Optical Materials 23 (2003) 529–538

www.elsevier.com/locate/optmat

Optimization of a photopolymerizable holographicrecording material based on polyvinylalcohol using

angular responses

S. Blaya *, L. Carretero, R.F. Madrigal, A. Fimia

Departamento de Ciencia y Tecnolog�ııa de Materiales Division de Optica, Universidad Miguel Hern�aandez,Av. Ferrocarril s/n Apdo. 03202, Ed., Torrevaillo, Elx (Alicante), Spain

Received 12 December 2001; received in revised form 18 December 2002; accepted 7 January 2003

Abstract

Acrylamide-based holographic recording materials have significant advantages and the composition of these ma-

terials has been optimized in terms of energetic sensitivity and diffraction efficiency. As a result, diffraction gratings with

an efficiency of almost 80% for energetic exposures of 35 mJ/cm2 and a spatial frequency of 1000 lines/mm in pho-

tosensitive films 65 lm thick have been obtained. In this paper we present the effects of intensity, thickness, andvariation in the concentration of each component by studying the angular responses of the diffraction gratings recorded

in each composition.

� 2003 Elsevier B.V. All rights reserved.

1. Introduction

In recent years a great effort has gone into the

research of photopolymerizable compositions as

holographic recording materials. Compared with

other holographic materials, such as dichromated

gelatins or holographic emulsions, these materialshave the great advantage of recording and reading

holograms in real time [1]. Photopolymer systems

have many characteristics that are necessary or

useful for holographic data storage since they ex-

* Corresponding author. Tel.: +34-9-66658612; fax: +34-9-

66658497.

E-mail address: salva@dite.umh.es (S. Blaya).

0925-3467/03/$ - see front matter � 2003 Elsevier B.V. All rights resdoi:10.1016/S0925-3467(03)00018-1

hibit a high dynamic range and photosensitivity

and can be easily processed [2–4]. Recently, the

holographic properties of polyvinylalcohol-based

films has been improved [5]. These materials are

normally composed of methylene blue as photo-

initiator, triethanolamine (coinitiator) and acryla-

mide derived monomers, all dissolved in apolyvinylalcohol (PVA) matrix. Holographic re-

cording takes place when two coherent beams

spatially overlap within the material. As a conse-

quence of the interference pattern, photopoly-

merization reactions takes place at the bright

areas, creating a refractive index modulation de-

termined by the difference in refraction index be-

tween the formed polymer and the unreactedmonomer. The resulting diffraction grating exhibits

erved.

530 S. Blaya et al. / Optical Materials 23 (2003) 529–538

high efficiency with low energetic exposures [5] and

a high signal-to-noise ratio. In order to improve

the performance of the material, the composition

has been optimized with regard to energetic sen-

sitivity and efficiency of the diffraction gratings

formed [5].Once the hologram has been recorded, the an-

gular variation of the diffraction efficiency gives a

great deal of information about the properties of

the material such as thickness, index modulation,

scattering and grating profile [6]. In this paper we

present a study of the angular responses of the

diffraction gratings when parameters such as the

concentration of the components, thickness andintensity are changed.

2. Experimental

The photopolymerizable film was prepared by

coating a 20� 40 cm2 glass plate (BK7) with thephotosensitive solution, using a TLC coater sup-plied by CAMAG, and allowing it to dry for 20 h

under normal conditions (65� 5% relative hu-midity and 22� 1 �C). The basic concentration ofthe photosensitive solution was methylene blue

(BM) 2:6� 10�4 M; acrylamide (AA) 0.33 M; tri-ethanolamine (TEA) 0.20 M and PVA 7.4% by

weight. The resulting thickness of the film was

measured with a Penning ionization gauge (PIG)455 supplied by Neurtek [5].

