“Photonics West_2002”, 4659B-59.
gelatin thick layers:
manufacturing and control
Nadya Reinhandb, Yury Vigovskyc,
Yulia Zagainovaa, Serge Malovd,
State University, Gagarin bul., 20, Irkutsk, 664003, Russia
of Mechanical Engineering Problems of the Russian Academy
V.O., Bolshoi pr., 61, St.Petersburg,
Novoslobodskaja str. No 31/1, Moscow, 103055, Russia
Institute of the Siberian Branch Russian Academy of Science,
a/ya 4038, Irkutsk, 664033,
Physical Technical Institute of the Russian Academy of Sciences,
Politechnicheskaya, 26, St.Petersburg,
gelatin (SD DG) is holographic photosensitive medium that
possesses the number of unique properties. Being illuminated
by interferometric pattern it records the energy distribution,
and in such way the gratings with high diffraction efficiency
can be obtained. The absence of postexposure processing allows
to use it for in-situ experiments. And besides the selfdeveloping
property of the material allows to create new kinds of optical
elements based on very thick gratings that possess very high
The mechanism of the hologram
formation in the material is proposed and the possibilities
to control this process are considered. The recorded hologram
is the result of conformational changes in the structure of
a gelatin system. It is the result of hierarchy
of sequential structural gelatin macromolecules modifications.
Characteristics of these processes can be effectively controlled
at the levels of the primary (the chemical composition of
emulsion) and ternary (coil-globule transitions for the entire
macromolecule) structures of the SD DG system. We analyzed
the influence of IR
laser annealing and/or special highly hardened gelatin sublayer
on the gelatination acceleration and resulting diffraction
efficiency. of layers with thickness more than 1 mm. Salt-doped
SD DG layers were experimentally tested. The
properties of new version of SD DG emulsion containing organic
high-molecular-weight polymer are discussed.
gelatin, glycerol, selfdeveloping, volume holograms, red sensitivity.
The holograms registered
in photosensitive layers of several millimeters thickness
have a number of specific properties, the most important
of which are extremely high spectral and spatial selectivities1.
These highly selective gratings are used for the development
of modern optical information processing devices, in optical
memory systems with multiplex information recording and
for interfiber connectors in optical telecommunication networks,
for the variety of spectral filters for imaging, etc.2,3.
Usual holographic materials
can not be applied for recording of grating of millimeter
thickness, because it is impossible to maintain the material
uniformity during the post-exposure processing, Differential
shrinkage caused by post-processing leads to the reduction
of diffraction efficiency, and besides to the asymmetry
and widening of selectivity contour of output signal1.
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gelatin4 is one of the solutions to overcome
these problems because it does not require post-exposure
processing. The developed photosensitive medium differs
from the wellknown dichromated gelatin by the addition of
certain amount of glycerol. Glycerol in these layers serves
as the developing component and also as the plastic component.
It keeps some amount of water molecules that are carrying
out the developing process due to their hydrogen ties. And
besides glycerol provides enough plasticity to the material
to make layers of millimeter thickness.
However several problems
arise at synthesis of superthick (more than 0,5 mm) layers.
We have to find the reasonable limit of the emulsion drying
time. Very long drying (more than three day) is preferable,
it leads to better gelatination and "ripening" of colloidal
medium. However, long term drying is not suitable from the
technological point of view. Our attempts to solve this
by conventional methods such as ventilation by hot air or
alcohol dehydration were not successful, they result in
originating of a strong gradient of optical refraction in
the layer depth. The processing of colloidal layers by microwave
radiation resulted in their flaking from the substrate and
to unmonitored change of the conformation gelatin macromolecules
- GELATIN STRUCTURE AND HOLOGRAM RECORDING
It is wellknown that highly
efficient holograms are recorded by means of creation of
refractive index modulation inside the materials. Usually
it is achieved by specially organized phase transition in
a substance. There are two main requirements to any recording
media: high photosensitivity and high spatial resolution.
