Doped poly(methyl methacrylate) photopolymers for holographic data storage

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c

World Scientiﬁc Publishing Company

DOPED POLY(METHYL METHACRYLATE) PHOTOPOLYMERS

FOR HOLOGRAPHIC DATA STORAGE

SHIUAN HUEI LIN∗, JUNE-HUA LIN†, PO-LIN CHEN†, YI -NAN SHIAO‡ and KEN Y. HSU†

∗_{Department of Electrophysics, National Chiao Tung University,} HsinChu, 30050, Taiwan

[email protected]

†_{Institute of Electro-Optical Engineering,} National Chiao Tung University, HsinChu, 30050, Taiwan

‡_{Department of Material Science and Engineering,} National Chiao Tung University, HsinChu, 30050, Taiwan

Received 29 March 2006

In this paper, we report our investigations on thick holographic recording material of the phenanthrenequinone doped poly(methyl methacrylate) (PQ:PMMA) photopoly-mer. The design strategy and fabrication technique for making thick polymer samples with negligible shrinkage and good optical quality are presented. The physical mech-anism for holographic recording in PQ:PMMA material is described, and methods for improving are proposed. Based on these methods, photopolymer samples with diﬀerent compositions are fabricated and experimentally characterized. The results show that by modifying compositions, the material sensitivity and dynamic range for volume holo-graphic recording have been improved.

Keywords: Volume hologram; holographic data storage; holographic material; photopolymer; PMMA polymer.

1. Introduction

Holographic data storage has been considered as one of the next generation

infor-mation storage technologies because of its distinct page oriented data format for

parallel data recording/retrieval.

1

–

3

_{It is known that the operation principle of}

this technology is based on the Bragg condition for thick gratings. In order to

achieve high storage density, viz to record multiple holograms in the same

spa-tial location of a storage volume, a recording material of several-millimeter thick

and with high photoinduced refractive index change is necessary. Recently,

pho-topolymer materials are of considerable interest for development of write-once

material due to their high sensitivity, large modulation in refractive index, and

easy fabrication characteristics.

4

–

7

_{The physical mechanism of holographic}

record-ing in these materials typically involves photo-induced polymerization of acrylic

239

J. Nonlinear Optic. Phys. Mat. 2006.15:239-252. Downloaded from www.worldscientific.com

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monomers in a polymeric ﬁlm. A number of photopolymer materials for holographic

applications have been developed since the ﬁrst holographic photopolymer was

reported in 1969.

8

–

15

_{However, practical volume holographic materials were not}

commercially available until recent years when they were announced by companies

such as InPhase and Aprilis. The main issues of these kinds of materials are limits

in thickness and high shrinkage during holographic exposure. In our laboratory, we

have developed a novel technique for fabricating bulk phenanthrenequinone doped

poly(methyl methacrylate) (PQ:PMMA) photopolymer of a centimeter thick.

16

Holographic experiments using these PQ:PMMA samples showed that

PMMA-based polymer could be an attractive material due to its good optical quality

and negligible photochemical shrinkage. Investigation on the recording mechanism

anticipated that by changing the compositions of PMMA-based photopolymers, the

performance of holographic recording materials could be further improved. In the

following we ﬁrst review our investigations on PQ:PMMA material. The

design-ing strategy and the fabrication method are described. In Sec. 3, the sensitivity,

dynamic range, and reliability for volume holographic recording are deﬁned. These

parameters are used to characterize the recording materials. Optical experiments

are also presented. Based on these results, we propose some strategies to improve the

recording material by changing compositions. Section 4 presents the fabrication and

characterization of new PMMA-based photopolymers with diﬀerent compositions.

Experimental results conﬁrm our ideas of improvement. Finally, some conclusions

are given in Sec. 5.

2. PQ:PMMA Photopolymer Material

The thick holographic recording material fabricated in our laboratory is PQ:PMMA

photopolymer, which consists of the host matrix PMMA doped with PQ molecules.

