Stability Issues in DCG
All of us know something about gelatin and the way it ages and holds up in various environments. It is present in many foods we eat and in leather and furs we wear. It is a fibrous felt like substance made up of dozens of amino acids arranged in long polypeptide chains. In leather it has been tanned for strength and lubricated to better stand the flexing. In photographic films it has also been tanned or cross linked to resist abrasion and absorption of water which would cause it to swell and become very soft and perhaps a meal for micro-organisms. In holographic phase only films the degree of hardness plays a key role in determining the clearness of the gel after it has undergone rapid dehydration. We begin to harden it first by cooking it for several hours with water vapor present, then by continued dark reaction with chromate ions, then by exposure to actinic or blue radiation to harden a pattern into it, then through a low ph bath of water and salts such as Kodak fixer with hardener and finally with a post process dry bake out at temperatures up to 120 deg C . This last treatment will increase the density of the film and decrease the legendary affinity it has for water vapor, but it is still far from water proof.
Hologram formation in gelatin
A DCG hologram is first formed optically by absorbing light and cross linking at the sites where the light is most intense. Then the whole film is swollen in water with the most heavily exposed regions swelling a little less than the unexposed or lightly exposed regions. At this point we have a dimensional magnification of the original fringe pattern where the water has decreased the density of the gel periodically corresponding to the original interference pattern. If it were possible to dry the film at this point without having it change dimensions then there would be a very large difference in the densities of light and dark areas which translates into a large change in permittivity or index of refraction at optical wavelengths. Unfortunately the surface tension of the water causes the structure to collapse to a small index change if left to dry at standard pressure and any temperature. People involved in making aerogels come up against this problem constantly and must resort to the use of pressure chambers, miscible solvents and other tricks to dry out their gels without shrinkage. We have found empirically that a mixture of water and alcohol with a specific gravity of .86 will remove much of the water without collapsing the structure by replacing some of it with alcohol. When the specific gravity is adjusted to about .84 then the structure begins to shrink and the gelatin begins to become more rigid. When we get to .76 to .78 the whole structure is rigid and the manner in which this is done determines the final index modulation of the film and some other properties. Normally the film is soaked with agitation in the .86 bath for a minute or more and then plunged into a much drier bath followed by an even drier and perhaps hotter bath from which it is slowly drawn as a finished piece. Alternatively several more baths may be used to gradually get to the driest possible bath and temperatures should be in the 50 to 70 degree C range for highly modulated broadband films or at room temp or lower for uniformly small modulations. The higher temperatures enable more rapid diffusion of the alcohol into the gel where it can more rapidly dilute, displace and expel the water leaving substantially higher differences between light and dark fringes.
Heat, Cold and Moisture
Nothing short of laminating the film between two pieces of glass, with a 3 to 6 mm border of gelatin removed from the substrate, will prevent the gel from eventually absorbing enough moisture, when stored at high humidity or near dew point conditions, to cause a total collapse of the holographic fringe pattern. All permanent DCG holograms made since the late sixties have faded to oblivion unless they were stored continually below 90% RH or have been capped with glass. A few thick and dense plastics have also been successful but most coatings and plastic laminates merely act like sponges and then like osmotic pumps carrying water molecules through their surfaces and into the gelatin, which readily accepts it. We always bake our gratings and hoes prior to capping with warm glass, to further insure that the water content trapped in the lamination is small enough to never become a threat to the fragile fringe structure. Trapped moisture that does no harm at room temperature will upon heating become active enough to wet and soften the delicate expanded structure that makes up the diffracting fringes, which will then collapse and join together in a more homogeneous layer, exhibiting only very weak diffraction. The baking is absolutely necessary for stability in extreme environments. We have tested many holographic scanners and outdoor hoes to destruction by baking after capping until the glue holding the parts together decomposes and turns dark. The gelatin and the recorded fringes usually survive up to 230 deg C where they begin to darken and carbonize. Some of this darkening can be attributed to the release of chromium from bonds in the gelatin and its subsequent reduction, along with some chrome salts that were never completely rinsed out during processing. Longer soak times in ever cleaner water baths will remove most of the residual chrome compounds and greatly reduce yellowing by heat and also by UV radiation. The gel does not seem to be affected by low temperatures and the adhesion to glass is good throughout all temperature ranges, provided that the glass was clean and final washed just prior to coating.
The effect of high energy radiation on gelatin is also important to anyone intending to put DCG hoes into space or into solar collectors outdoors or into creative lighting designs in buildings where the windows are DCG gratings. Gelatin, like all organic and many inorganic substances will be damaged by photons of more than 4 or 5 electron volts. Eventually all the bonds will be broken if the flux and energy are high enough for long enough. The gel will then be decomposed into loose atoms and molecules incapable of maintaining the original physical form. Fortunately the process takes a very long time to complete, if the gel is protected between two pieces of glass. We have tested holographic scanners sandwiched between two pieces of 1.5 mm thick glass by placing them near a 250 watt low pressure mercury arc lamp for over 24 hours or until the glass itself solarized and became dark near the exposed surface. The gelatin was unchanged in diffractive properties but the glue line and the gel were slightly yellowed which would reduce transmission in the blue region. Less severe tests on display holograms were carried out from 1977 to 1979 where the test pieces were placed out in the open to take on whatever the weather in Salt Lake city and also in New Jersey could dish out. In all cases, for a year or for a month, the glue that we were using then became noticeably yellow but the gelatin and glass remained unchanged within the limits of our ability to measure it optically. Another test was carried out on DCG and three other recording materials that were all prepared in our Paradise lab in 1989 for Northrop electronics division. Several Sample gratings were recorded on thick fused silica substrates and then baked and capped with another silica plate. This time the wave-fronts were measured and photographed before and after the radiation treatments. Neither the efficiency or any other property of the hologram changed from high energy radiation equivalent to several years of direct exposure in space. Somewhere a formal report exists to back up this statement but it did not yield to my searches and the individuals that carried out the work are no longer at Northrop. They did send me some photographs of the wave-fronts and I have those but they cannot tell me what the radiation doses were. In this case the silica could not absorb much of the radiation so it was a truer test of the durability of the gelatin itself. A few of our customers have put our large gratings into green houses as panes of glass and they appear to have survived at least since 1986 or so. Several 16 inch diameter hoes have gone to NASA Goddard where they are used in outdoor LIDAR since about 1991. One early reflection hoe which was not baked out has drifted a little to the blue, nothing that was baked thoroughly before sealing has changed.
Our experience with holograms and hoes recorded in DCG since 1974 has been varied and more or less all encompassing with respect to the variety of product and applications possible with this material. We can speak with some authority to the issue of stability and durability in hostile environments. We have made thousands of hoes that have endured 10 or more years of industrial or commercial environments without failing, as long as they were sealed between two pieces of glass at least one half mm thick with at least 3 mm of cleared area near the outer rim of the sandwich. We have also had a lot of failed product that was sent out without that total protection. The O ring type seal formed by the glass to glue to glass bond in the cleared region is absolutely a must for longevity as is the final bake-out before capping. To neglect the O ring will most likely result in a faded recording over time, if ample water vapor is also present. At a minimum the edges of the recording will disappear for several mm into the sandwich. Capillarity pulls the water ever farther into the plates until most or all of the gelatin softens and collapses. Radiation does not seem to be any more hazardous to gelatin than it is to glass and it holds up better than any plastic we have embossed into. Most of the horror stories of disappearing hoes or blue shifting hoes or the loss of some initial efficiency have their origin in less than optimum preparation before capping. We think we know how to do it right and now so do you.
Last modified on 2/18/99