Volume Hologram Formation in Photopolymer Materials

W. S. Colburn and K. A. Haines

Holograms have been constructed in photopolymer materials which give bright, low-noise images. Theseholograms are of the volume type and have no surface variations in all but a few special cases. Theyare constructed in virtually real time and in situ, requiring no processing. Materials sensitive to bothuv and blue-green radiation have been used. In this paper, the mechanism of hologram formation isexamined. Experimental results on sensitivity, spatial frequency response, particle scattering noise,and nonlinearities are discussed. A few holographic applications of the material are presented.

Introduction

An ideal material for many holographic applicationsis capable of recording the intensity variations in theincident radiation as index of refraction variationsthroughout its volume. Only this type of hologramcan achieve diffraction efficiencies of up to 100%.'The material has great potential for holography if inaddition it is capable of forming holograms in essen-tially real time and in situ. Some photopolymer ma-terials have these characteristics. Furthermore, theyare practically grainless, contributing to an image freefrom particle scattering noise. They display poor lowspatial frequency response, a factor that can greatlyreduce intrinsic and material nonlinearities that mightotherwise be evident when the hologram is relativelythin. Characteristics that limit the widespread useof present photopolymer materials are their irreversi-bility and their low sensitivity, which is usually lessthan that of the slowest silver emulsions (649F).

Previous work on volume holograms has been carriedout by Friesem and Walker,2 although these hologramswere of the absorption type. Volume phase hologramsthat require processing and are not in real time havebeen recorded in dichromated gelatin by Shankoff.'Holograms have been recorded on photoresist, 4 -6

although these have been thin-phase holograms thatdiffract light due to surface modulation. Close et al.7and Jenney8 have constructed good holograms on self-developing photopolymers in which the holographicinformation is recorded mainly as a thickness variation.These holograms are of the thin-phase rather than

Both authors were with Holotron Corporation, Wilmington,Delaware 19898, when this work was done. K. A. Haines isnow with the Electrical Engineering Department, University ofCanterbury, Christ Church, New Zealand.

Received 5 January 1971.

volume type. Furthermore, the intensity of thediffracted image of their holograms is quite dependenton the rate of polymerization during hologram con-struction. This dependence was not particularlyevident in the construction of our volume holograms.

The materials that we used were prepared by E. I.du Pont de Nemours and Co., Inc. As described byWopschall,9 they consisted of three parts: a photo-polymerizable monomer, an initiator system whichinitiates polymerization upon exposure to light, and apolymeric binder. The initiator is sensitive primarilyto uv radiation near 360 nm but can be sensitized to theblue and green regions of the visible spectrum. Thebinder forms a matrix to hold the liquid monomer.Typically the mixture is coated on a substrate of glassor film.

The mechanism of hologram formation in photo-polymer is a complicated process involving polymeriza-tion and monomer diffusion. In the next section, wepresent a hypothesis of this mechanism and a descrip-tion of experiments that we performed to validate thishypothesis. We also measured several holographiccharacteristics of the Dupont photopolymer, includingthe spatial bandwidth that the material is capable ofrecording, scattering noise, and material nonlinearities.

Mechanism of Hologram Formation

In general, hologram formation in photopolymers isa three-step process. First, a normal exposure is madeto the interference pattern to be recorded; this initialexposure polymerizes part of the monomer, with theamount of polymerization being a function of theintensity of the illumination. Monomer concentrationgradients, caused by variations in the amount ofpolymerization, then give rise to the diffusion of therelatively small monomer molecules from regions ofhigher concentration to regions of lower concentration.With the completion of the diffusion step, the photo-

1636 APPLIED OPTICS / Vol. 10, No. 7 / July 1971

polymer is exposed to light of uniform intensity untilthe remaining monomer is polymerized.

Certain changes occur in the material upon poly-merization. When it is polymerized in the bulk, thematerial shrinks and its refractive index increases. Theinitial exposure of a hologram might then be expectedto produce surface relief due to variations in shrinkagecorresponding to variations in polymerization.Furthermore, the mass transport due to diffusion mightalso be expected to produce surface relief in the finalhologram. At typical spatial frequencies, however, nosurface relief is observed, indicating that instead, theseeffects must create density and thus refractive indexvariations that cause diffraction of light by the holo-gram.

HypothesisLet us consider the steps of exposure in greater detail.