The doped element is photosensitive. Under light illumination, PQ molecules induce

the photochemical reaction such that the refractive index of the material changes,

and the refractive index pattern follows that of light intensity pattern during

holo-graphic recording. This material is suitable for write-once holoholo-graphic applications.

In conventional materials of photopolymerizable PMMA, the fundamental issue is

the material shrinkage induced by the photochemical reaction. As a result, the

Bragg condition for volume hologram is mis-matched and the recorded information

could not be retrieved completely. Our idea for alleviating the shrinkage problem is

to make a strong polymer matrix to support the material structure, such that it is

not aﬀected by light exposure during holographic recording. Hence, the shrinkage

problem can be minimized. On the other hand, in order to record holograms, a

small percentage of photosensitive molecules, in our case PQ, are doped and

dis-tributed uniformly inside the polymer matrix during material fabrication. These

PQ molecules together with a small portion of unreacted monomer molecules are

responsible for holographic recording. Thus, the material will be capable of

record-ing phase holograms, whereas the basic photopolymer structure will not be aﬀected

by holographic recording. The key to achieving this goal lies in the technique to

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separate the photochemical reaction during holographic recording from the

poly-merization of the host monomer molecules during material preparation. This can be

accomplished if most of the MMA molecules are polymerized to form a host polymer

matrix PMMA during material preparation in a way that only a few percentages

of unreacted MMA monomer molecules are left for holographic recording.

We have found a two-step thermo-polymerization procedure that can achieve

this goal.

17

In the ﬁrst step, the thermal initiator, azobisisobutyronitrile (AIBN,

∼ 1%) and PQ molecules (up to 0.7%) are dissolved in solvent MMA. The solution

was stirred in an ultrasonic water bath at 30

◦

C for at least 80 hrs until the solution

turned homogeneously viscid. In the second step, the solution was poured into a

glass container and baked at 45

◦

C for 48 hrs until polymerization was complete and

the sample became a solid bulk. The actual shape of the sample was determined

by the geometry of the glass container. We have found that, careful control of

the ﬁrst step is key to obtaining high quality samples. During this stage, nitrogen

gas is released from the thermo-decomposition of AIBN and heat is produced from

thermo-polymerization of MMA monomers. In order to produce a bulk sample with

no residual air bubbles, the N

2

gas molecules and heat should be released completely

from the liquid in a slow pace. Thus, this step should be kept at low temperature, in

our case room temperature (

∼ 25

◦

C). We have also found that slow stirring is very

helpful for releasing N

2

gas and heat, thus producing samples with high uniformity.

Our PQ:PMMA samples appear yellow in color. We have measured the optical

transmission of diﬀerent samples in the visible range. The sample possesses strong

absorption below the blue wavelength (< 450 nm), and is transparent for red and

near infrared wavelengths (> 540 nm). In the holographic experiments we used an

argon laser with wavelength 514.5 nm. At this wavelength the absorption

coeﬃ-cient is 2.7 cm

−1

. We also measured surface ﬂatness of the samples. A disk sample,

5 inches in diameter and 2 mm thick, was placed into one arm of a Mach-Zehnder

interferometer. The interferogram was imaged onto a CCD camera which showed

that there was only one fringe across the 5-inch diameter. This indicates that our

technique can produce large samples with high uniformity for holographic data

storage application.

3. Holographic Characteristics of PQ:PMMA Samples

In general, the desired characteristics for a volume holographic material include

high sensitivity for optical exposure, large refractive index change, easy fabrication

of large area or volume with high optical quality (or, low scattering noise). From

the point of view of holographic data storage, the most important parameters are

material sensitivity, dynamic range and reliability.

3.1. Methodology for characterizing three material parameters

Optical sensitivity is related to material capability to support data recording speed.

