Holographic recording in acrylamide photopolymers:thickness limitations
Mohammad Sultan Mahmud,1,2,* Izabela Naydenova,1,2 Nitesh Pandey,1,2 TzwetankaBabeva,1 Raghavendra Jallapuram,1 Suzanne Martin,1 and Vincent Toal1,2
1Centre for Industrial and Engineering Optics, Dublin Institute of Technology, Dublin 8, Ireland2School of Physics, Faculty of Science, Dublin Institute of Technology, Dublin 8, Ireland
Photopolymers [1–10] are considered one of the mostversatile holographic recording media because oftheir high sensitivity, wide dynamic range, and rela-tively low cost. Many photopolymers have the advan-tage that they are self-developing and need no wetprocessing or thermal treatment. However their ap-plications are restricted by the need for thick layersand the material shrinkage that occurs during andafter recording [2,11–13]. In data storage applica-tions this can change the fringe spacing and there-fore the reconstruction angle, resulting in no lightbeing diffracted at the expected reconstruction angleand the stored data page cannot be recovered. Very
thick photopolymer layers (500 μm or more) are es-sential for high capacity holographic data storagemedia [14–18]. We focus our efforts on developingthick acrylamide-based photopolymer layers withsuitable holographic recording properties [18].Other research groups have reported studies ofholographic recording in polyvinyl alcohol (PVA)acrylamide-based photopolymer layers with differentcompositions and thickness of around 1mm [19,20].Recording in thick photopolymer layers has twodrawbacks, namely, increased layer thickness leadsto an increase in the difference between the effectiveoptical thickness and the physical thickness in thematerial that is due to the absorption of light [21]and increased losses that are due to scattering andabsorption. We report characterization of thicklayers of acrylamide-based photopolymers developed
at the Centre for Industrial and Engineering Optics[5,6], by measuring holographic parameters such asdiffraction efficiency (DE) and angular selectivityand by studying the diffraction patterns obtained.The influence of layer absorbance on the maximumthickness with acceptable optical losses in the firstdiffraction order is discussed.
2. Theory
The acrylamide-based photopolymer is a self-developing dry layer that consists of monomers, anelectron donor or initiator, a photosensitizer, and apolymer binder that acts as a matrix in which theother components are suspended. Holographic re-cording in this material is based on photopolymeriza-tion reactions in the areas illuminated by laser lightof appropriate wavelength. The dye molecules absorbphotons and enter into excited states in which theyreact with the electron donor molecules to generatefree radicals which then initiate the polymerizationprocess. When a spatially modulated light field isused, polymerization uses monomers producing amonomer concentration gradient and monomermolecules diffuse from the unexposed regions tothe exposed regions where they are polymerized asdescribed. These polymerization and diffusion pro-cesses lead to a spatially modulated refractive-indexchange in the material and a diffraction grating isrecorded. Recording can continue until no monomeris left in the unexposed region or it can be intermit-tent so that several recordings can be made with dif-ferent spatial frequencies of illuminating lightpatterns. There are different theoretical models thatexplain the formation of holographic gratings inphotopolymer material [22,23]. Diffusion studies ofour photopolymer [24,25] show that diffusion isfaster than in other photopolymer systems [26,27].Depending on the thickness of photopolymer
layers, noise gratings can be observed that are dueto scattering from inhomogeneities [2,28–30] in therecording material. The scattered field can be treatedas the superposition of a large number of planewaves, each of which interferes with the recordingbeams, producing weak planar parasitic gratings.When illuminated with a reconstruction beam, theweak gratings diffract some of the incident light.Under exactly the same readout conditions as for re-cording, parasitic gratings that satisfy the Bragg con-dition will give rise to scatter. Any deviation from therecording conditions, either in wavelength, angularrotation [30–32], or polarization state [33], increasesthe number of parasitic gratings for which the Braggcondition is violated, decreasing the scattered inten-sity. Of course, the reconstructed data beam is alsoadversely affected. As the thickness of the layer in-creases, the amount of optical inhomogeneities alsoincreases, implying a critical thickness above whichthe diffracted beam completely disappears because ofthe scattering effect. Different compositions of photo-polymer layers are expected to have different criticalthicknesses. The DE is given by
DE ¼ I1I0
× 100%; ð1Þ
where, I1 is the first-order diffracted beam and I0 isthe incident probe beam intensity.