After the initial exposure, areas corresponding tohigher-intensity illumination (regions I in Fig. 1) aremore greatly polymerized than areas corresponding tolower-intensity illumination. The absence of surfacerelief suggests that shrinkage in those regions of greaterpolymerization is less than would occur in bulk form.We hypothesize that this results in a decrease in refrac-tive index rather than the increase that would occurfor polymerization of the material in bulk form.Shrinkage in regions of less polymerization, on theother hand, is as much as or more than would occur inbulk form, resulting in an increase in refractive index.Thus in Fig. 1 (a), nj < n,.

Gradients in monomer concentration due to differen-tial polymerization by the initial exposure give rise todiffusion of monomer molecules from regions of higherresidual concentration to regions of lower concentration[from regions II to regions I, as shown by the arrows,in Fig. 1(b)]. Diffusion reduces the gradients, prob-ably to some level between uniform material densityand uniform monomer concentration, and causes therefractive index of regions I in Fig. 1 to increase and theindex of regions II to decrease.

When the diffusion process is complete, the remainingmonomer is polymerized, usually by exposure to light

tll * till ** ll3X I II r M I X

(a )

I T. 1E I IC I II(b

nI< nl

no INCREASES

nz DECREASES

I I I

nr > nErr I Or x rt :

(C)

Fig. 1. Index variations during hologram formation.

DIFFERENTIALINDEX an

EXPOSUREINTENSITY

K

U I TIM?

I II II I

1 1 _ 1 t_ti t2 t 3 TIM9

Fig. 2. Differential index during hologram formation.

of uniform intensity which has one or more wave-lengths within the sensitivity range of the material.Because of the mass transport during diffusion, ahigher polymer concentration exists in the regionsthat were originally exposed to the higher-intensityillumination [regions I in Fig. 1(c)]. In the absence ofsurface relief, a higher polymer concentration impliesa higher refractive index.

The photopolymer diffracts light due to a variationin refractive index, which we will define as a = n -

nil according to Fig. 1. According to our hypothesis,this differential index An varies during hologram forma-tion as shown in Fig. 2. The initial exposure, lastingfrom zero to t, creates a negative an. During diffusionof monomer (tl to t), An returns to zero and may reachsome positive level. The final polymerization, be-ginning at t2 and lasting until t3, increases n to itsfinal positive value.

Experimental VerificationOur hypothesis is supported by several experiments.

Figure 3 shows a typical curve of diffracted lightintensity as a function of time during hologram forma-tion. Formed by the interference pattern of two planewaves having an angular separation of about 250, thishologram was made at 364 nm. Since this photo-polymer sample was insensitive to red light, a dif-fracted wave could be observed continuously duringformation of the hologram when it was illuminated withlight from a helium-neon laser. In Fig. 3 the dif-fracted light intensity rises during and immediatelyfollowing the initial exposure, drops to zero and risesslowly with monomer diffusion, and rises again with thefinal exposure to a value higher than the initial peak.For convenience, the final exposure in this as well asmost of our experiments was made with the referencebeam. Note that this curve of diffracted light in-tensity is actually the square of the differential indexcurve shown in Fig. 2, as predicted.

The hypothesized mechanism assumes that nosignificant surface relief exists during hologram con-struction. If it existed, surface relief would causediffraction of a wave reflected from the photopolymersurface; during the entire process of hologram forma-tion virtually no such wave can be observed. Theabsence of surface relief in the final hologram is also

July 1971 / Vol. 10, No. 7 / APPLIED OPTICS 1637

demonstrated by gating the hologram with an index-matching liquid. This gating produces no significanteffect on the holographic image. Furthermore, holo-grams were made on photopolymer which was lami-nated between two sheets of stiff film that preventedformation of relief at high spatial frequencies.

A key assumption in the hypothesis is the initialdecrease rather than increase of the index of refractionof the more heavily exposed areas with respect to theindex of the less heavily exposed areas. The validityof this assumption was examined by measuring therefractive index before and after, and the shrinkageduring, polymerization of bulk photopolymer, andcalculating the index for the hypothetical case in whichit is assumed that the polymerized and shrunken ma-terial is reexpanded to its original volume. Thisapproximates hologram formation in which little or noshrinkage occurs. In the bulk, when exposed to auniform intensity beam (which is not normally the casefor holography, where interference fringes are recordedin the material), the material shrinks upon polymeriza-tion. This shrinkage was measured with a modifiedTwyman-Green interferometer by observation of thetransient fringe pattern during the polymerizationprocess. The final thickness of the material was alsomeasured. The refractive index of both the unexposedmonomer and heavily exposed polymer was calculatedfrom Brewster angle measurements made at 633 nm.

