blue laser-sensitized photopolymer for a holographic high density data storage...

Blue laser-sensitized photopolymer for a

holographic high density data storage system

Yong-Cheol Jeong, Bokyung Jung, Dowon Ahn, and Jung-Ki Park*

Department of Chemical and Biomolecular Engineering

Korea Advanced Institute of Science and Technology

373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701 Korea

*[email protected]

Abstract: We present a new blue-sensitized photopolymer to achieve a

higher storage density compared to green/red-recordable media.

Photopolymers are prepared based on a two-chemistry system and their

holographic recording properties are investigated. A matrix of long and

flexible ether units of an epoxy precursor and a multi-crosslinkable amine

hardener enhances energetic sensitivity and suppresses volume shrinkage

effectively. Page-wise recording of 961 bits/page of digital data is

demonstrated and long term recording stability is also verified for a period

of roughly 2 months.

©2010 Optical Society of America

OCIS codes: (090.0090) Holography; (090.2900) Holographic recording materials; (090.7330)

Volume holographic gratings.

References and links

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#136757 - $15.00 USD Received 21 Oct 2010; revised 10 Nov 2010; accepted 10 Nov 2010; published 16 Nov 2010(C) 2010 OSA 22 November 2010 / Vol. 18, No. 24 / OPTICS EXPRESS 25008

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1. Introduction

Optical disks are considered suitable media for distribution of audio/video content owing to

their cost-effective replication. However, optical disks must now meet the demands of higher

storage density for ultra high definition or 3D movies. To date, in efforts to achieve this goal,

two approaches have been predominantly taken: (i) submicron-sized recording size, and (ii)

volumetric recording. In the former case, near-field recording [1,2] and a scanning probe

microscope [3] have been utilized to overcome the diffraction limit of the recording size,

leading to increased storage capacity. Nonetheless, these methods yield a low

recording/reading speed, presenting an obstacle for practical application. In the latter

approach, volumetric recording simply by two-photon absorption [4] or holography [5]

provides huge storage capacity. In particular, a page-based holographic recording technique

provides a high recording density of more than 100 bits/μm2 and a fast readout data transfer

rate of Gbytes/s, since two-dimensional data arrays are recorded into and retrieved from

media, and are multiplexed in the same volume [6]. However, there remains a barrier of

inevitable volume shrinkage, and other properties including crosstalk between multiplexed

pages, signal to noise ratio, recording size, and long term stability should also be addressed in

order to verify its commercial feasibility.

Among these issues, recording volume size and long term stability have seen relatively

little research. Recording size is an essential factor in terms of determining the storage

density, which is more significant in volumetric recording of holography than in surface

recording of a conventional 2D optical disk [6–8]. The size of the recording volume is

determined by the diffraction limit, including coefficients of both the wavelength and

numerical aperture (NA) of lens as represented in Fig. 1. There is a price to pay for high NA

objectives, which contain sophisticated multi-lens elements to correct for common optical

aberrations, and thus a shorter wavelength laser is preferable. Recording size generated by a

blue laser (405nm) is reduced to 54% and 74% with respect to green (524nm) and red lasers

(633nm), respectively. Thus it is apparent that there is strong motivation to develop a blue

laser-sensitized photopolymer [9].

In this work, we present a new blue laser-sensitized photopolymer system as a photo

imagable media for higher density storage. Diffraction efficiency and volume shrinkage were

investigated with an asymmetric recording angle to the normal direction of the photopolymer.

Recording of a digital data page (961 bits/page) was demonstrated by using an amplitude

spatial light modulator (SLM). The UV post-fixing effect on long term stability was also

examined.


Fig. 1. Schematic illustration of page-wise holographic recording and decisive parameters (λ:

wavelength, NA: numerical aperture) of recording volume (ν).

2. Preparation of epoxy-resin based photopolymer

A blue laser-sensitized photopolymer was prepared based on a two-chemistry system: The

matrix was formed by thermal curing of epoxy-resin, wherein the photo-polymerizable

acrylate monomer and photoinitiator do not react until holographic irradiation. Rather than

employing diverse epoxy precursors and amine hardeners for the matrix, as reported in

previous studies, we prepared a matrix consisting of poly (propylene glycol diglycidylether)

(PPGDGE, Mn ~380) and pentaethylenehexamine (PHA) for robust fabrication of recording

media and to attain holographic recording properties. By virtue of the low initial viscosity of

the matrix during mixing, undesirable bubbles were not entrapped and a highly transparent,

thick photopolymer was easily obtained without any solvent. Furthermore, long and flexible

ether units of PPGDGE between each epoxy end group enable faster diffusion of the photo-

reactive monomers, thereby enhancing the energetic sensitivity. In contrast to binary-

crosslinkable amine hardeners, PHA is a multi-crosslinkable hardener, and enhances the

mechanical strength of the matrix and therefore effectively suppresses volume shrinkage.

For the preparation of photopolymers, the polymer matrix: monomer (benzylmethacrylate,

n = 1.512): initiator (Irgacure819, λmax = 370 nm) was formulated in a ratio of 89.6:9.5:0.9

wt% without any solvent. This makes it possible to easily control the thickness up to 1.5 mm,

which is essential for realizing a high dynamic range of multiplexing [10,11]. The matrix

consists of a 1:1 molar ratio of amine to epoxide groups of PPGDGE and PHA, and the

refractive index of the matrix was calculated to be 1.475 by the Lorentz-Lorentz equation.