3. Experiment
A. Preparation of Thick Layers
The components of the photopolymer are acrylamidemonomer (0:6 g) and N,N’-methylene-bisacrylamidecross-linking monomer (0:2 g), triethanolamine in-itiator (2ml), 10ml PVA binder (20 wt./vol. % waterstock solution) and Erythrosin B sensitizing dye(0.11 wt. % stock solution) [5,6]. The amount ofdye added to the layer was adjusted to maintain con-stant absorbance independent of the thickness. The20% concentration of PVA was used to enable fasterdrying of the photopolymer layer although the higherviscosity of the stock solution required more time toprepare. To obtain thick layers the photopolymer so-lution was deposited in a Petri dish. Once dry thelayer was removed and placed on a glass slide foruse in holographic recording. To measure thicknessusing a white light surface profilometer (MicroXAM S/N 8038), the layer was cut back to the glasssurface and part of it was peeled off.
B. Experimental Setup
A two-beam holographic optical setup (Fig. 1) with anangle of 15° between beams was used to record un-slanted transmission gratings using a 532 nm Nd :YVO4 laser. The gratings were recorded in layersranging in thickness from 250 to 1000 μm at a record-ing intensity of 5mW=cm2 and a spatial frequencyof 1000 lines=mm. The recording intensity wascontrolled by a variable neutral density filter. The ab-sorption of the photopolymer at 633nm is negligibleso a 633 nmHe–Ne laser was used as the probe beamat the Bragg angle to monitor the diffracted andtransmitted intensities in real time. The probe andrecording beams were TE polarized. To measurethe diffracted intensity dependence on the incidentangle of the probe beam, we placed the grating ona precision rotational stage (Newport, ESP 300).The intensity was read by an optical powermeter(Newport 1830-C) and the data were transferred toa computer. The measurement accuracy of the Bragg
Fig. 1. Experimental setup: S, shutter; BE, beam expander; BS,beam splitter; M, mirror; D, optical powermeter.
angle was �0:001 deg; exposure was 100 s long. Theangular dependencies were measured 60 s after therecording was finished. The Bragg condition was in-itially ensured by adjusting the angle of incidence ofthe probe beam to maximize the photodetector out-put using a thin sample layer in which the gratingwas not overmodulated.
4. Results and Discussion
A. Ultraviolet–Visible Spectra for the Same OpticalAbsorption but Different Layer Thickness
The absorption spectra were measured by use of anultraviolet–visible–near-infrared absorption spectro-meter (PerkinElmer Lambda 900). Figure 2 showsthe absorption spectra at absorbances of 0.42, 0.17,and 0.10 at 532nm for thicknesses from 250 to1000 μm, �10 μm. We determined that absorbanceA for layers with the same thicknesses in the rangefrom 250 to 1000 μm varied by only 5%. The slightlyhigher absorbance of the thicker samples is due tothe increased scattering in these samples. Suchsmall variation indicates a generally low level ofscattering in unexposed photopolymer.
B. Diffraction Efficiency Measurements
Figure 3 shows the DE versus exposure time forthree different absorbances at different thicknesses.The DE increases with the exposure and reachesmaximum for all thicknesses. As the thickness
Fig. 2. Ultraviolet–visible spectra for absorbance A532nm ¼ 0:42at different thicknesses: ▪, 250; ∘. 350; ▴, 450; ∗, 550; +, 800; ♦,1000 μm. Absorbance of 0.17 and 0.10 shown here for comparison.