High sensitive material enables holographic recording using low power lasers with

short exposure time. This is crucial to achieve a compact data storage system with

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high recording speed. There are several ways to deﬁne the sensitivity of holographic

materials. Basically, they indicate how much exposure energy is required to achieve

the requested diﬀraction eﬃciency. For holographic data storage, the sensitivity of

a material is defined to be equal to the square-root of diffraction efficiency of one

hologram divided by the corresponding exposure energy. The square-root value of

the diﬀraction eﬃciency is named as the grating strength of the hologram.

Dynamic range describes the total change in refractive index of the material

that can be produced by optical exposures. In holographic data storage in

write-once material, hundreds or thousands of holograms are multiplexed and recorded

at one location of the material until it is photo-chemically exhausted. Each of these

holograms shares the refractive index change of this spot. For a speciﬁc value of

hologram diﬀraction eﬃciency, the larger dynamic range of the material means more

holograms can be recorded at one spot. The dynamic range of a volume holographic

material can be conveniently represented as M # =

N_i=1

√

η

_i

, where η

_i

is diﬀraction

eﬃciency of each hologram and N is the total number of holograms that can be

recorded in the material as it is exhausted. In general, the requested diﬀraction

eﬃciency η

_i

is determined by the sensitivity of the output photodetector array and

laser power of the recording beams. A typical value of η

_i

is in the order of 10

−4

.

Thus, the larger dynamic range of the material means the larger the N that can be

recorded, resulting in a larger M # and higher data storage density. The M -number

is therefore a systemic parameter of the material’s dynamic range.

18

The above two parameters can be experimentally determined by performing

multiple plane-wave hologram recording at a single spot in the material. First, a

series of holograms were recorded at one location by use of a multiplexing technique

until the material was exhausted. Then after each recording, the diﬀraction

eﬃ-ciency of each hologram was measured, and the summation of the square roots of the

diﬀraction eﬃciencies formed a running curve of the cumulative grating strength,

which is plotted as a function of cumulative exposure energy, viz C(E) =

n_i=1

√

η

_i

,

where E is the cumulative exposure energy and n is the total number of holograms

that have been recorded by E. The curve indicates the dynamics of the build-up

process of the multiple hologram recording. When n approaches N , the material

became exhausted and no more holograms can be recorded, thus C tends to be

saturated, and its saturation value is equal to the M /#. On the other hand,

accord-ing to the deﬁnition of material sensitivity, it can be calculated as change of the

cumulative grating strength divided by the corresponding exposure energy of that

hologram, viz S =

∆_∆C(E)_E

_{one hologram}

, where ∆C(E) is the grating strength of one

hologram with energy ∆E. For a write-once photopolymer in multiple hologram

recording, the recording dynamics C(E) will be a nonlinear function of exposure

energy E. Without loss of generality, we will consider the sensitivity as S calculated

for a fresh material, or, when the material is under a few exposures of a multiple

storage experiment.

Reliability relates many aspects of volume holographic data storage, such as

archival lifetime and temperature stability, etc. For volume holographic storage

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F L1 2F F F CCDL To TV Monitor & Host Computer

SH1 BS _SF/BE SH2 ST/M1 L1 L2 L3 L4 Polymer Cube Control Signal From Computer Control Signal From Computer LCTV Input Image Laser S-Polarized

Conditions:

x

y

x

1

y

1 Focal length : 100 mm Thickness : 5 mm Wavelength : 514 nm Image size : 10x10 mm2

Fig. 1. Schematic diagram of the optical setup to store 250 Fourier holograms using angular multiplexing technique. L1–L4: lens; SH1–SH2: shutter; BS: beam splitter; LCTV: liquid crystal television; SF/BE: spatial ﬁlter/beam expander.

using thick materials, good dimensional stability is the most stringent requirement.

Photo-chemical reaction induced material shrinkage during holographic recording

will result in a mis-match of Bragg condition such that the recorded information

cannot be read out completely.

