"bad" batch of dichromate chemical?

[Joe Farina]

Been having some problems with my DCG, and was wondering if it has to do with the potassium dichromate. Several plates (with two different kinds of gelatin) had strange swirls and other markings on the plate which I haven't seen before. My first thought was that it's unlikely that the potassium dichromate itself could be a problem, but other factors have been ruled out. It's the only thing I can think of at the moment, since the problems appeared when this potassium dichromate was used.

The finished hologram shows odd patterns, a lot of random diffraction, but absolutely no image. The parameters were simple for the test made today: 3-30-200 potassium dichromate/gelatin/water, exposed at 488nm, light bulb "fix," water wash, alcohol dehydration. The dichromate wasn't from a big chemical company and was inexpensive:

https://www.sciencecompany.com/Potassiu ... 00gm-P6661

I still have trouble believing this is due to the dichromate, and was wondering if anyone has experienced something similar. Have to make a new batch with a different dichromate and see what happens.

Thanks.

[Din]

There are several possibilities.
One is that the cheap pot. dichromate had impurities in it that blocked the polymerisation process, leading to a lack of image-specific cross-linking.
Another possibility is that the coating was too deep/shallow, ie the emulsion depth was too high/low, and/or there was a variability in the solution concentration. This may have led to an increased absorption within too few layers, leaving insufficient light and low modulation. This may be enhanced by the fact that pot dichromate has a significantly lower solubility than amm. dichromate - 13% as opposed to 36%. This may lead to increased scattering due to increased reticulation.Therefore, if you used the same procedure, with the same temperatures, you may have dissolved significantly less of the dichromate, also leading to low modulation.

For issues of emulsion depth, at POC we used a traveling microscope with a resolution of <~ 1micron; this is basically a microscope which can be raised or lowered with a knob in intervals of a micron or less. We used a marker to draw a line on the glass substrate, coated only the sections away from the line, and then put a line on the (dried) surface of the emulsion. We then focused on the line on the emulsion surface, translated the plate so the mark on the glass was under the microscope and turned the knob to focus on the glass-surface mark. The amount by which we had to lower the microscope, which could be read off on the scale, gave us the emulsion depth. Since our emulsion was typically ~15 u, we had an error of 1/15, or 6%. I then calculated the difference in efficiency for a 6% difference in emulsion depth, and found no significant loss.

For issues on the difference in solubility of pot dichromate to amm dichromate, see below from "Topics in Applied Physics - Vol 20 Holographic Recording Materials" ed H. M. Smith. Also https://www.sciencedirect.com/science/a ... 2621009542

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[Joe Farina]

Thanks for your help, Din. That's a good point about the lower solubility of the potassium dichromate. When I planned to mix the stock solution, I knew nothing about it (compared to ammonium) but decided to do a quick check. I was shocked to see that, as you said, the solubility is about 1/3rd of the ammonium. So I decided to make a weak 5% solution in water. My actual film formula was 12ml 5% potassium dichromate, 6 grams gelatin, and 50ml water.

I chose the potassium because I had previously used ammonium, and wanted to see if there was a difference, and because it may make an easier transition to dye-sensitized DCG (pH ~9.2) but I don't know. Jeff Blyth's formula used potassium chromate, and I may try using this (in "regular" DCG), because my chemical is proven good. There is some really cheap potassium dichromate on Amazon: https://www.amazon.com/PCSLLC-Potassium ... 218&sr=8-1 and I'm wondering if this is some impure stuff that may be akin to what I purchased (I like the Amazon seller's name: Pyro Chem Source). I will be trying ammonium dichromate (proven to be good) to see if the problem was with the potassium.

I don't know about the thickness of my coating, but my goal is a "standard" thickness of ~12 to 15u for regular "monochrome" DCG. I used a #24 bar with the above formula, which was basically the same as earlier good "regular" DCG holograms I did. It seems that problems can arise when the thickness goes much above or below this range. I was reading some of your old posts and I think I have a better understanding of a few things. For example, in some of the other reading I've done from the literature, thicker layers seem to be suggested for narrowband holograms. But when the thickness gets up to say 25u, it becomes very difficult (it seems to me, and as I gathered in your posts) to process these in liquid solutions (with all kinds of processes taking place) and have the final result be un-distorted fringes. I did a recent test with two identical setups, one with a thicker layer (using a #48 bar) and a somewhat thinner (#36 bar) and also a #24 bar (thinner still). The quality of the colors (i.e., saturated and bright, my idea of "narrowband') came out worst on the thickest layer, best on the #36 bar layer, and reasonably good with the #24 bar (thinnest) layer. So maybe in an ideal world, thicker may be better for narrowband, but in practice a medium-thickness layer (say ~12-15u) may be better.