One can control the material photosensitivity by preparing
the recording media where phase transitions are characterized
by an appropriate inversion degree of molecules (in the
first-order approximation, such molecules can be considered
as two-level quantum systems). Simultaneously, a threshold
character of the relevant photochemical reaction should
be ensured which is necessary for the pre-exposure storage
of the photomaterial. To achieve a high spatial resolution
of the recording medium, the condition of spatial localization
of the primary photochemical reaction should be met. If
the requirements specified above are met, optical radiation
being recorded plays the role of a "trigger" which actuates
the mechanism of changes in the phase state of the recording
medium at micro- and macrolevels.
In this paper, we invoke
the biophysical data5,6 to consider the mechanism
of the recording of optical data in SD DG layers as a set
of hierarchical interrelated phase transitions at various
levels of gelatin structure, including the intramolecular,
molecular, and macromolecular levels.
- Cascade of phase transitions
The main distinguishing
feature of dichromated gelatin as a polymer is associated
with the natural origin of gelatin, which is produced as
a result of the denaturation of collagen which macromolecules
have a structure of triple hyperhelix. Therefore, the native
state of gelatin macromolecule also has a biohelix-like
structure, which makes it possible to prepare DG layers
with an initially ordered (helical) molecular structure.
In the process of development, the phase transition from
the ordered (quasi-crystalline) state to disordered (quasi-amorphous)
state occurs under the action of light and water at all
the levels of structure organization of gelatin layer, which
leads to large photoinduced changes in optical characteristics
of a DG layer.
In other words, the synthesis
of dichromated gelatin layer before the recording process
is accompanied by the appearance of stresses applied to
"springs" representing the helical fragments of gelatin
macromolecules. Then, the action of light plays the role
of' a trigger, which sets the springs free. The energy stored
in these springs induces a cascade of phase changes in the
state of gelatin layer. These phase changes, in their turn,
give rise to variations in the macroscopic characteristics
of the recording medium. Note that, apparently, artificial
polymers, for example, polyvinyl alcohol, do not exhibit
such clearly pronounced native structure and therefore,
cannot compete, at least in holography, with dichromated
2.2. The structural organization
of the dichromated gelatin systems
Gelatin is a highly asymmetric
linear polypeptide polymer of protein nature. Macromolecule
of gelatin consists, on the average, of 500-600 amino acid
radicals. The molecular mass of gelatin macromolecules ranges
from 40,000 to 1000,000. Similar to all the proteins, gelatin
is a high-molecular compound characterized by its ability
to form various supermolecular structures5.
There are several basic
levels in the structure organization of proteins. The primary
structure is determined by the composition and the sequence
of amino acid radicals in a macromolecule of the polypeptide
chain. The secondary structure is determined by the configuration
and relative spatial arrangement of protein macromolecules,
which have the shape of helixes, folds, or globules. The
hypersecondary structure is determined by the aggregation
of secondary -structure elements into a single macromolecule,
which is manifested, in particular, as the phase state of
the globular nucleus. The domain structure is determined
by external parameters (the thickness of layer, the degree
of adhesion of the layer to a substrate, etc.) responsible
for the formation of separate and relatively weakly bonded
globular segments of macromolecule. The ternary structure
is determined by the conformation of protein helixes in
the form of fibrils or globules. The quaternary structure
is determined by die aggregation of protein fibrils and
globules and similar to the domain structure, it is highly
sensitive to external conditions (the conformation of a
collagen-like triple hyperhelix is the limiting case of
the quaternary structure for gelatin macromolecules).
2.3. The control of the
dichromated gelatin structure
Macroscopically, in the
preparation of a DG layer, gelatin as a polymer system passes
through a sequence of different aggregate states. The initial
state in this sequence is the dilute solution of gelatin
in water, where macromolecules reside in the state of Gaussian
coil or globule. In the process of DG layer preparation,
when the solution is poured onto a substrate, the interaction
of chain macromolecules gives rise to the formation of a
gel. This gel may exhibit properties of a liquid crystal
in the case of rigid-chain molecules or properties of a
strong solution with equal volume portions of the solvent
and polymer5. As the solvent (water) is evaporated
out of the emulsion poured onto the substrate, macromolecules
return to their native state, forming a collagen-like triple
helical structure. The deformation degree of this structure
is determined by the conditions of film formation. In the
case of gelling, the forces acting from the side of the
substrate and conditions of drying lead to the unfolding
of macromolecules into linear structures with simultaneous
twisting of segments of these structures into helixes. Such
a renaturation in the course of gelling has a statistical
and local character and occurs through the linking of segments,
giving rise, in its turn, to a clearly pronounced short-range
order in the gelatin film. The long-range order, which is
characteristic of collagen, is not observed in this case5.