The index of refraction of the polymerized materialn, varies with its molecular density as'"

np = ( + 2aN)(1 - aN), (1)

where a is a constant, dependent on the atomic resonantfrequencies of the material. The molecular densityvaries inversely as the volume or, approximately, thethickness t of the material. The relationship betweenrefractive index and thickness is then

np = (1 + 2/t)/( - lt),where 0 = aNt.

DIFFRACTED LIGHTINTENSITY

0

EXPOSUREINTENSITY

0

(2)

t=OTIME, 5 SEC / D IV

Fig. 3. Diffracted light intensity during formation of typicalhologram.

DIFFRACTELIGHT

I NTENSIT)

EXPOSURIINTFENSET

ED-cut- F ' - I,

'VVLi. IL - I | I ; ! I I

.

t=O TIME, 5 SEC/DIV

Fig. 4. Diffracted light intensity during hologram formation forcontinuous exposure at low intensity.

We can predict the decrease in refractive index ofthe polymer due to a material expansion by differen-tiating Eq. (2), giving

An = At[(np' - 1)(n,2 + 2)]/6nt, (3)

where An is the decrease in refractive index due to themeasured thickness change At. If shrinkage were pre-vented during polymerization, the refractive index ofthe polymer n' would be less by an amount An thanthe index n, which was measured for polymerization inthe bulk, or n,' = n - An. A calculation of n,' for atypical sample having a polymer refractive index n =1.497 0.003, a thickness t = 39.6 nm -I- 2.5 nm, anda shrinkage At = 1.50 nm A 0.03 nm gives n' =

1.475 0.008. The measured monomer refractiveindex before polymerization on the same sample wasnm = 1.490 d 0.003. Although the variations in indexand degree of polymerization during the formation ofa hologram are much smaller than those calculated forthe special case above, this result demonstrates that aninitial decrease in refractive index of the more heavilypolymerized areas during hologram formation is areasonable assumption.

Examples

The transient behavior of the diffraction efficiencyof a photopolyrner hologram is dependent on a numberof factors, including the rate of diffusion of monomerinto the more heavily polymerized areas and the rate ofpolymerization on exposure to radiation. Three exam-ples using different exposure methods demonstrate thisdependence and add credence to the hypothesis ofhologram formation.

In a continuous exposure of low intensity in whichthe rate of diffusion exceeds the rate of polymerization,unpolymerized monomer diffuses into areas of higherpolymer concentration rapidly enough to continuallyincrease the refractive index of those areas. If there isany initial decrease in the index of the more heavilypolymerized areas, it is too small to be observed, dueto the low intensity of the exposure. In such an ex-posure, the diffracted light intensity is a monotonicallyincreasing function, as shown in Fig. 4. (In the exam-ple shown in Fig. 4, there is a significant delay betweenthe start of the exposure and the beginning of diffrac-tion; this delay is the induction period, a characteristicof photopolymer materials.8 )

1638 APPLIED OPTICS / Vol. 10, No. 7 / July 1971

| I I I I I I I I I I I I I I I 1 + 1 1 I

W Hl l l l l l l l l l l l l l l l l l

W j | titSiTi

)W are i

I I l l l I ,1, 1 1 1 1 1 i

l ,, l 1, 1 1 , 1 l l l l l l l l l l l l

._ lE:#

0

In a continuous exposure of high intensity, the rate ofpolymerization exceeds the rate of diffusion. In such acase little monomer diffusion can occur before polymer-ization is complete, resulting in little ultimate variationof refractive index throughout the photopolymer andthus low diffraction efficiency. An example of thistype of exposure is shown by the curve of Fig. 5. Theinitial peak in the figure is due to the variation in theamount of polymerization corresponding to the intensityvariation in the illumination; once polymerization iscomplete in the regions of greatest illumination, thevariation in the amount of polymerization decreases asthe remaining areas also approach complete polymeriza-tion, reducing the differential index and thus diffrac-tion. The final low diffraction efficiency indicates thata small amount of diffusion did take place during theexposure.

At low spatial frequencies, diffraction is due tosurface relief. In this region, there is no significantdiffusion, because the grating period is large comparedto the distance over which diffusion can occur. This isillustrated in the following experiment. A hologramwas formed by the interference of two plane waves at aspatial frequency of 33 lines/mm. The photopolymerwas laminated between two sheets of clear film whichprevented the formation of surface relief at this fre-quency. The results are shown by the curve in Fig. 6.