Thanks for the information on the traveling microsope, and also for that quote about the solubility/crystallization of the dichromate. That's a very interesting paper about the real-time potassium dichromate DCG. It's good to see a recent paper about DCG. I was rather shocked to see their report of 19% real-time DE recorded at 532nm. (If I understood correctly, that is). I would have expected to see something like a maximum of 4%.

[Din]

The quality of the colors (i.e., saturated and bright, my idea of "narrowband') came out worst on the thickest layer, best on the #36 bar layer, and reasonably good with the #24 bar (thinnest) layer. So maybe in an ideal world, thicker may be better for narrowband, but in practice a medium-thickness layer (say ~12-15u) may be better.

The thicker the layer, the narrower the bandwidth, ie the range of wavelengths diffracted. However, the narrower the bandwidth, the dimmer the 'brightness'. The eye needs a minimum bandwidth to resolve color, and if the bandwidth is too low, the eye cannot resolve the image as a colour. It's often forgotten that a hologram is peculiar in that it's recorded, and reconstructed, radiometrically, but, for display holography, it's evaluated photometrically. Display holographers often confuse 'brightness' for DE. Brightness is the value of the integral over the bandwidth, multiplied by the photopic function - a photometric quantity. If the bandwidth is very narrow, the value of the integral is also very low. But DE is the ratio of intensity diffracted and intensity presented to the diffractive structure, and, as such, is a radiometric quantity.

bright_de.jpg (88.43 KiB) Viewed 6213 times

[Martin]

That's a good point about the lower solubility of the potassium dichromate. When I planned to mix the stock solution, I knew nothing about it (compared to ammonium) but decided to do a quick check. I was shocked to see that, as you said, the solubility is about 1/3rd of the ammonium. So I decided to make a weak 5% solution in water. My actual film formula was 12ml 5% potassium dichromate, 6 grams gelatin, and 50ml water.

As you can see by https://en.wikipedia.org/wiki/Solubility_table, potassium CHROMATE water solubility is pretty high. So I wonder if it would make sense adding some alkali agents (ammonium hydroxide, potassium hydroxide?) to convert at least part of the dichromate into chromate. I vaguely remember this subject has been discussed here on the forum some years ago.

Comparing the speed of the two agents at 488nm might be interesting. I recall there was not much of a difference between the two at 405nm.

[Joe Farina]

Thanks for the clarification, Din.

Martin, thanks for the solubility table, I've added it to my favorites. I was surprised to see potassium chromate have such a high solubility compared to ammonium dichromate, not to mention potassium dichromate. Since I have the potassium chromate available, I may try this instead of dichromate, though it's said to have less sensitivity.

I've been looking at my previous results with regular DCG and color (dye-sensitized) DCG. I'm trying to compare what's happening in each system, and get some understanding of why the results are so variable. As has been mentioned so many times by yourself and others, I think pre-exposure hardening is the critical issue. The best DCG I've done (many years ago) was regular DCG aged for a specific time period on the coated plate.

In a high-pH (~9.2) dye-sensitized system (with an extremely low chromate concentration, i.e., Jeff's formula), I think the hardening situation is entirely different. I don't really think the gelatin gets hard (on its own) at this high pH. Years ago, I did some resistance tests on coated (pH = 9.2) plates (using a megohm meter and copper pads clamped to the emulsion surface). I made the assumption that, as the gelatin hardens, its ability to hold water (i.e., swell) goes down. The emulsion surface is a weak conductor of electricity, and as the amount of water in the emulsion determined the conductivity, I felt fairly confident that I was measuring the hardness of the gelatin layer. Based on those tests, the high-pH/low-chromate layer actually seemed to get softer with time.

So I'm trying to re-think the idea of color DCG. So far, I haven't had great results with trying pre-harden a liquid emulsion prior to coating. I get the feeling that, ideally, the DCG needs to harden naturally on the glass (the glass serving as a kind of "anchor" to keep the emulsion in place as it hardens).

[Joe Farina]

My thermostat for the water bath was malfunctioning, and I was cooking the gelatin without knowing it. (No problem with the potassium dichromate.)