Red Sensitive DCG

During the latter half of the 1980s I took up the challenge of trying to make DCG holograms with low power HeNe lasers (633nm).

The consequent discovery of the useful compound TMG and the theoretical side of it is described in ref. (1 ) in the piece below which was mostly written back in 1989.    Does it work?  It sure does ,  but patience is not a virtue-- it is an essential here.  However could anyone get a more enthusiastic email than I got in 2002 from a student in South America?  Here it is:-

Date: Wed, 5 Jun 2002 11:18:51 -0700 (PDT)
From: M

Subject: wow!
To: Jeff Blyth <jeff@biotech.cam.ac.uk>

I can only say, Oh my goodness!  WOW,WOW,WOW!!!!!!    

thanx so much, I'll send u a picture of the beautiful DCG that I got yesterday.

thanx, M.

However while I was in 1989 satisfied to have removed a little of the monopoly power of Agfa-Gevaert as the supplier of red sensitive holographic emulsions, things have long since moved on a lot as we all know.

I switched over to only working on silver halide based emulsions and because DCG was relatively so photographically slow, I put it on the back burner.    So M's and others and correspondences about 12 years after I last did any work on it was a surprise.

(What follows was nearly all written in1989 for DIY holographers at the time.)                

How to make clear and very bright DCG holograms with a HeNe.

(633nm) by Jeff Blyth  

Some private research of mine has uncovered a readily available chemical which enables one to obtain a dazzling bright reflection hologram of an object of say 1 inch in diameter after an exposure of only 1minute from a 10 mw. HeNe. (i.e. with the beam spread to about 4 cm.diameter to give even illumination of object.).      This opens up the way for holographers to produce their own holographic plates and get brighter single-beam results than they could get by purchasing expensive silver halide plates. It also enables them to coat curved surfaces such as wine glasses. The recipe below  really does work and can be great fun.

 Unlike conventional DCG  recording material, this one remains sensitive for weeks at room  temperature and yet it contains only a small fraction of the quantity  of dichromate normally used. The approach taken in the instructions below is to enable one to produce remarkably good coatings at minimal cost. There is however one item that would be very helpful. Control of the pH to a value of about 9 is really important in this system and therefore a pH meter would be much easier than using pHpapers.

Procedure for preparing DCG to record with a HeNe laser

The following instructions will produce approximately 100 ml of material. The new discovery centres on 1,1,3,3- tetramethylguanidine  ( TMG ).(Aldrich cat. no. 24176-8.).  This is a considerable advance over other similar systems [2] possibly because this molecule exists as a number of resonance structures in dynamic equilibrium.[1 ]

Once TMG is neutralized with acetic acid and added to the gelatin formulation it is a reasonably innocuous substance. However, the pure chemical is a volatile ,  smelly and very alkaline liquid  which should not be inhaled and eye protection should be worn when using it.

Preparation of stock solution of 25% v/v  TMG (  1,1,3,3-tetramethylguanidine): 50 cc.Deionized water: add until total volume reaches 200 cc.

 Preparation of stock solution of TMG acetate.

25% stock solution of TMG :   80 cc.

Glacial acetic acid  8 cc. (approx).    This quantity should be added very slowly while stirring and adjusted if necessary to give pH of roughly 7. (A bit of  fuming occurs here as the two vapours combine.)

 Preparation of stock solution of 5% w/v potassium chromate

Potassium chromate : 5.0 gmade up to 100 cc. with deionized water.

Preparation of stock 0.4% methylene blue solution

Methylene blue : 2.0 g.Deionized water: 500 cc.Any undissolved impurity can be filtered out.

Preparation of coating solution.

Cold de-ionized water  80 cc.

Gelatin  12 g. (A good quality cow gelatin rather than pig gelatin)(It needs to have the lower "Iso-electric point" of cow gelatin.)  However,  good catering gelatin has given excellent results.

Place container in a water-bath and heat up contents to between 45 and 50 C. while mechanically stirring slowly. This may require 20minutes or more. Check that no undissolved gelatin particles remain.

Filter the solution through a nylon mesh, eg. a  stocking. Maintaining the temperature between 40 and 45C, continue as follows:

Stock 5% potassium chromate solution :  1.0 cc.

Stock solution  TMG acetate :  6 cc. **

**(Note that this quantity should produce an orange replay  color in reflection holograms made with HeNe laser light, at an ambient relative humidity of 60%. To obtain a green replay color this quantity should be increased to 8 cc. This will also have the advantage of increasing the light sensitivity but the disadvantage of making the surface rather tacky because TMG is hygroscopic.

pH adjustment step. 