Fig. 3. DE versus exposure time for A532nm of (a) 0.10, (b) 0.17, (c), (d) 0.42 for different layer thicknesses: ▪, 250; ∘, 350; ▴, 450; ∗, 550; +,800; ♦, 1000 μm at 1000 lines=mm and intensity of 5mW=cm2.
increases the maximum DE decreases mainly due tothe noise grating caused by scattering. Further in-crease of the exposure leads to an oscillatory evolu-tion of DE, typical for overmodulated volumegratings [34] and none of the gratings reach 100%DE that is due to scattering. The final value of theDE is obtained at exposure times longer than 55 s.In the case of 250 μm at the A ¼ 0:11 layer, the DEreaches a maximum followed by a decrease and thenan increase. This might be due to a faster polymer-ization process that leads to the diffusion process.Figure 4 shows the dependence of the maximumachieved DE on the absorbance of layers with differ-ent thicknesses. The first maximum was chosen asrepresentative of the highest DE that could be ob-tained. For 250 μm at A ¼ 0:11, the final DE valuewas used. We determined that, in 250 μm thicklayers, A532 > 0:17 is required to achieve higher than80% DE. Optical losses are relatively low at thisthickness, <5%.At 350 μm thickness, the first maximum of the dif-
fracted beam is practically independent of the absor-bance in the range from 0.1 to 0.4, which allows forflexible adjustment of the layer sensitivity by vary-ing the dye concentration without significant reduc-tion in the DE because of holographic scattering.Significant reduction of the DE is found in 450 μmthick layers with absorbance above 0.17. At this par-ticular thickness an absorbance lower than 0.17would only lead to a reduction in sensitivity of thelayers. The first-order DE decreases almost linearlywith increasing absorbance. Using this dependenceone can determine the appropriate layer propertiesthat depend on the tolerance to losses in a particularapplication. For example, for 800 μm thick layers, theoptimum absorbance is between 0.17 and 0.4 but theoptical losses are very high as shown in Fig. 4.Figure 5 shows the maximum achieved DE versus
the thickness of the layers for three different valuesof layer absorbance. It is seen that, for a smallerthickness, the maximum DE was reached at thehighest absorbance value. At low absorbance thepolymerization might not be sufficiently effectivebecause of the low absorption. As the thicknessincreases, the DE decreases with increased
absorbance. The probable reason is that, at higherabsorbances, the influence of the noise grating thatis due to scattering is more pronounced. At a layerthickness of 1000 μm the DE is significantlyreduced for all three absorbances because of the in-creased scattering. The critical thickness is definedas the maximum thickness of a layer with specifiedabsorbance whose scattering loss is less then a spe-cified acceptable level. For example, if we were to ac-cept losses of 50% , from Fig. 5 we could determinethat the critical thickness for A532nm ¼ 0:10 is550 μm, for A532nm ¼ 0:17 it is 500 μm, and 450 μmfor A532nm ¼ 0:42.
Figure 6 shows the angular selectivity curves forgratings for three absorbances. It is seen that themaxima of the central and sidelobes decrease withan increase in layer thickness. These sidelobes arisefrom the conventional Bragg diffraction theory [34]and completely disappear at thicknesses greaterthan 550, 800, and 1000 μm for A532nm ¼ 0:42, 0.17,and 0.10, respectively. Results from other photopoly-mer compositions suggest a thickness above whichsidelobes vanish because of noise gratings [20,28–30]. This limits the performance of any thick photo-polymer layer. Another possible reason for theattenuation of the sidelobes in the Bragg selectivitycurves is the nonuniformity of the refractive-indexprofile through the depth of the layer [21]. At thebeginning of the recording, the light is attenuatedin the material because of the absorption by thesensitizing dye that leads to attenuation of therefractive-index modulation with depth. As a resultthe sidelobes of the angular selectivity curvedisappear [21].