Diffraction gratings were obtained using a He–

Ne laser with a wavelength of 633 nm, a spatial

frequency of 1000 lines/mm, total intensity of 4

mW/cm2, and beam ratio of 1/1 using a typical

unslanted holographic setup [5]. The diffracted

intensity was monitored at real time with a He–Cd

laser positioned at the Bragg�s angle tuned at 441nm where the material does not absorb. Angular

response measurements were performed using

spatially filtered and collimated laser reading beam

from a He–Cd laser tuned at 441 nm. The sample

was mounted on a monitorized rotation stage and

the angle of the sample plane relative to the

Bragg�s angle to either the diffracted beam wascomputer controlled via a motion controller,model PMC200P from Newport with an angular

resolution of 0.1�.

3. Results and discussion

Holographic recording in this material is based

on the photopolymerization reactions produced at

the bright areas. Normally, radical polymerizationis divided into three stages: initiation, propagation

and termination [7]. A possible mechanism of po-

lymerization for the composition of this material is

proposed in (1)–(10) where the constants (kn) arethe respective rate coefficients. In the initiation

stage, when the dry photopolymerizable film of

thickness d is illuminated with a non-uniformmonochromatic light of intensity I0 (mW/cm2) at awavelength where methylene blue (BMþ) absorbs,

this molecule is converted into the triplet form

(BMþ�), as can be showed in (1). This compound

can be reduced by triethanolamine (TEA) ac-

cording to (2)–(4) giving rise to the formation of

leuco-methylene blue (BMH) and the resulting

aminic radical (TEAð�HÞ�). This highly reactivespecie can initiate the polymerization reactionbonding with the acrylamide (AA) as shown in (6).

BMþ þ hm ! BMþ� ð1Þ3BMþ þ TEA! BM� þ TEA�þðk0Þ ð2ÞTEA�þ ! TEAð�HÞ� þHþðkb1Þ ð3ÞBM� þHþ ! BMH�þðkb2Þ ð4ÞBMH�þ þ TEA! BMH þ TEA�þðkcÞ ð5ÞTEAð�HÞ� þAA! AA�

1ðkaÞ ð6ÞAA�

1 þAA! AA�2ðk1Þ ð7Þ

AA�2 þAA! AA�

3ðk2Þj ð8Þ

..

.

AA�i þAA! AA�

iþ1ðkiÞ ð9ÞAA�

k þ TEAð�HÞ� ! PiðktÞ ð10Þ

(7)–(9) show the growth of the polymeric radicalchain (propagation). It has been assumed that the

rate constants of these processes do not depend on

the chain length, then k1 ¼ k2 ¼ k3 ¼ � � � ¼ ki ¼ kp[8–10], where kp is the constant rate of propaga-tion. Finally, the termination process gives the

polymeric chain (Pi) as shown in Eq. (10). In thiscase we have assumed that the most important

contribution to this stage is made by the reactionbetween the macroradicals and the amino radical

(termination by primary radicals) instead of the

S. Blaya et al. / Optical Materials 23 (2003) 529–538 531

coupling of two polymeric radicals (bimolecular

termination). This is a a reasonable assumption

when the mobility of macroradicals is low and the

concentration of primary radicals is high. In our

case this assumption may be made because theviscosity of the reaction medium (polymeric matrix

of polyvinylalcohol) is very high.

Angular response or selectivity curves have

been widely used in photopolymers [6,11]. For

example these curves have been used to study the

effect of thickness on the performance of pho-

topolymerizable films. For example in cationic

ring-opening photopolymers Waldman [6] dem-onstrated that uplift of the nulls from zero dif-

fracted intensity is caused by a non-uniform

grating profile. To do this, he assumed an expo-

nentially decaying of the index modulation in the

direction perpendicular to the grating n1ðzÞ, as inthe model for non-uniform grating using the cou-

pled wave theory proposed by Uchida [12].