 From this point on, before adding the methylene blue, it is VERY IMPORTANT to maintain the pH between 9.0 and 9.5. 

The adjustment is carried out by very carefully adding stock 25% TMG solution dropwise while the solution is well stirred. If the pH rises above 9.5 it should be brought back at once by a drop of acetic acid. It is not possible to give a precise value of the quantities of either of these reagents at this point because it will depend on the batch of gelatin being used. I found that it took 1.8 cc. of stock  25% TMG to get pH 9.1.  It is a big advantage to use a pH meter rather than pH paper strips.[A useful calibrating solution is 3.80 g. borax crystals in a litre of distilled water. This has a pH value of 9.2 at room temperature.]

 Finally, under green or dim yellow safe-light:  6 cc.  stock methylene blue solution.  Note that this quantity is for a coating thickness  between 4 and 5 microns. If you are going to double the thickness of coating by using methods other than the simple one described below, then the methylene blue quantity might need to be reduced. This is because the dye in the dried coating needs to be low enough to allow as much light as possible through to the object. However if you do not see any sign of a blue coloration when your dried, unexposed film is resting on white paper, then that probably means you have let the pH drop too low and it will have no sensitivity to red light.  [  In my earliest formulation I used ammonium dichromate, but found that it caused a wide variation in the pH of the coating depending on its state of dryness and how much free ammonia had escaped from that salt. Methylene blue forms a useless precipitate with dichromate ions unless the pH is above about 8.5.  Accordingly this problem was eliminated by using the alkaline salt potassium chromate instead of ammonium dichromate.]

This formulation can remain active for several weeks at room temperature although it is best to store it in a cold place, but not actually allowed to freeze.

Simple coating method.  "Veil Coating"  for a 300mm x 400mm glass plate

(The plate should be very clean eg. by leaving overnight in neat domestic bleach solution.  Then well rinsed in  de-ionized water.  Thankfully a silane pre-subbing is not essential unlike the case of silver halide gelatin work.)  

Under green safelighting (Can be much brighter than would have to be used for silver halide holographic recording).

1) Warm the solution to between 40 and 45 C. in a beaker with a good spout.   Do not overheat.

2) Hold a clean and warm  glass plate at an angle of about 30   degrees to the vertical with its base resting in a flat and clean tray.

3) Position the spout of beaker at a point about 1 cm. from the top   of the glass and 1 cm from the edge closest to you .

4) Start to pour out the solution at a constant rate while moving your arm at a constant rate a centimeter from the top of the plate.   The liquid should fall like an evenly spread veil on that side of   the glass.

5) After the plate is covered,  allow it to gel in this position for a few minutes.     ( The balance between gravitational and surface tension   forces can give a surprisingly even coating.)

6) After the coating has gelled, the thick layer at the bottom edge   of plate can be either wiped off or removed by dipping the bottom  edge in warm water. Pour the tray contents back into the beaker.

   The plate is now best left to dry for at least 24 hours in a  darkened room before cutting it up into reasonably sized pieces.

eg. 4cm squares. if you only have a 4 mw laser.

Exposing       To make a bright  Denisyuk hologram of an object of one inch in diameter may need about 1  min. exposure to a 10 mw. HeNe laser with beam expanded just sufficiently to give a reasonably uniform illumination of object. Firstly, a trial-and-error test on exposure levels is essential. A large bright coin is the standard test object.

The initial trials should preferably contain a  range of exposure levels on one plate by masking off the spread laser beam for various times . Please remember that doubling the beam  diameter may increase the exposure time needed by a factor of 4 and not 2.

The biggest cause of failure in new DCG work is either insufficient exposure time or insufficient settling time to allow micromovements of the gelatin to cease.

(My policy was always to set it up and to go away and do something else for an hour or three.! ) 

Processing.  It is very helpful to keep the following mechanical model in mind. If one hangs weights on to the end of a steel extension spring, then removing those weights will result in the spring recovering its original un-extended length unless the weight exceeds a critical value. When a weight is of that critical value, the material of the spring has reached just beyond what is known as the "yield point". When that weight is removed the spring would normally never completely recover its original un-extended length but will remain permanently distorted. Had the material of the spring been a little stiffer, then that particular weight would not have stretched the spring beyond its yield point. If the temperature of the stiffer spring were to be raised however, then that weight may again take the spring beyond its yieldpoint.

