FEG (ferric gelatin) holograms

Martin · Post by **Martin** » Sun Dec 14, 2008 9:30 am

Thanks to a_k (thank you, Ahmet!) I got access to a 405nm (Bluray) laser diode. Though its coherence length is rather limited, it is definitely a very exciting wavelength for a great many holographic recording materials.
Among the things I tried were ferric systems, based on ferric ammonium citrate and ammonium ferric oxalate.

I started with a similar mixture as the one described in the CHIBA system (see http://spitbite.org/carbon/papers/TheChibaSystem.pdf and http://holographyforum.org/phpBB2/viewt ... a&start=15). The exposed layer was “fixed” for 30s in a ~1% hydroxy peroxide solution (I actually diluted a peroxide household bleach). Then the plate was washed and put in a (water) swelling bath. Subsequently the usual alcohol dehydration took place, followed by hot air drying. So this is exactly the same processing as for DCG.
Although speed was not as high as with DCG (ammonium dichromate), the FEG holograms revealed an excellent DE. So the good news is, ferric compounds may successfully replace DCG – at least at a blue-violet recording wavelength. In an analogy with DCG we may call these materials FEG (= ferric gelatin).
The holograms look very similar to DCG. I actually believe they may even perform better than DCG when it comes to issues like noise and resolution. Ferric systems seem to differ from DCG with regards to their specific absorption spectra. Ferric ammonium citrate and ferric ammonium oxalate seem to be sensitive from UV to blue only.
One might wonder about spectrally sensitized ferric systems also...

The highest speed levels were achieved with ammonium ferric oxalate. These layers were well on par with ammonium dichromate-DCG. One drawback was the short shelf life of the sensitizing solution.
I assume more ferric components ought to be tested, I am particularly thinking of ferric oxalate and alike.

Ed Wesly · Post by **Ed Wesly** » Mon Dec 15, 2008 7:20 pm

Were these reflection or transmission holograms? it would be great to see some results.

It is interesting to hear that this works in making phase gratings. I have been demonstrating the 19th century cyanotype technology in my photo-history class, which uses Fe Ammonium Citrate, but yields a brilliant blue image, so I thought the end result of a Fe system would be an amplitude holo! My hat's off to you!

Do you think or know if it works at 532 nm?

Martin · Post by **Martin** » Tue Dec 16, 2008 1:51 pm

Ed Wesly wrote:Were these reflection or transmission holograms?

I did both transmission and reflection holograms.

it would be great to see some results.

OK. The results don't differ greatly from DCG.

Its interesting to hear that this works in making phase gratings. I have been demonstrating the 19th century cyanotype technology in my photo-history class, which uses Fe Ammonium Citrate, but yields a brilliant blue image, so I thought the end result of a Fe system would be an amplitude holo!

I understand all (?) kinds of blueprints rely on the conversion of ferricyanide to ferrocyanide. According to Kosar, Light-sensitive systems, this is a process “in which the original ferric salts or the reduced ferrous salt react with another substance to form a colored reaction product.” Thanks. I've yet to see if it can be used at 532nm. At first glance I'd suspect it can't but then, wasn't that what we also used to say about DCG at 532nm?

FEG is much simpler. It's based on “the ability of ferric salts to render gelatin and other organic colloids insoluble.” (Kosar) So essentially only one ferric compound is required for this system. FEG relies on FAO (ferric ammonium oxalate), FAC (ferric ammonium citrate) and possibly on other, yet to be tested, ferric compounds. On exposure to light they are reduced to ferrous compounds. Then, putting the exposed layer into a weak peroxide solution is likely to generate free radicals. What's taking place there in detail goes beyond my head. I'd love to hear about it from a chemist's perspective. It seems clear though that a light exposure creates a structure of different local hardening. The alcohol dehydration step amplifies this structure whereby micro voids are produced. So on a practical level the process comes very close to DCG.
By the way, the effect of the peroxide developing bath may be analogue to the fixing bath in DCG.