DIFFRACTED

INTENSITY

t -O TIME, I SEC/DIV

Fig. 5. Diffracted light intensity during hologram formation forcontinuous exposure at high intensity.

DIFFRACTED XLIGH4TINTENSITY

EXPOSURE m IINTENSITY

0E

TIME.,5 SEC/ DIV

Fig. 6. Diffracted light intensity during hologram formationwith low spatial frequencies.

U.Lu

o -a

10 100 1000

SPATIAL FREQUENCY (Iln"/mm)

Fig. 7. Spatial frequency response of photopolymer.

After the initial exposure, light is diffracted due to avariation in refractive index between lightly and moreheavily exposed regions; but with no diffusion, the finalpolymerization eliminates nearly all the refractiveindex variation, causing the diffraction efficiency toapproach zero.

The unrestricted formation of surface relief at lowspatial frequencies is apparently due to differentialshrinkage corresponding to the intensity variation ofthe interference pattern. 8 The initial exposure poly-merizes a sufficient amount of photopolymer to makethe material somewhat rigid, so that further significantshrinkage during the final polymerization is prevented.

Measurements of HolographicRecording Characteristics

SensitivityThe energy density required for the initial exposure

of the Dupont photopolymer at 364 nm is about 1 mJ/cm'. Most of the increase in diffraction efficiencyduring the final polymerization occurs with 1-5 mJ/cm2 additional exposure, although approximately 50mJ/cm' are required for complete polymerization.Energy requirements of the initial exposure can bereduced by exposing the photopolymer to uniformillumination during the induction period. Althoughmost of our experiments have been in the uv, we havealso recorded holograms on photopolymer sensitized to514 nm with about 10 mJ/cm' for the initial exposure.

Spatial Frequency Response

Figure 7 shows the frequency response of the ma-terial. This is a curve of diffraction efficiency plottedas a function of spatial frequency for a series of planewave gratings. The gratings were made at offsetangles between 0.5 and 50 degrees at 364 nm; thereference-to-object beam ratio was 15. The diffractionefficiency was the fraction of the incident light intensitydiffracted into the first order.

July 1971 / Vol. 10, No. 7 / APPLIED OPTICS 1639

NOT_ .. GATED

Below a spatial frequency of 100 line pairs/mm,diffraction is due primarily to surface relief. This isshown by the dashed portion of the curve. When thesurface of these holograms is gated with an index-matching liquid (xylene), most of the diffraction iseliminated, as shown by the solid curve below 100lines/mm. The drop-off in response of the surfacerelief with increasing spatial frequency appears to bedue to surface tension forces, which prevent the photo-polymer surface from changing significantly over dis-tances shorter than 10 um.

Above 100 lines/mm, the frequency response rises toa peak at about 1100 lines/mm; it then falls to half thispeak at about 1900 lines/mm. In this region, diffrac-tion is due to refractive index variations within thematerial; the curve here is essentially the same for thegated and nongated holograms. The response falls offfor frequencies below the peak because the gratingperiod becomes large compared to the distance overwhich diffusion occurs.

Scatter Noise

One of the salient features of photopolymer is that itis grainless. Noise due to scattering by Dupont'sphotopolymer was compared with that due to scatteringby 649F plates. Both materials were exposed by thereference beam only in a normal holographic setup;the photopolymer was completely polymerized, and the649F was exposed and developed to an amplitudetransmittance of 0.5. Scatter noise was measured witha detector at an off-axis position which would normallyhave been occupied by a reconstructed image had anobject beam been used in construction. At a wave-length of 647 nm, scatter noise intensity due to thephotopolymer was measured to be less than one-tenththat of 649F plates.

0 .02 .04 .06

SIGNAL BEAM INTENSITYREFERENCE BEAM INTENSITY

Fig. 8. Nonlinearity of photopolyn

Fig. 9. Real-time interferometry.

NonlinearitiesNonlinearities in phase materials may be divided into

two categories': those that result from a nonlineartransfer of the incident intensity into phase in thematerial and those that result from the fact that aperfect phase recording cannot give rise to a perfectdiffracted amplitude function. The first type is dueto the material; the second is intrinsic to the phaserecording process.