19

The thicker the material is, the more serious the

shrinkage eﬀect. This eﬀect can be evaluated by recording an image into a volume

hologram and checking the quality of the retrieved image. Consider a Fourier

holo-gram recording with 90 degrees geometry, where the reference and signal beams are

incident into the recording medium at the adjoin sides, as shown in Fig. 1. A

pho-topolymer block with dimensions 5

× 5 × 5 mm

3

_{is used as the recording medium.}

The input image is presented on a LCTV, and the readout image is detected by a

CCD camera. The two-dimensional distribution of the reconstructed image can be

expressed as

20

g(x

1

, y

1

)

∝ f

−

1 (1 + α

_x

)

x

1

+

λf

2π

∆K

x

,

−

1 (1 + α

_y

)

y

1

+

λf

2π

∆K

y

× t sin c

t

2π

∆K

_z

+

2π

λf

2

x

2₁

+ y

₁2

−

(1 + α

_z

)

(1 + α

_x

)

2

x

1

+

λf

2π

∆K

x

2

+

(1 + α

z

)

(1 + α

_y

)

2

y

1

+

λf

2π

∆K

y

2

+ α

_z

f

2

(1)

where α

_x

, α

_y

, and α

_z

are material shrinkage coeﬃcients along the x-, y-, and

z-directions, respectively, λ is the wavelength, t is the thickness of the medium,

f is the focal length of Fourier lenses L3 and L4, and ∆K

_x

and ∆K

_y

represent the

diﬀerence of grating wavevector induced by the shrinkage distortion of hologram.

Assume that t = 5 mm, f = 10 cm, and λ = 514.5 nm; then the reconstructed

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Table 1. The reconstructed images under different material shrinkage coefficients (computer sim-ulation). The second row represents the original and reconstructed images with different shrinkage coefficients. The third row represents the envelope profile of the reconstructed image if a plane-wave grating is recorded.

× −5 × −5 × −4 × −4

image can be calculated if the shrinkage coeﬃcients are given. Table 1 shows the

simulation result with shrinkage coeﬃcient α = α

_x

= α

_y

= α

_z

. It can be seen that

when the shrinkage coeﬃcient is larger than 5

× 10

−5

, then the retrieved images

become seriously distorted. For the 5-mm thick medium, the shrinkage coeﬃcient

should not be larger than 10

−5

.

3.2. Experimental characterization of the material

In order to measure dynamic range and sensitivity of the material, we have recorded

300 plane-wave holograms on a spot of the material with a peristrophic

multi-plexing system, each hologram with equal exposure energy (

∼ 40 mJ/cm

2

). After

each recording, the diﬀraction eﬃciency of each hologram was measured, and the

cumulative grating strength C(E) of the material was plotted as a function of the

total exposure energy that has been illuminated on the material during recording.

Figure 2 shows the experimental results for photopolymer samples with thickness

of 1 mm. If we carry out a curve ﬁtting by the function C(E) = M #[1

− (E/E

_τ

)],

then the M # and the exposure energy constant E

_τ

of the material are obtained.

From these results, sensitivity can be evaluated. As suggested in the previous

para-graph, we consider the recording sensitivity of a fresh sample. During this stage,

the cumulative exposure energy is small, thus the exponential function C(E) can

be linearized and the material sensitivity S can be found to be

S =

dC(E)

dE

E→0

≈

M #

E

_τ

.

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From Fig. 2, it is calculated that for the 1-mm thick sample, M/# = 2.06 and

E

_τ

= 4.76 J/cm

2

_{. By inserting these values into Eq. (2), the sensitivity of our}

PQ:PMMA sample is found to be 0.43 cm

2

/J. The M # of our PQ:PMMA material

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0 0.5 1 1.5 2 2.5 0 2 4 6 8 10 12 C u mu la ti v e Gr at in g Str e n g th Exposure Energy (J/cm2) M# = 2.0612; E_τ = 4.7552J/cm2

Fig. 2. The running curve of the cumulative grating strength as a function of total exposure energy for a 1-mm thick PQ:PMMA photopolymer sample.

is about comparable with that of LiNbO

3

crystals, whereas our material sensitivity

is an order of magnitude higher.

