DCG Hardening (Dinesh)

JohnFP · Post by **JohnFP** » Fri Apr 23, 2004 11:52 am

I am realy starting to visualize this now.

So one more question and I will leave you alone (for the time being).

If I have two samples of a gelatin that are exactly the same, especially polypeptide density, except one has more crosslinking then the other. The one with the higher amount of cross linking has a different index of refraction then the other.

So gelatin with different amounts of cross linking have differnt index of refractions with all other variables the same?

JohnFP · Post by **JohnFP** » Fri Apr 23, 2004 11:55 am

This would also mean DCG is not surface relief.

JohnFP · Post by **JohnFP** » Fri Apr 23, 2004 11:57 am

So the cooling of gelatin does not cause a constriction?

Sergio · Post by **Sergio** » Fri Apr 23, 2004 5:34 pm

But holographically, since we are interested in write holograms, the unique way to transfer an electron to the right region (fringe lines)is by actinic radiation, your question is interesting, imagine that excited dichromate molecule with blue light will stay excited for a time period that vary with the energy level, during this period chemical reactions (exciplex reactions?) electron transfer may occur, this electron source would come from another region connected to another wavelenght absorption, RED, another electron injection mechanism could be introduced to very fast electron injection during the dichromate excitation time, so the coupled blue light would trigger the reduction. In resume, excited by the blue but reduced by the "red" electron.

Sergio · Post by **Sergio** » Fri Apr 23, 2004 5:45 pm

"Surely one way to test this would be to fill the voids with some other material with an index between air and gelatin. It seems that the DE of the DCG hologram could be used to find the exact modulation, a la Kogelnik, and this would settle the question. I've not seen any other papers address this"

"The mechanism of hologram formation in DMP-128 photopolymer"

R.T Ingwall and M. Troll Proc Spie vol 883, pp 94-101(1988)

describes the void structure in polymers and links it to the Kogelnik theory, i believe that it could be adapted to gelatin, the even filled the voids with liquids!

Dinesh · Post by **Dinesh** » Fri Apr 23, 2004 7:35 pm

"So gelatin with different amounts of cross linking have differnt index of refractions with all other variables the same?"
Yes. In any hologram, whether DCG or not, there is an overall increase in the altered physical property, in this case the index of the gelatin. On top of this overall increase, there is a further increase in the bright parts of the fringe. The overall increase is the (actinic) bias hardening and the further increase in the fringes is the modulated index. Think of it that there is a "DC amplification" and an "AC amplification on top of it.

Dinesh · Post by **Dinesh** » Fri Apr 23, 2004 7:36 pm

"This would also mean DCG is not surface relief."
No. It can be. In fact it's an old printing processs where it was used as "surface relief" at the turn of the last century.

Dinesh · Post by **Dinesh** » Fri Apr 23, 2004 7:40 pm

"So the cooling of gelatin does not cause a constriction?"
That's a little tricky. All cooling causes constriction. It's a balancing act between the thermal agitation of the molecules and the bonds between them. When cooled, the thermal agitation slows down and the intermolecular distances decrease, when heated the opposite happens. However, gelatin is a polar molecule and so is water. because of this polarity effect, cooling may actually cause expansion. Living in baltimore, I'm sure you're aware of this fact every time the water pipes freeze and crack!

Tom B. · Post by **Tom B.** » Sat Apr 24, 2004 12:43 am

Interesting idea. It makes me think of the use of high-frequency AC bias in analog tape recording. The bias I think sort of softens up the magnetic domains and makes them easier to flip by the desired signal. I wonder if pulsing the blue light might have some similar advantage for red recording? So many things to try ...

Tom B. · Post by **Tom B.** » Sat Apr 24, 2004 5:42 am

I later realized that in magnetism you have three states (North, South and unmagnetized) whereas in film you've got only two (exposed or not) so the analogy likely doesn't apply. - Still, it might be interesting to experiment with pulsing the blue light (on the microsecond time scale) - the idea being to make the emulsion more receptive to the red light + Methylene Blue excitation while minimizing the additional unwanted crosslinking due to the blue light.

Wishful thinking + total ignorance of photochemistry = breakthrough!