I was amazed by the speed levels of FEG. General wisdom used to say ferric systems were far behind DCG. Obviously this isn't the case – at least not at that wavelength.
As for the problems of shelf life regarding FAO sensitization I mentioned earlier, they may actually turn out to be far less dramatic. I sensitized a pure gelatin layer with a 5% FAO solution (a solution I had prepared some three years ago). It did work nicely. 24 hours later, these sensitized gelatin layers still didn't lose any light sensitivity.
The shelf life for a gelatin/FAO solution, ready for coating, remains to be seen though.

My hat's off to you!
Do you think or know if it works at 532 nm?

Thanks. I've yet to see if it can be used at 532nm. At first glance I'd suspect it can't but then, wasn't that what we also used to say about DCG at 532nm?

Martin · Post by **Martin** » Tue Dec 16, 2008 1:54 pm

: The left side shows a DCG, the right side a FEG hologram.; 1.jpg (5.51 KiB) Viewed 3693 times

Joe Farina · Post by **Joe Farina** » Tue Dec 16, 2008 2:09 pm

Congratulations, Martin! Thanks for the picture, that's really amazing. There seems to be a dramatic improvement to the health/environmental factor regarding ferric compounds vs. dichromate. And those ferric compounds might be a lot easier for some people to obtain -- I noticed there is an eBay seller who is offering a pound of ferric ammonium oxalate for $35, and international shipping is available.

By the way, what kind of coherence were you seeing with the diode?

Martin · Post by **Martin** » Tue Dec 16, 2008 2:34 pm

Joe Farina wrote:Congratulations, Martin! Thanks for the picture, that's really amazing. There seems to be a dramatic improvement to the health/environmental factor regarding ferric compounds vs. dichromate. And those ferric compounds might be a lot easier for some people to obtain -- I noticed there is an eBay seller who is offering a pound of ferric ammonium oxalate for $35, and international shipping is available.

By the way, what kind of coherence were you seeing with the diode?

Thanks, Joe.
Yes, I also think handling ferric compounds may involve less health hazards than dichromates.

The last time I took a look into the coherence length of the laser diode, it was about 2cm. But this may vary with the temperature in my lab.

Colin Kaminski · Post by **Colin Kaminski** » Wed Dec 17, 2008 4:57 pm

Could this be used to make a surface relief hologram? Perhaps one could wash out the un-hardened gelatin before drying?

Joe Farina · Post by **Joe Farina** » Wed Dec 17, 2008 5:54 pm

Colin Kaminski wrote:Could this be used to make a surface relief hologram?

Since people like Ahmet and Martin are proving that a $10 violet diode can produce excellent dichromate or ferric gelatin holograms, maybe the surface relief idea would be worth looking into. But it seems more likely to get better results by simply using a standard photoresist. I'm not sure, but I'm guessing some photoresists would have excellent sensitivity at 405nm. This might make small-scale embossing a reality (at low cost). Although I've never done any electroforming myself, it doesn't look like a very big deal (actually I'm getting ready to do it for another application). The power supplies (rectifiers) seem to be getting inexpensive these days. I bought a new 0-30 volt and 0-10 amp variable rectifier (which can be run either constant current or constant voltage) for about $130 on eBay recently. This would be the most expensive part of an electroforming setup.

Sergio · Post by **Sergio** » Wed Dec 17, 2008 6:28 pm

Joe Farina wrote:. But it seems more likely to get better results by simply using a standard photoresist. I'm not sure, but I'm guessing some photoresists would have excellent sensitivity at 405nm.

I don' know about the standard photoresistor used in embossing but actually the famous SU-8 photoresist can be sensitized to 470nm and 532 for sure, (I must remember where is the paper about it and repass to who is interested

).

Sergio

Colin Kaminski · Post by **Colin Kaminski** » Wed Dec 17, 2008 8:34 pm

Sergio wrote:
Joe Farina wrote:. But it seems more likely to get better results by simply using a standard photoresist. I'm not sure, but I'm guessing some photoresists would have excellent sensitivity at 405nm.
I don' know about the standard photoresistor used in embossing but actually the famous SU-8 photoresist can be sensitized to 470nm and 532 for sure, (I must remember where is the paper about it and repass to who is interested ).

Sergio

With all the papers Joe has collected I am hoping he leaves them to a library one day.