21

For the shrinkage evaluation, we performed a Fourier hologram storage

experi-ment on a (5

×5×5)-mm

3

_{photopolymer cube. The schematic diagram of the optical}

setup is the same as that shown in Fig. 1. The reference and signal beams were

incident into the cube at the adjoin sides of the cube. The intensity of each beam

was 2 mW/cm

2

_{. An angle-multiplexing scheme was used in the experiment. Two}

hundred and ﬁfty Fourier holograms of a chessboard pattern, which was shown on a

liquid crystal television (LCTV) with resolution of 320

× 240 pixels, were recorded

on a single location. The original and one of the retrieved images together with the

linear scan of the gray level of the corresponding images are shown in Fig. 3. It

is found that the reconstructed images have as good ﬁdelity as the original image.

Compare the experimental result with that of the analysis presented in the

previ-ous paragraph; we can estimate that shrinkage in this material is as low as 10

−5

,

because the whole image can be reconstructed from such a 5-mm thick block.

We have investigated possible physical mechanisms for holographic recording

in our samples.

22

Chemical analysis showed that when our photopolymer sample

was fabricated, most of MMA monomer was thermally polymerized and only about

10% of it was left as the residual monomer in bulk samples. At holographic

record-ing, photons excite the o-quinone double bond on the carbonyl functional group

of PQ molecules, and then the PQ radicals react with the vinyl group on MMA

molecules to form one PQ molecule to one MMA molecule attachment. These

photo-compounds become less conjugated than the original molecular structure such that

the refractive index of the sample has been modulated. Consequently, a diﬀerence

between the refractive index of the polymer matrix in the dark region and that of

PQ-MMA compounds in the bright area is created, i.e. a phase grating. In other

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Original image 0 50 100 150 200 250 0 50 100 150 200 250 Position 0 50 100 150 200 250 0 50 100 150 200 250 Position 0 50 100 150 200 250 50 100 150 200 250 Position Gr ay l e v e l 0 50 100 150 200 250 0 50 100 150 200 250 Position Gr ay l e v e l Retrieved image

Fig. 3. The original and one of the retrieved images together with the linear scan of the gray level of the corresponding images.

words, hologram recording in our samples is mainly due to refractive index change

induced by the structural change of the PQ-MMA attachment. Further, analyses

of the molecular weight distribution indicated that the excited PQ molecule also

induced a little polymerization of MMA molecules, in which one PQ was linked with

four to several units of MMA to form an oligomer. But this eﬀect was relatively

small compared to that of PQ-MMA attachment. Such small amounts of the

photo-polymerization does not aﬀect the backbone of the polymer matrix. Since there is

negligible inﬂuence on the matrix structure of PMMA chains, thus the volume

shrinkage induced by photo-polymerization, which always happens in conventional

photopolymer materials, can be minimized.

4. Improve the Performance of PQ:PMMA Photopolymer

The above results show that the two-step thermo-polymerization technique is

capa-ble of fabricating thick holographic recording material with negligicapa-ble shrinkage.

For the holographic recording, the attachment of the o-quinone double bond on the

PQ molecule and the vinyl group on the MMA molecule plays a key role. We can

apply this knowledge to further improve material sensitivity and dynamic range of

the doped PMMA photopolymer materials. The strategy is to modify the

mate-rial compositions while maintaining the two-step fabrication technique. Since our

PQ:PMMA photopolymer is synthesized from PQ, MMA and AIBN, there are at

least three possibilities for improvement.