The material nonlinearities in the photopolymerwhich we used for our experiments were considerable.A plot of the square root of diffraction efficiency as afunction of object beam strength is shown in Fig. 8.For comparison purposes, a plot of the intrinsic non-linearities only is shown by the dashed curve. For athick hologram, these intrinsic nonlinearities follow asine curve,' as has been adequately described else-where in the literature. The photopolymer curvefollows the intrinsic nonlinearities curve to 1-2%diffraction efficiency.

It can be theoretically demonstrated that non-linearities for a phase recording material that haspoor low-frequency response, such as the material usedhere, may be considerably reduced over the same typeof material having good low-frequency response. Thisis because the low-frequency signal cross-product termsin the hologram which normally distort the recon-structed image are no longer present.

ApplicationsWe have examined briefly two of the possible appli-

cations which demonstrate some of the characteristicsof this material. The capability of forming hologramsin situ is useful for real-time holographic interferom-etry.2 Figure 9 shows the result of one such experi-

.08 .10 ment. The hologram was constructed with the objectin its unstressed condition; upon completion of holo-gram formation, the object was stressed. The photo-

mer. graph shows fringes created by interference of light

1640 APPLIED OPTICS / Vol. 10, No. 7 / July 1971

reflected from the stressed object with light diffractedby the hologram. Since the experiment was carried outat 364 nm, the image was displayed through a closed-circuit television system using a uv-sensitive vidicon.

Good quality images have been reconstructed intheir original color when thick holograms were re-illuminated with incoherent white light. An exampleof this is shown in Fig. 10, a photograph of the image(a pinch bottle) obtained from a hologram 400 mthick. This hologram was recorded on visible sensitivephotopolymer at 514 nm with a 25° offset angle. Theresolution of this single-color (green) image is indicativeof volume hologram formation. Furthermore, thediffraction efficiency of this hologram is high; when itis illuminated with a monochromatic source, 46% of theincident light is diffracted into the image.

Conclusions

All of our experimental evidence of volume hologramformation in photopolymer materials is consistent withthe hypothesized mechanism, involving partial poly-merization of the monomer after a short exposure, thediffusion of residual monomer, and finally completepolymerization after a fixing exposure.

The hologram construction process is practically realtime; this aspect of the material has been useful in theapplication to holographic interferometry. Certain

Fig. 10. Reconstructed image in incoherent white light fromhologram constructed at 514 n.

types of photopolymer other than those used to generatethe exposure curves of Figs. 4, 5, and 6 are charac-terized by diffusion times of less than 5 msec, thusshortening considerably the delay time before which apermanent holographic image may be viewed. Be-cause holograms are recorded in photopolymer asvolume phase holograms, it has been possible to achievehigh diffraction efficiencies. These thick photopolymerholograms when illuminated with white light recon-struct high-quality images in the original colors of theobjects. Furthermore, there is practically no noisescattered into the images from the photopolymeremulsions, since the material is essentially grainless.

Much of the experimental work on measuring thephotopolymer characteristics was performed by R. A.Richardson. The visible holography work was doneby E. T. Kurtzner, and it is his experimental resultthat we show in Fig. 11. The photopolymer materialsfor holographic applications were developed and sup-plied by Dupont's Photo Products Department, thiswork being carried out primarily by R. H. Wopschall,T. R. Pampalone, and W. D. Feathers.

References1. H. Kogelnik, in Proceedings of Symposium on Modern Optics,

J. Fox, Ed. (Polytechnic Press, Brooklyn, 1967), pp. 605-617.

2. A. A. Friesem and J. L. Walker, Appl. Opt. 9, 201 (1970).

3. T. A. Shankoff, Appl. Opt. 7, 2101 (1968).

4. N. K. Sheridan, Appl. Phys. Lett. 12, 316 (1968).

5. H. J. Gerritsen, W. J. Hannan, and E. G. Ramberg, Appl.Opt. 7, 2301 (1968).

6. M. J. Beesley and J. G. Castledine, Appl. Opt. 9, 2720 (1970).

7. D. H. Close, A. D. Jacobson, J. D. Margerum, R. G. Brault,and F. J. McClung, Appl. Phys. Lett. 14, 159 (1969).

8. J. A. Jenney, J. Opt. Soc. Amer. 69, 1135 (1970).

9. R. H. Wopschall, presented at OSA meeting, Tucson,Arizona, April 1971.

10. M. Born and E. Wolf, Principles of Optics (Pergamon, NewYork, 1970), p. 93.

11. J. C. Urbach and R. W. Meier, Appl. Opt. 8, 2269 (1969).

12. B. P. Hildegrand and K. A. Haines, Appl. Opt. 5, 172 (1966).

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