In this work, as a first approximation we willuse Kogelnik�s theory [13], which assumes at thatthe modulation index is not attenuated in the di-

rection perpendicular to the grating vector and

that there is no bending of the generated fringes. It

is assumed that the grating is perpendicular to the

Y -plane (the incidence plane of the object andreference beams), therefore the grating vector ~KKforms an angle / with the Z-axis. From Maxwell�sequations, expression (11) is obtained.

r2~EE þ k2~EE ¼ 0 ð11Þ

where x is the angular frequency of the oscillatingelectric field (~EE), r is the electrical conductivity, eris the relative dielectric constant, c, is the lightspeed in vacuum, j ¼

ffiffiffiffiffiffiffiffiffiffið�1Þ

pand l is assumed

equal to that of free space and k is given by:

k2 ¼ x2e2rc�2 � jxrl ð12Þ

When two laser beams overlap at a photosen-sitive plate, the result is an interference pattern in

the plane of the material that originates a sinu-

soidal grating. Then the spatial modulation of eand r is given by

er ¼ e0 þ e1 cosð~KK~rrÞ ð13Þ

r ¼ r0 þ r1 cosð~KK~rrÞ ð14Þ

where e1 and r1 are the amplitudes of the spatialmodulation, while e0 and r0 are the average di-electric constant and average conductivity, re-

spectively. ~rr is the position vector given by thecoordinates ðx; y; zÞ and ~KK is the grating vector,whose components are ð2p=KÞðsin/; 0; cos/Þ,where K is the fringe spacing and /. CombiningEqs. (12)–(14), we obtain:

k2 ¼ b2 � 2jab þ 2jbfexp½j~KK �~rr� þ exp½�j~KK �~rr�gð15Þ

where

b ¼ 2pffiffiffiffie0

p

kð16Þ

and where k is the wavelength in the free space.The absorption coefficient a, is given by the ex-pression:

a ¼ lcr02ffiffiffiffie0

p ð17Þ

and finally the coupling coefficient j, is expressedas

j ¼ 2pe1k

�� jlcr1

��ð4 ffiffiffiffi

e0p Þ ð18Þ

Normally the coupling constant is expressed as:

j ¼ pn1k

� ja12

ð19Þ

where n1 and a1 are deduced by comparing Eqs.(19) and (18). Assuming that only two waves are

propagated in the material (the incident R anddiffracted S waves), the total electric field (E) canbe expressed as

E ¼ RðzÞ exp½�j~qq �~rr� þ SðzÞ exp½�j~vv �~rr� ð20Þ

where the propagation vectors ~qq and ~vv fulfill theBragg condition given by

~vv ¼~qq � ~KK ð21Þ

Substituting Eqs. (15) and (20) in expression (11),

taking into account the condition of Eq. (21), by

rearranging the terms that multiply the exponen-tial functions exp½�j~qq �~rr� and exp½�j~vv �~rr� the fol-lowing two coupled differential equations are

obtained:

532 S. Blaya et al. / Optical Materials 23 (2003) 529–538

kR00=ð2pn0Þ � 2jR0 cos h � 2jaR

þ 2ðpn1=k � ja1=2ÞS ¼ 0 ð22Þ

kS00=ð2pn0Þ � 2j½cos h � 2 cos/ cosð/ � hÞ�S0

þ ½4pn0kh sinð2hÞ=k � 2ja�S

þ 2ðpn1=k � ja1=2ÞR ¼ 0 ð23Þ

where n0 ¼ ðe0Þ1=2 is the refractive index and Dh isa parameter that takes into account the deviations

of Bragg condition (21) and h is the Bragg�s angle.R0 ¼ dR=dz and S0 ¼ dS=dz. After effecting thesubstitution, waves appear in the directions ~qq þ ~KKand ~vv � ~KK. These have been disregarded togetherwith all other higher diffraction orders.

If we neglect R00 and S00 [13], then we are left

with the following two coupled differential equa-tions:

�2jR0 cos h � 2jaRþ 2ðpn1=k � ja1=2ÞS ¼ 0 ð24Þ

Fig. 1. Experimental and fitted angular selectivity curves of the diffr

composition of the material is: ½AA�0 ¼ 0:33 M; ½TEA�0 ¼ 0:20 M; ½BMis 4.3 mW/cm2.