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The ﬁrst idea is to play with diﬀerent photosensitive molecules with o-quinone

double bonds. When the side link functional groups of the o-quinone based molecule

are modiﬁed, the recording sensitivity and dynamic range of the sample may

be improved. We have fabricated photopolymer samples with four kinds of

dop-ing elements, includdop-ing the original 9,10-phenanthrenequinone (named as PQ),

and 1-isopropyl-7-methyl-9,10-phenanthrenequinone (named as PQ1),

4,5-dinitro-9,10-phenanthrenequinone (named as PQ2) and 11,12-dihydrochrysene-11,12-dione

(named as PQ3). Table 2 lists their chemical formulas and the corresponding

infor-mation. Each material was made to 1-mm thick, and doped with the saturated

doping concentration of the corresponding photosensitive element, as indicated in

Table 2. The holographic recording characteristics of the diﬀerent samples are

mea-sured according to our measuring methodology described in Sec. 3. For PQ and

PQ1 doped PMMA samples, three hundred plane-wave holograms, each with equal

exposure energy (

∼ 40 mJ/cm

2

), have been recorded at a single location of the

poly-mer samples. For PQ2 and PQ3 doped PMMA samples, one hundred plane-wave

holograms have been recorded, each hologram was recorded with exposure energy

of 400 mJ/cm

2

_{and 800 mJ/cm}

2

_{, respectively. The running curves for the}

cumula-tive grating strength of diﬀerent polymer samples are given in Fig. 4. It can be seen

that all samples demonstrate the similar recording behavior. By exponentially curve

Table 2. M#, Eτ and sensitivity of 1-mm thick PMMA photopolymer samples doped with PQ, PQ1, PQ2 and PQ3, respectively.

Doping Exposure Energy

Chemical Molecular concentration Constant Sensitivity

Formula Structure (wt%) M# (J/cm2) (cm2/J) PQ 0.7 2.06 4.76 0.433 9,10-phenanthrenequinone O O PQ1 0.6 2.13 2.07 1.029 1-isopropyl-7-methyl-9,10-phenanthrenequinone C O O H3C C CH3 CH3 H PQ2 0.26 0.74 21.17 0.035 4,5-dinitro-9,10-phenanthrenequinone O O NO2NO2 PQ3 0.25 0.5 60.9 0.008 11,12-dihydrochrysene- 11,12-dione-9,10-phenanthrenequinone O O

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0 0.5 1 1.5 2 2.5 0 2 4 6 8 10 12 Cu m u la tiv e G ra ti n g S tre ng th Exposure Energy (J/cm2₎ PQ1 PQ PQ2 PQ3

Fig. 4. The running curves of the cumulative grating strength as a function of total exposure energy for 1-mm thick PMMA photopolymer samples doped with PQ, PQ1, PQ2 and PQ3, respectively.

ﬁtting the running curves of Fig. 4, the material M # and E

_τ

are estimated and

summarized in Table 2. Then, sensitivity of each sample is calculated and listed.

It can be seen that the best result is obtained by the PQ1 doped PMMA samples,

which has the M # of

∼ 2.13 and sensitivity of 1.029 cm

2

_{/J, respectively. The results}

show that an improvement in both recording sensitivity and dynamic range can be

achieved if a suitable dye is chosen.

The second idea is to change the compositions that form the polymer matrix. It

can be achieved by increasing the number of vinyl functional groups on the monomer

and/or changing the side functional group of the vinyl group on the monomer.

The purpose is to enhance the combination ability of monomer molecules with

PQ radicals, so as to improve the material dynamic range and sensitivity. Thus,

in addition to the original MMA monomer, two kinds of additional monomers,

including trimethylolpropane triacrylate (TMPTA) and acrylic acid 2-phenoxyethyl

ester (PEA) are added, one at a time. The TMPTA molecule has three vinyl

groups, while the PEA molecule has an additional benzene side functional group.

Table 3 shows their chemical formulas and the corresponding information. Thus,

two types of new photopolymer samples are fabricated. In each type, two monomers

are introduced during the fabricating procedure, with a weight-ratio of 2:8 for

both TMPTA:MMA and PEA:MMA, respectively. These samples are named as

PQ:PTMPTA-co-PMMA and PQ:PPEA-co-PMMA, respectively. The doped

con-centration of PQ molecule was ﬁxed at 0.5 wt% for both samples. For comparison,

the PQ:PMMA sample was also made with equal concentration of the PQ molecule.

