobic polymers as described elsewhere 6.

Conclusions

The diffusion method offers the following great advantages over the emulsion method.

1. Ability to produce silver-based holographic gratings in ready-made films of polymeric materials (even when somewhat hydrophobic ) which would otherwise have been impossible.

2. Ease and rapidity of operation.

3. A readily achievable, controlled degree of panchromatic sensitivity.