� 2j½cos h � 2 cos/ cosð/ � hÞ�S0

þ ½4pn0Dh sinð2hÞ=k � 2ja�Sþ 2ðpn1=k � ja1=2ÞR ¼ 0 ð25Þ

By solving the coupled differential equations we

can obtain one of the fundamental parameters of

holography, diffraction efficiency, defined as

g ¼ Id=I0, where Id is the diffracted intensity (jSj2)and I0 is the initial intensity (jRð0Þj2).In the case of pure phase holograms with loss of

absorption (a1 ¼ 0), the expression of the diffrac-tion efficiency near the Bragg angle obtained by

Kogelnik is given by Eq. (26). Where h is the Braggangle at the reading wavelength (441 nm), d is thethickness of the film and Tsfa is the coefficient re-lated to scattering losses. It is important to notethat n1 is related to the conversion of monomerinto polymer, because this important parameter

that determines the efficiency of the grating is the

difference between the concentration of acrylamide

in the bright and non-illuminated fringes. Finally,

action efficiency in films of different thicknesses. The chemical

�0 ¼ 2:1� 10�4 M and PVA: 7.4% by weight. The intensity used

Fig. 2. Tsfa coefficient (a) and modulation index (b) as a func-tion of thickness. Results obtained from the non-linear fit of

angular responses of the diffraction efficiency in materials with

composition: ½AA�0 ¼ 0:33 M; ½TEA�0 ¼ 0:20 M; ½BM�0 ¼2:1� 10�4 M and PVA: 7.4% by weight. The intensity used is4.3 mW/cm2.

S. Blaya et al. / Optical Materials 23 (2003) 529–538 533

information regarding the scattering is given by

Tsfa, the higher this value, the lower the scatteredlight, since it is related to the length of polymeric

chain

g ¼ Tsfa

sin2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

pn1dk cos h

� �2 þ 2Dhpn0d sinðhÞk

� 2r !

1þ Dhn0 sinð2hÞn1

� 2 ð26Þ

Due to the complexity of the mechanism pro-

posed in (1)–(9), in order to obtain information of

the species involved in these reactions, all of the

components were varied and the angular response

curves of the holographic gratings studied. The

fittings were carried out using the Levemberg–

Mardquart method [14–16], taking n0 ¼ 1:55which corresponds to a pure polyvinylalcohol film[17]. In this way, the parameters Tsfa, d and n1 wereobtained as a function of each component.

3.1. Effect of the film’s thickness (d)

Thickness is one of the most important pa-

rameters in holographic storage due to fact that

determines the sensitivity and the capacity ofstorage [11]. We studied gratings generated with

different thicknesses in order to evaluate the effect

of this variable on the parameters obtained from

the angular response. In Fig. 1 angular selectivity

curves and the respective fittings are shown for

gratings of different thicknesses. As can be showed

good agreement between experiment and theory

are observed (the regression coefficients are higherthan 0.998). It is important to note that due to the

concordance with Kogelnik�s model, we can con-clude that the effects of the bending and non-uni-

formity of the fringes are negligible in these

conditions. As can be seen, the theory predicts

important effects produced by the thickness, be-

cause as the thickness of the film increases the

bandwidth of the angular response decreases. Thevalues of the parameters fitted with the model (Tsfa,n1 and d) can be seen in Fig. 2a and b where theaverage of 18 fitted curves is represented. It can be

seen that both modulation index and scattering are

constant when the thickness varies. Therefore, in

the range studied the effect of thickness on the

kinetics of polymerization is not important be-

cause the fitted parameters do not change signifi-

cantly.

3.2. Effect of the concentration of triethanolamine

(TEA)

Normally, the systems used to initiate vinylic

polymerizations, such as radical sources, are

compounds with the ability to produce electron-

transfer reactions with the dye (coinitiators),

thereby increasing the polymerization rate [10,18].Tertiary amines have the ability to produce

photoreduction reactions in dyes, as shown in

Fig. 4. Tsfa coefficient (a) and modulation index (b) as a func-tion of the concentration of TEA. Results obtained from the

non-linear fit of angular responses of the diffraction efficiency in

materials with composition: ½AA�0 ¼ 0:33 M; ½BM�0 ¼2:6� 10�4 M and PVA: 7.4% by weight. The intensity used is4.3 mW/cm2 and the thickness is 64� 7 lm.