Again, we measured the material M # and sensitivity for 2-mm thick samples. 175

plane-wave holograms, each with the same exposure energy (

∼ 0.6 J/cm

2

_{), were}

recorded at a single location of the sample. The running curves of the cumulative

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Table 3. M#, Eτ and sensitivity of 2-mm thick PQ/PTMPTA-co-PMMA and PQ/PPEA-co-PMMA photopolymer samples.

Doping Exposure

Chemical Molecular concentration Energy Constant Sensitivity

Formula Structure of PQ (wt%) M# (J/cm2) (cm2/J) MMA 0.5 0.62 20.0 0.03 methyl methacrylate H2C C C O O CH3 CH3 H2C C C O O CH3 CH3 H2C C C O O CH3 CH3 TPMPTA 0.5 1.20 16.2 0.074 trimethylolpropane-triacrylate O H2C C H C O H2 C C O H2 C C C H O CH2 O H2 C C C H O C H2 O H2C C H C O H2 C C O H2 C C C H O CH2 O H2 C C C H O C H2 H2C C H C O HH22 C C CH3 CH2 O H2 C C C H O CH2 O H2 C H2 C C C H O CH2 CH2 O H2 C C C H O C H2 O H2 C C C H O C H2 PEA 0.5 1.06 10.6 0.1 acrylic acid 2-phenoxyethyl ester O O O O O O

grating strength for diﬀerent samples are shown in Fig. 5. Curve ﬁttings are

per-formed and the M #, E

_τ

, and S are calculated and listed in Table 3. It is seen

in the table that the M /# of PQ:PPEA-co-PMMA is 1.06, which is 1.5 times

larger than that of PQ:PMMA (M#

∼ 0.62). On the other hand, the dynamic

range of PQ:PTMPTA-co-PMMA has been improved by about 2 times to become

1.20. Furthermore, the exposure energy constant, E

_τ

of PQ/PTMPTA-co-PMMA

and PQ/PPEA-co-PMMA are 16.21 J/cm

2

and 10.60 J/cm

2

, respectively. Both are

slightly lower than that of PQ/PMMA (E

_τ

∼ 20 J/cm

2

_{). Using these parameters,}

sensitivity can be calculated, as shown in last column in Table 3. It can be seen

0 10 20 30 40 50 60 70 80 0.0 0.2 0.4 0.6 0.8 1.0 1.2 PQ/PMMA PQ/ PTMPTA-co-PMMA PQ/PPEA-co-PMMA Cumulative Grating Strength Exposure Energy (J/cm2₎

Fig. 5. The running curves of the cumulative grating strength as a function of total expo-sure energy for 2-mm thick PQ/PTMPTA-co-PMMA and PQ/PPEA-co-PMMA photopolymer samples.

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that S has been improved by about 2 and 3 times, respectively. The results conﬁrm

our idea that by adding other components to form a copolymer structure, the

holographic recording characteristics of PQ:PMMA photopolymer can be improved.

The third alternative for improving PQ:PMMA material is to add another

dop-ing element, so as to provide a catalyst to speed up the photoreaction between

the PQ and MMA molecules. Zinc methacrylate (ZnMA) has been chosen in

our investigation. It was added up to 0.35 wt% into PQ:PMMA photopolymer to

make ZnMA and PQ co-doped PMMA samples with thickness of 2 mm, which is

named as ZnMA/PQ:PMMA. In order to measure the material M # and

sensitiv-ity, 175 plane-wave holograms were recorded at one location of the material, each

with equal exposure energy (

∼ 0.31 J/cm

2

_{). Figure 6 shows the running curves for}

photopolymer samples with diﬀerent doping concentrations of ZnMA molecules.

