A layer of gelatin containing a small amount of dichromate (ammonium or potassium), can be modulated by a photo- chemical process. The unexposed preparatIon is insoluble in water, but portions which have been sufficiently exposed, become soluble. The process has been known for some con- siderable time and forms a basis to many photoetching tech- niques. It is usual to expose the material by a photographic process using dark blue or ultraviolet light, but the sensitivity can be extended to wavelengths of about 0.5 micrometre. The resolution obtainable with this grainless material is high, . possibly exceeding 4000 lines/mm.
The potential advantages offered by the dichromated gela- tin for holography have been exploited at the Bell Telephone Laboratories and diffraction efficiencies of over 32%, an eight-fold improvement on a simple silver-image grating without further processing are reported. Unlike photo- graphic materials, dichromated gelatine has a limited life between preparation and usage, sometimes as low as 2-3 hours. As a result of the short shelf-life, plates are often prepared just prior to use. The process is not difficult, but care is required to obtain optimum conditions. Repeatability can be assessed from the diffraction efficiencies of holograms made under the same conditions by involving materials from different batches. To prepare plates, a gelatin layer of about the correct thickness is required. One suitable way is to mix a 7% by weight solution of gelatine and pour it to form a free flowing layer on a piece of clean glass. On drying and hard- ening in the photographic fixes and again washing, a stable layer of hard gelatin about 3 micrometre thick, remains de- posited on the glass. Thicker deposits may be obtained with higher concentrations, but multiple layers seem to be more satisfactory. (The dry gelatine thickness of Kodak 649 F is about 12 micrometre.) Finally, the gelatine layers are sensi- tized by soaking in a 5% solution of ammonium dichromate for 5 minutes at 20°C and drying slowly in darkness. This
HOLOGRAPHIC MATERIALS 29
PattErns on the surface of a hologram plate. On illumination with a laser this will turn into an image
preparation brings a good compromise between sensitivity and stability.
The preparation produces a hard fibn not soluble in water.
Exposure of the hologram and subsequent processing do not remove any material, but the recording appears to be made in terms of gelatine swelling where water absorption takes place; hence the amount of swelling decreases for an increase in exposure. Subsequent rapid dehydration in isopropanol locks the swell pattern into the layer. Recordings can be made without any development at all, which although optically unstable and of low efficiency, do have advanfages in some tasks of holographic interferometry where the avoidance of disturbance is of prime importance.
30 HOLOGRAPHY
A unique advantage of dichromated gelatine is its ability to accept a series of exposures. The gelatine can be sensitised and used a number of times without destroying the informa- tion previously recorded. The material is likely to find in- creased use, as the argon laser becomes more popular and easily available.
Holoe:raphv with Photochromics
The holographic recording on dichromated gelatine is es- tablished by changing the material properties after exposure in the development and fixing stages. The process for photo- chromics is very different, the final record being gradually formed throughout the exposure and no subsequent devel- opment being required. Such an arrangement seems ideal for holographic interferometry but unfortunately the hologram cannot be fixed and is slowly removed by the reference beam when used for reconstruction. This shows promise for eras- able holograms because of their reversible characteristics.
Their utility is in real-time processing, as in data handling and storage.
The hologram is stored in a photochromic by either a positive or negative process, in accordance with the laser wavelength used. The material darkens when exposed to short wavelength light and gets bleached again on exposure to longer wavelengths. The material is grainless, and is typi- cally darkened with ultraviolet light and selectively bleached by interfering wave fronts from a helium-neon laser.
The photochromic material may be in the form of a doped crystal, (crystals with specific impurities added deliberately) glass or gelatine or any other binder matrix which can be used to hold a suspension of photosensitive organic dye.
With all photo chromic holograms, the reconstruction beam will destory the hologram, so that the actual read-out must be carried out as quickly as possible and with minimum possible beam intensity. The storage life of the hologram may
HOLOGRAPHIC MATERIALS 31
be several weeks, or even longer tinder dark refrigerated conditions.
Electrical and Magnetic Recording of Holograms Considerable interest has been shown in improving holo- graphic techniques which are independent of chemical proc- essing. One possibility is to generate a direct electrical or magnetic image on a charged selenium plate or on a magnetic tape. So far, the former method has not produced usable results but a technique using a thin recording film of fer- roelectric barium titanate and lithium niobate crystals has been tried. Recording at present is restricted to pulsed lasers.
Reconstruction can be carried out with a helium-neon laser, but the efficiency is very low. The Japanese have experi- mented with photochromic films and liquid crystal area de- tectors to record infrared energy holograms at 10.6 micrometre wavelengths .