Fig. 3. Variation in thickness obtained from the non-linear fit

of the angular selectivity curves. The composition of the ma-

terial is: ½AA�0 ¼ 0:33 M; ½TEA�0 ¼ 0:20 M; ½BM�0 ¼ 2:1� 10�4M and PVA: 7.4% by weight. The intensity used is 4.3 mW/cm2.

534 S. Blaya et al. / Optical Materials 23 (2003) 529–538

Eqs. (1)–(5) [19,20], resulting radicals. Normally,

in holography, triethanolamine has been used ascoinitiator, producing an increase in the energetic

sensitivity [21]. Furthermore, the amine acts as

plasticizer favoring the solution and stability of the

other components inside the matrix and, conse-

quently, the performance of the material. This ef-

fect can be clearly seen in Fig. 3 where the average

of the thicknesses obtained from the fitted angular

selectivity curves is represented. It can be seen thatan increase in thickness is produced, as the con-

centration of triethanolamine is raised, thus dem-

onstrating the plasticizer properties of this

compound. Regarding the values of index modu-

lation and scattering, in Fig. 4 it can be seen that

triethanolamine does not produce a significant

variation in these parameters. Thus, in the range of

concentrations tested, TEA does not give pro-nounced variations in the conversion of monomer

in polymer or in the length of the chains.

3.3. Study of the concentration of dye (BM)

The photoinitiator determines the zone of ab-

sorption and, therefore, the wavelength used to

record and read the grating. Normally, in photo-polymerizable compositions photoreducible dyes

are used, in which xanthenes and phenothiazines

are the most common, specially in holographic

materials based on acrylamide [22–27]. From the

fittings of the angular responses, information

about the effects of the concentration of dye can be

obtained. In Fig. 5 the angular selectivity curves

are represented with the respective fittings at high

concentrations of methylene blue and the ones that

produce higher energetic sensitivities. In all cases

practically symmetrical responses with reference to

Bragg�s angle are shown. Low variations are ob-served, which are not signifivative because these

differences are lower than the magnitude order of

the error of the measurements. Due to the good

correspondence between theory and experiment it

can be concluded that there are no modulation

Fig. 5. Experimental and fitted angular selectivity curves of the diffraction efficiency in films of different concentrations of methylene

blue. The chemical composition of the material is: ½AA�0 ¼ 0:33 M; ½TEA�0 ¼ 0:20 M and PVA: 7.4% by weight. The intensity used is4.3 mW/cm2 and the thickness is 61� 2 lm.

S. Blaya et al. / Optical Materials 23 (2003) 529–538 535

index gradients produced by differential absorp-

tion [28,29].

In Fig. 6a the results for the coefficient of

scattering (Tsfa) are shown. It can be seen thatwhen concentration of methylene blue is raised the

amount of scattered light increases (decrease in

Tsfa). This behavior may be caused by the forma-tion of longer polymeric chains or incomplete

bleaching of methylene blue. Regarding the mod-

ulation index of the diffraction gratings, it can be

observed in Fig. 6b that this parameter increases

with the concentration of methylene blue, reachinga saturation value for values higher than 2 �10�4 M.

3.4. Variation of the intensity (I0) used to record thediffraction gratings

The relationship between the photoinitiator and

incident intensity is very important. In previousstudies we studied the effects of the concentration

of dye for a constant value of intensity, since when

this parameter is changed the time necessary for

the photobleaching process to take place and for

the diffraction grating to be formed varies. As can

be observed in Fig. 7a coefficient Tsfa is practicallyconstant, therefore higher intensity will not pro-duce long polymeric chains. However, the index

modulation increases as the intensity is raised, and

so more acrylamide conversions will be converted

into polymer (Fig. 7b).