Depending on the thickness of emulsion applied to a substrate,
films obtained by molding through gelling on solid substrates
exhibit planar orientation of their structure elements.
Obviously, the state of structure elements in the film depends
on the state of these elements in the emulsion solution.
Specifically, gelatin in a film obtained from solution at
a temperature higher than 35 °C has a conformation of a
Gaussian coil4-6 and does not display any features
of ordering and planar orientation of structure elements.
We analyzed the possibility
to control the emulsion adhesion to the substrate. The experiments
showed that it can be controlled with the use of a gelatin
sublayer with a thickness of 0.5-1 micron with different
hardness. Thus, in synthesizing and pouring of SD DG layers,
one can control the structure of gelatin layer.
It is important to take
into account the influence of substrate properties in variety
of applications, particularly, for interfiber connectors,
where the substrate role is played by bridged fibers.
2.4. The water structure
and it’s control by means of the IR laser annealing
The SD DG system contains
water in quantity sufficient for the hologram development.
The water in such system is held with the help of glycerol
and consequently the holographic properties depend on glycerol
concentration in a system7. This system contains
great many of water, and, therefore it is possible to consider
it almost as aqueous solution, to which the quasicrystal
liquid structure role is essential8. The collective
character of the water molecule motion is confirmed by the
molecular dynamics results9. Under the IR radiation
action the water structure energy increases and weakens
hydrogen bindings, the oscillation frequencies increase,
the concentration of defects which are generating at molecules
translations from the site of a crystal lattice in an adjacent
potential well is augmented. The water molecules form a
The interaction of water with solutes is described
thus through aquacomplex, the aggregation of the
material molecule with several water molecules. The water
molecules, which are included in the complex structure,
interact with adjacent water molecules. And it is possible
to consider aquacomplexes as elements of hydrogen bindings
net in a water5,9.
It is possible to influence
on SD DG structure in all volume by means of the laser IR
radiation. IR electromagnetic radiation causes molecular
rearrangement in the material. This effect consists in the
loss of coordinate oscillations stability: the part of molecules
does not return in the position with initial coordinates.
It is impossible to estimate the optimal radiation wavelength
for such annealing because of composite emulsion structure.
Therefore, basing on the known data on absorption spectrum
of water we chose 1 micron wavelength radiation. Besides
it has another advantage that the IR near range radiation
passes through the glass substrate and consequently in thicker
layer there are no undesirable effects of an interference
of IR radiation.
Thus the energy pumping
should be by very selective in order to change only tertiary
and quaternary gelatin structures, causing their “elongation”.
Therefore it is expedient to use laser radiation with small
quantum energy and which is not absorbed by gelatin and
single water molecules. The IR electromagnetic field action
on an aqueous solution of gelatin macromolecules results
in disturbance of equilibrium system condition. After the
irradiation discontinuance there is a dielectric relaxation.
Every relaxation, including dielectric, is the process of
the system returning to equilibrium or system transition
to the new status. After action of IR radiation, the water
molecules start to be rebuilt according to Le Chatelier-Braun
principle. Being bound in a unified hydrogen binding net,
the molecules can be rebuilt only with the help of preferable
movements. It means, that the time of this rearrangement
(Debye time) should be much more, than time of one transition.
The molecular movement in a three-dimensional
spatial net of hydrogen bindings descends more often, than
gaps of its connections to adjacent molecules. Thus the
frequency of absorption 1000 cm-1 (wavelength
is equal to 1 micron) can be interpreted as the sum of two
frequencies: one of which is 200 cm-1
, the frequency of personal rotary oscillations, and ~ 800
cm-1, vibration frequency of water molecules
in aquacomplex or collective module.
3. Experimental technique
3.1. Synthesis of selfdeveloping
dichromated gelatin layers.
The procedure of the sample
preparation is the following11.