C. Scatter Measurement
Scatter was characterized by using an integratingsphere (PerkinElmer Lambda 900) over a wide rangeof wavelengths (400–700nm). Figure 7 shows thescatter percentage at 532nm as a function of thick-ness for three absorbance values. It is seen that thescattering losses increase both with increased layerthickness and dye concentration. Scattering was stu-died using diffraction patterns (Fig. 8) that are due toa single probe beam as a function of thickness and
Fig. 4. Maximum of the first-order DE versus A532 for differentlayer thicknesses: ▪, 250; ∘, 350; ▴, 450; ∗, 550; +, 800; ♦, 1000 μm.
Fig. 5. Maximum of the first-order DE versus layer thickness forabsorbances: ▪, 0.10; ∘, 0.17; ▴, 0.42.
absorbance. A screen was placed at a distance of ap-proximately 10 cm from the samples to obtain theseimages. The spot on the right-hand side of each im-age corresponds to the transmitted beam and thespot on the left-hand side is the first-order diffractedbeam. The transmitted beam forms a ring patternthat increases in intensity with thickness and absor-bance, indicating the presence of a noise grating[28–30,35].Forshaw [35,36] explained the rings in terms of
Ewald sphere construction. The intersection of the
Ewald sphere with the hemisphere of the primaryimage and its conjugate, (i.e., the region in reciprocalspace spanned by the recorded grating vectors), cre-ates two cones [35–37] whose projections on a screenproduce rings whose brightness increases as thenoise grating increases in strength. To estimatethe strength of the recorded noise gratings we mea-sured the intensity of the transmitted light in thezero order and the light diffracted in the first orderand observed the change in their sum during thetime of recording. The decrease of the sum of these
Fig. 6. Angular selectivity (Bragg) curves for A532nm of (a) (b) 0.10, (c) (d) 0.17, (e) (f) 0.42 for different thicknesses: ▪, 250; ∘, 350;▴, 450; ∗,550; +, 800; ♦, 1000 μm of photopolymer layers at an exposure intensity of 5mW=cm2.
two intensities was attributed to the appearance ofnoise gratings. It was estimated that the contribu-tion of the noise gratings varies from 1% for250 μm thick layers with absorbance of 0.11–15%for 800 μm layers with absorbance of 0.42.As explained above, the strength of the noise grat-
ing in thick photopolymer layers is decreased by low-ering the absorbance of the layer. Our studies of1000 μm thick samples showed that optimal absor-bance at 532nm is around 0.1. Further decrease ofabsorbance requires long exposure, because of re-duced concentration of photosensitive dye. If thenumber of dye molecules is low, there is reduced pro-duction of free radicals and hence of polymer. Scatter-ing from inhomogeneities is the main cause of thenoise gratings and a possible way to increase DEfor thick layers is to reduce the number of these in-homogeneities by improving the photopolymer com-position. The main target would be the PVA matrixwhose scattering properties could be altered by anappropriate choice of the molecular weight and thepercentage of hydrolysis.
5. Conclusion
We investigated the holographic characteristics of ac-rylamide-based photopolymer layers ranging inthickness from 250 μm to 1mm with the same absor-bance. Unslanted diffraction gratings were recordedand real-time diffraction efficiency growth curvesand angular selectivity profiles were obtained. Thedependence of the DE on the layer thickness and ab-sorbance was determined to find the optimum absor-bance for a given layer thickness. By measuring thediffraction efficiency growth and studying the dif-fraction pattern, the influence of scattering on thediffraction efficiency of thick volume holographicgratings was analyzed. Critical thicknesses for acry-lamide-based photopolymer layers were determinedfor different absorbances and were found to decreasewith increased absorbance. Scattering effects are animportant reason for the thickness limitations thatshould be taken into account when different applica-tions are envisaged. Based on our studies to date webelieve that the layer to be used for phase-coded re-ference beam recording with an absorbance of 0.1 at532nm can have a thickness of 450 μm. The potentialuse of thicker layers characterized by low scatteringlosses is part of our continuing research.
The Dublin Institute of Technology (DIT) providedfinancial support for this project. The authors thankE. Leite and Q. Cheng, DIT, for their assistancein preparing the thick samples and the insightfuldiscussions.
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