3.5. Study of the monomer concentration (AA)

Acrylamide photopolymerization reactionshave been studied, specially when they are sensi-

tized in the red zone by methylene blue [30–33]. In

holography this monomer has been widely used

due to the high rate of reaction and its solubility in

water [34–36]. It has been shown that diffraction

efficiency and energetic sensitivity depend mainly

on the concentration of monomer, because this

determines the difference in refractive index of theexposed and non-exposed zones [37]. This effect

can be clearly seen in Fig. 8, where temporal

variation of the diffraction efficiency can be seen.

Fig. 6. Tsfa coefficient (a) and modulation index (b) as a func-tion of the concentration of methylene blue. Results obtained

from the non-linear fit of angular responses of the diffraction

efficiency in materials with composition: ½AA�0 ¼ 0:33 M andPVA: 7.4% by weight. The intensity used is 4.3 mW/cm2 and the

thickness is 70� 10 lm.

Fig. 7. Tsfa coefficient (a) and modulation index (b) as a func-tion of incident intensity (I0). Results obtained from the

non-linear fit of angular response of the diffraction efficiency in

materials with composition: ½AA�0 ¼ 0:33 M; ½BM�0 ¼2:6� 10�4 M; PVA: 7.4% by weight and thickness of 70�10 lm.

536 S. Blaya et al. / Optical Materials 23 (2003) 529–538

When the concentration of acrylamide is raised the

maximum value of diffraction efficiency moves to

the left (low time), reaching values of 84% of effi-

ciency with an exposure of 35 mJ/cm2.

At low concentrations of acrylamide, diffractionefficiency reaches the value of 60%, but at higher

quantity of monomer overmodulation is produced

(the modulation index has a higher value than that

the necessary to obtain maximal efficiency at this

thickness). This situation can be clearly seen in

Table 1, where the results of the fitted curves are

presented. The index modulation increases when

the concentration of monomer is raised, but thescattering is practically constant. However, mate-

rials with higher concentrations of acrylamide in-

side the film cannot be obtained due to the limited

solubility of this compound in the polyvinylalco-

hol matrix.

4. Conclusions

Diffraction gratings recorded in a photopoly-

mer based on acrylamide in a polyvinylalcohol film

have been characterized by using the angular re-

sponses curves. Symmetrical curves has been ob-tained without uplift in the non-diffracted intensity

Fig. 8. Temporal variation in the diffraction efficiency in materials with different concentrations of acrylamide. The chemical com-

position is: ½BM� ¼ 2:6� 10�4 M; ½AA� ¼; ½TEA� ¼ 0:20 M; PVA: 7.4% by weight and thickness of 65� 2 lm. The intensity used is 4.7mW/cm2.

Table 1

Effect of the concentration of acrylamide on the parameters

obtained from the non-linear fit (modulation index, scattering

coefficient and thickness)

Concentration

of AA (M)

Index modulation

(n1)Scattered light

(Tsfa)

0.20 0.0023� 0.0001 0.90� 0.020.33 0.0038� 0.0007 0.83� 0.050.45 0.0045� 0.0004 0.90� 0.01

The chemical composition of the material is: ½BM� ¼ 2:6� 10�4M; ½AA� ¼; ½TEA� ¼ 0:20 M; PVA: 7.4% by weight and thick-ness of 65� 2 lm. The intensity utilized is 4.7 mW/cm2.

S. Blaya et al. / Optical Materials 23 (2003) 529–538 537

zones. Good agreement between the Kogelnik�stheory and the experimental data is observed.

Therefore, the material can be optimized in terms

of the quality of the grating formed. In this case a

sinusoidal profile is obtained with a constantgrating in the propagation direction. Conse-

quently, the effects of each component have been

studied, showing that the concentration of meth-

ylene blue produces an amount in scattered light,

whereas the modulation index increases signifi-

cantly with intensity and concentration of acryla-

mide, reaching a saturation value when the

concentration of photoinitiator is increased.

Acknowledgements

This work was financially supported by the

Comision Interministerial de Ciencia y Tecnolog�ııa(CICYT) of Spain (projects MAT2000-1361-C04-

03 and MAT99-0622).

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