12: the gelatin is solved during
1hour in water (1 g of gelatin on 5 ml of distilled water)
at temperature 50oC, then glycerol (0.8 ml in
5 ml of water) was added, and the solution was maintained
at temperature 40oC within 2 hours.
Then we added ammonium bichromat of
necessary concentration (20 % of the weight of dry gelatin
or 0.2 g in 5 ml of a water) in the obtained homogeneous
solution. Then ammonia for achievement of pH=9.0 was added,
then the stain methylene blue (MB) as solution of 10 mg
of the stain in 100 ml of water was added. The prepared
emulsion solution filled up the transparent pan with lateral
walls of the necessary altitude (from 0.5 up to 3mm), and
it was covered by special glass. Gelatination procedure
was carried out at the temperature of 25oC within
24 hours. In outcome the layer of the given depth was received.
KCl doped emulsions were
tested to investigate the possibility of chemical control
of the SD DG structure. KCl was added during the emulsion
preparation in the amount of several percent.
We used the gelatin sublayers
to investigate the influence of adhesion from substrate.
The substrate was covered with 1% gelatin water solution.
After drying the sublayer of about 1 micron thickness was
obtained. Then it was hardened in the Orwo-400 solution
during the various time intervals.
3.2. Technique of the laser
After holding the emulsion
layer during a day the quasicrystal selfdeveloping dichromated
gelatin structure was formed. Then the plate was subjected
to IR laser radiation with 1 micron wavelength and with
known values of power in the pulse. In order to eliminate
the influence of all transition processes, the hologram
was recorded 10-12 hours later.
3.3. The holographic characteristics
We illuminated samples with
interference pattern constructed by two plane light waves
from the HeNe laser. The spatial frequency of the recorded
diffraction grating was hundreds lines/mm. The energy of
two beams was 6mW. Diffraction efficiency was measured as
the relation of intensity values of diffracted and input
- EXPERIMENTAL Results
4.1. The IR laser annealing
The variable parameters for
the annealing IR pulses are the pulse energy, duration, and
shape. It should be mentioned that maximal diffraction efficiency
obtained in SD DG layers is about 70%. However when we analyzed
the effect of different factors on the diffraction efficiency
we were working in the range of 5-10% efficiency in order
to eliminate the influence of non-linear properties of medium.
The experimental results for
IR laser annealing are shown in Fig. 1 for holographic grating
of 600 lines/mm recorded in selfdeveloping dichromated gelatin
layer of 1mm thickness by radiation of HeNe laser. We analyzed
the dependence of diffraction efficiency on the amount of
IR laser annealing pulses and
full pulses energy (Ean)
and the best result occurred for six annealing pulses. Figure
1 shows the obtained diffraction efficiency of holograms increasing
– m=[(DEan – DE0 ]/DE0 )100%,
where DEan is the diffraction efficiency of the
IR laser annealing SD DG test hologram and DE0 is
the same for SD DG test holograms without laser annealing.
One can see from this figure that there is an area of considerable
diffraction efficiency increasing. The best result received
at an annealing by six pulses with duration 3 msec at full
impulses energy equal to 18-30 J. However, the exact energy
level depends on the material thickness, on the concentration
of gelatin in emulsion, and on the type and series of gelatin.
It should be pointed out that all obtained values of diffraction
efficiency for the annealed materials are higher (on average
2-3 times) than they are for the plates not subjected to the
annealing. It is visible
that the diffraction efficiency curve (Figure 1) has the brightly
expressed peak (for annealing energy more than 20 J) at small
pulse duration. Further increase
of pulse energy leads to considerable
diffraction efficiency increasing.
1. Diffraction efficiency of holograms recorded in selfdeveloping
dichromated gelatin layers with different energy of IR laser
annealing pulses with 3 msec duration of six annealing pulses.
The pulse irradiation regime
for laser annealing is preferable, because it allows to estimate
precise energy of effect on the layer structure. The pulse
irradiation is to some extent like sharp system “shaking”,
and the necessary macromolecules packing is reached by the
following relaxation processes in the system.
In our opinion IR electromagnetic
field causes impulsive disturbance of water at the level of
collective modules while single water molecules and gelatin
squirrels do not absorb radiation of 1 micron wavelength9.