From this ﬁgure, the M #, E

_τ

and S for the diﬀerent samples are estimated and

summarized in Table 4. It shows that when the doping concentration of ZnMA

molecule is increased from 0 to 0.35 wt%, the M /# is increased by about 3 times,

from 2.83 to 8.81, and the exposure energy constant is changed slightly. As a result,

the sensitivity is improved by 2.5 times (1.23 cm

2

_{/J for 0.35 wt% ZnMA), compared}

0 10 20 30 40 50 0 2 4 6 8 10 Cumulative Grating Strength Exposure energy (J/cm2₎ ZnMA 0.35 wt% +PQ 0.7wt% ZnMA 0.175 wt% +PQ 0.7wt% ZnMA 0.07 wt% +PQ 0.7wt% PQ 0.7wt%

Fig. 6. The running curves of the cumulative grating strength as a function of total expo-sure energy for PQ/ZnMA:PMMA photopolymer samples with diﬀerent concentrations of ZnMA molecule. Notations on the right corner indicate the doping concentrations of ZnMA and PQ molecules, respectively.

Table 4. M#, Eτ, and sensitivity of 2-mm thick PQ/ZnMA: PMMA photopolymer samples with diﬀerent concentrations of ZnMA molecule.

Conc. ZnMA (wt%) 0 0.07 0.175 0.35

M# 2.83 4.63 5.59 8.81

Eτ(J/cm2) 5.53 7.49 6.75 7.15

Sensitivity (cm2/J) 0.51 0.62 0.83 1.23

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with that of the PQ singly-doped PMMA sample (0.51 cm

2

_{/J). These experimental}

results demonstrate that by adding the catalyst type molecules, such as ZnMA, into

PQ:PMMA photopolymer, it is possible to improve both the M # and sensitivity

of the material for volume holographic recording.

5. Conclusions

We have presented our investigations on the holographic recordings in doped

poly(methyl methacrylate) photopolymers. The design strategy and the fabrication

technique for making thick polymer samples with negligible shrinkage and good

optical quality are described. The physical mechanism for holographic recording in

this material is presented. In terms of material dynamic range and sensitivity, a

methodology for characterizing the material for holographic data storage has been

proposed. For the PQ:PMMA samples, the shrinkage coeﬃcient of the samples is

estimated to be smaller than 10

−5

, the M /# of a 1-mm thick sample is measured to

be 2.06, and the sensitivity is calculated to be 0.43 cm

2

_{/J. By modifying the}

com-ponents in materials, we then presented three possibilities to improve the material.

The ﬁrst method is to fabricate a photopolymer doped with diﬀerent

photosen-sitive dye molecules. The best result is achieved with a PQ1:PMMA sample, of

which the M # of the 1-mm thick sample is slightly improved, and the sensitivity is

about 2.4 times higher than that of PQ:PMMA sample. The second method is to

fabricate a photopolymer sample with PTPMPTA-co-PMMA or PPEA-co-PMMA

copolymers. The M # have been improved by 1.5

∼ 2 times, and the sensitivity of

these materials has been improved by 2

∼ 3 times. The third idea is to add catalyst

molecules, such as ZnMA, into the composition to form a co-doped

photopoly-mer so that the photo-chemical reaction can be speeded up. The material M #

of the ZnMA/PQ:PMMA sample was measured to have improved by 3 times and

the material sensitivity by 2.5 times. The above results have conﬁrmed our idea of

changing material composition for enhancing the holographic recording

characteris-tics. However, they are by no means the optimal conditions yet. We anticipate that

by suitable selection of photosensitizers, co-doped monomer molecules and catalyst

molecules, the material dynamic range and sensitivity can be further improved.

Acknowledgment

The authors are grateful for helpful discussion about material preparation from

Prof. Wha Tzong Whang. We gratefully acknowledge the ﬁnancial support from

the National Science Council, Taiwan under contracts 89-E-FA06-1-4 and

NSC92-2112-M-009-010.

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