DCG in water behaves similarly to the stretched spring. At a certain temperature, the stretching forces due to swelling in water may take the gelatin beyond its yield point. Had the water temperature been lower, then the gelatin would not have reached its yield point.

The art of making the most noise-free and yet bright DCG holograms, is to take the gelatin as close as possible to its yield point without exceeding it. Stretching gelatin beyond its yield point creates noise in the finished hologram. Using the onion layer analogy of Curran and Shankoff, the holographic fringe layers might resemble dried out onion with crinkled-up layers if the yield point is exceeded. A sign that you have got the swelling  just right is no milkiness  in the exposed area but milkiness in the unexposed area.

 A Simple Processing Method

 (It is strongly recommended that this simple method is tried before other methods you may know of.) 

No meaningful tests should be tried until the coating is at least 24 hours old.

The first trial hologram is best processed so that the yield point of the gelatin is just exceeded, and some noise or scatter is generated deliberately in the finished hologram. This is necessary to be sure that no sign of a hologram after processing, is not due to insufficient swelling in the first water bath and therefore must be due to either relative movement between object and holographic plate or insufficient exposure. Once one has established the necessary exposure level then one can produce holograms devoid of any noise by processing at a level which takes the gelatin just below its yield point.

After exposure, the sample should be left for a few minutes before processing.

1) Thoroughly rinse hologram in COLD running tap water for several

minutes.

Important note written in 2002:-

It is essential to wash out all traces of that TMG compound to stop it getting into your alcohol baths where it would finish up attracting in water into the finished holograms and this will fatally affect their long term survival.

2) The plate is then swollen in water at a temperature of 26 deg.C   for about 1 minute.

3.) Shake off excess water droplets and plunge plate into cold   iso-propyl alcohol. Agitate vigorously for at least 1 minute. (The   water content of this bath can be allowed to be anything from zero  to 10%.)

4.) Put plate into another iso-propyl alcohol bath but this    time insure that its water content is less  than 1%.

Agitate for 2 minutes at least. (Insufficiently dry alcohol in this bath will    result in drying marks  which replay in the blue.) It must be said that particularly good results and rapid image-appearance come from having this bath rather warm, but there are fire safety risks involved and a tepid bath can work very well.

5.) Blow alcohol off the plate with a strong  blast of tepid air.

Prolonged inhalation of the alcohol vapour is harmful and hence the drying process should be carried out with air extraction. The second alcohol bath should be protected from ambient air as soon as possible as it rapidly absorbs moisture.

No image may be visible until all the alcohol has escaped from within the fringes; a process helped by leaving the sample in a warm, dry place.

If the finished result is completely clear and shows neither scatter nor hologram, then the chances are that the fringe voids or cracks have not formed properly in the alcohol bath by the "Shankoff effect"[3] and the hologram simply needs reprocessing using water at a higher temperature in the first bath.  If the hologram is noisy or "milky" then that means that the gelatin has had its yield point exceeded and nothing can be done about this noise except to shoot another plate and process it in water at say 18 deg.C so that the yield point is not exceeded. Plates which are a month old and have been left at room temperature,  may need a water temperature of about 35 deg. C.

Getting the water temperature just right, can produce the most noiseless and bright single-beam holograms.

Finished holograms should be thoroughly dried for several hours at around 60C. in a dry place space. Once treated like this, they seem able to withstand ambient relative humidity below 70% indefinitely. (I used to leave my holograms for several days in a laboratory desiccator containing violet crystals of freshly roasted self indicating silica gel). One should then seal them if they are going to be carried around, otherwise a cold hologram suddenly brought into a warm room can create surface moisture and cause the notorious disappearing trick of DCG holograms. (They can be reprocessed in water and alcohol if that happens.) Note that sprays should not be generally used as sealants since they can penetrate into the fringes.  Viscous UV curing resin and a glass backing sheet seems to work well.

Acknowledgement. I am very grateful for the help of George Clare who made many independent tests of formulations.

References.

(1)  J.Blyth. Methylene Blue sensitized dichromated gelatine holograms: a new electron donor for their improved photosensitivity Appl. Optics 30,  1589-1602 (1991)

 (2)  R. Changkakoti., S.S.C.Babu, and S.V.Pappu, "Role of External Electron Donor in Methylene Blue Sensitized Dichromated Gelatin Holograms"  Appl. Optics 27, 324 (1988)