Water quasicrystal net rearrangement conducts the change of
electromagnetic field affecting gelatin macromolecules. These
macromolecules move under of this field action and change
therefore tertiary and quaternary conformation status to achieve
an energy minimum in new quasicrystal net. Thus spiral segments
do not change, and this means that the SD DG system photosensitive
properties do not change too. (Our conception consists in
the material photosensitivity based on untwisting of spiral
In general our experiments
showed that the variety of pulse parameters affect on the
resulting diffraction efficiency value. The main parameter
is, of course, the amount of energy absorbed by the material.
However, the pulse duration and its shape (this means the
sharpness of fronts) as well as the moment of the annealing
relatively the emulsion maturing time are also of importance.
It is obvious from the experiments that there exist several
solutions to reach the maximal diffraction efficiency by combining
the optimal pulse energy, shape, duration and amount of pulses.
However that concerns the moment of annealing, multiple experiments
showed that the optimum is around 24 hours after emulsion
pouring on substrate
4.2. The influence of adhesion
to substrate on the selfdeveloping dichromated gelatin structure
Experiments showed that SD
DG layer separation from glass substrate often occurs causing
the diffraction efficiency reduction. This effect is called
photoinduced collapse of the emulsion at recording. For elimination
of this phenomenon we proposed to utilize an additional gelatin
sublayer with the hardness higher than it is for the photosensitive
emulsion layer. We investigated what optimal value of the
sublayer hardness. The dependence of holographic properties
of the SD DG layer on the substrate hardness is shown in Fig.
2. One can see from the Figure 2 that the maximal diffraction
efficiency was observed for the sublayer subjected to hardening
in ORWO-400 during 4.5 munites (corresponds to H=4.5).
Fig. 2. Influence of
Н of sublayer on the SD DG exposition characteristics.
4.3. SD DG emulsions containing
We developed the new version
of selfdeveloping dichromated gelatin, in which 50 % of glycerol
in emulsion was replaced by natural polymer (polysaccharides
similar to stratch) obtained from plants, its molecular mass
is about 20,000. The experiments showed considerable reduction
of the layer gelatination time (Figure3). The formation of
quasi-crystal structure occurs faster in this case compared
with initial version of SD DG. Obtained holograms have improved
Fig. 3. Influence of
the gelatination time on the SD DG exposition characteristics
for 50% polymer replacement of glycerol.
4.4. The SD DG emulsion doped
We investigated the influence
of salt addition to the composition. We used KCl, because
it is known that K ions affect the aquacomplex structure.
The experimental results are shown in Figures 4, (a) and (b).
It is found that KCl essentially improves photosensitivity
of the emulsion. The shape of the diffraction efficiency contours
changes under the influence of KCl, however the character
of this change depends on gelatination temperature schedule.
Fig. 4. The hologram
diffraction efficiency for SD DG doped by KCl in the gelatination
time dependence at temperature 24о
С (a) and at temperature 00
We proposed different technologies
to improve the holographic characteristics (diffraction efficiency,
storage time) of selfdeveloping dichromated gelatin material.
It was shown that IR laser
annealing (series of pulses) before hologram exposure results
in increase of diffraction efficiency.
In order to overcome the
problem of the photosensitive layer separation from the substrate
we proposed to use strongly hardened gelatin sublayers. It
was shown experimentally the improvement of diffraction efficiency
in this case.
It was shown that the storage
time of hologram can be improved by replacement of part of
glycerol by natural polymer with high molecular weight.
Addition of salt to the emulsion
leads to the increase of the material photosensitivity and
to the diffraction efficiency improvement due the influence
of ions on water molecules inside the material.
It should be pointed out that
it would be reasonable to carry out further experiments with
the usage of two lasers, blue and red ones, simultaneously,
in order to substrate the influence of sensitizer.
Our thanks for help in the
experimental work to A.L.Antipov, S.A.Medvedeva, V.Yu.Molotcilo,
A.A.Petrov, S.N. Pidgurskii.
The work was supported by
Russian Basic Researches Fond project No 01-02-17141 and European
Office of Aerospace Development (USA) project No 2057P.
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