Theoretically, a 3.6 × 102 difference in the refractive index is sufficient to achieve 100%

diffraction efficiency with a 1mm thick volume grating [12]. All reactants were mixed well

and injected into the 1mm thick space between two glass substrates after undesirable bubbles

were removed. A highly transparent photopolymer film (over 95% at 633 nm), 2 × 3 cm, was

formed in a three-dimensional crosslinked state after thermal curing for 24 hrs at room

temperature. We measured the glass transition temperature of the photopolymer film by a

thermal analysis with a differential scanning calorimetry (DSC, TA Instruments DSC2010).

We obtained a value of 5.9°C, indicating that the monomer would diffuse readily across the

grating period through the free volume of the matrix [13]. This indicates that the matrix can

provide high holographic recording performance including diffraction efficiency and

photosensitivity with high resistance to volume shrinkage. Structures and refractive indices of

photopolymer components are presented in Fig. 2.


Fig. 2. Molecular structures of component materials for photopolymer, PPGDGE

(Polypropylene glycol diglycidyl ether), PHA (Pentaethylenehexamine), BzMA (Benzyl

methacrylate), I819 (Irgacure819), * calculated by Lorentz-Lorentz equation.

3. Holographic recording

A stable single longitudinal mode laser, CrystalLaser DL-405-040-S TEM00, with a

wavelength of 405nm was used to build up volume gratings by means of continuous laser

exposure. Output from the blue laser was spatially filtered and collimated to provide

holographic exposures. The laser beam was split into two secondary beams, a p-polarized

state, with an intensity ratio of 1:1. The working intensity of each beam was 3.65mW/cm2,

and both beams were recombined on the sample at an angle of 20° to the normal, which

results in a spatial frequency of approximately 1300 lines/mm. In order to elucidate the

grating period precisely, simple holographic lithography was carried out on a multi-epoxy

functionalized photoresist film. Although the refractive index of the photoresist is slightly

different from that of the photopolymer, the grating period was determined to be

approximately 780nm through atomic force microscopy (AFM). The diffracted and

transmitted intensity was monitored with an Ophir optical meter (model PD300-SH) through

a red laser, 633nm, at the re-calculated Bragg angle. The apparatus employed for the optical

experiment and to obtain the grating period result is shown in Fig. 3.

Fig. 3. Experimental apparatus of holographic recording, definition of diffraction efficiency,

AFM result of grating period (~780nm) are depicted.


3.1 Diffraction efficiency, photosensitivity, volume shrinkage

Diffraction efficiency and volume shrinkage were determined at different asymmetric

recording angles with respect to the normal direction of the photopolymer medium in order to

verify its angle multiplexing feasibility, which is decisive to high storage density. After

holographic recording, angular selectivity was measured by rotating the sample at steps of

0.005°. The experimental results, as shown in Fig. 4(a), were found to be well-correlated with

the Kogelnik equation [14], implying that sinusoidal modulation of the refractive index was

transferred.

Figure 4(b) shows that the diffraction efficiency was uniformly controlled at different

asymmetric recording angles, which is important to predict the required irradiation energy for

multiplexing at different angles. The average maximum diffraction efficiency at various

recording angles was about 80% with energetic sensitivity of 1.85 × 106. This is attributed to

vigorous photo-reactive monomer diffusion due to the free volume by using an epoxy

precursor (PPGDGE) having a long space unit, which is acceptable in holographic data

storage systems. Volume shrinkage, caused by perpendicular pressure due to micro-volume

extinction, was also investigated. A drastic increase was observed at higher asymmetric

recording angles, which can limit the applicability of photopolymer to commercial data

storage media. It is possible that the tilted grating is less resistant to perpendicular pressure

compared to a vertical grating. It is noteworthy that our photopolymer showed only a 0.025°

of Bragg angle deviation at an asymmetric recording angle of 20°. This would result in 18%

decrease in signal intensity, which is acceptable for application to angle multiplexing [15].

Furthermore, the angle deviation of 0.025° at an asymmetric angle of 20° is smaller than that

of a previously reported blue-sensitized photopolymer [9], which could be attributed to the

good mechanical modulus induced by the multi-crosslinkable amine hardner, PHA. In

summary, both the diffraction efficiency and sensitivity of our photopolymer were

comparable with those of a photopolymer widely used in holographic recording, and the

volume shrinkage was slightly lower than that of a previously reported photopolymer [9].

Table 1 lists experiment result of holographic recording of photopolymer.

Fig. 4. (a) Correlation between experimental and simulated results of diffraction efficiency

with angle rotation (b) diffraction efficiency () and Bragg angle deviation () at different

asymmetric recording angles: 0° indicates vertical grating vector.


Table1 Diffraction efficiency, refractive index modulation, energetic sensitivity, volume

shrinkage, and angle deviation, *, ** calculated from Ref [14], [9], respectively. E80:

required energy to reach 80% of maximum diffraction efficiency.

DE Δn* (104) S** (Δn/E80) Volume shrinkage Angle deviation

~80% @0-20° 1.3 1.85x106 <0.1% @0-20° 0.025° @20°

3.2 Page-wise recording with SLM

Page-wise recording was demonstrated by using the amplitude SLM, consisting of 31 × 31

pixels/ page, and its Fourier transformed (FT) image was illuminated to photopolymer to

produce colorful hologram (see the inset picture in Fig. 5(b)). After holographic recording,

photopolymer film was left in the dark until the active radical polymerization is terminated,

followed by UV post fixing. Finally, the recorded information was retrieved by reference

beam only without object beam.

Figure 5 represents the CCD captured image of transmitted and retrieved page through

photopolymer film, and the binary-converted retrieved page, respectively. Intensity profile of

the upper 18 pixels was calculated. Each pixel was completely recognized even at the gray-

scale level and this confirms the fine-tuned apparatus of FT holography recording and also

high transparency of photopolymer film. The inset in Fig. 5(b) shows an acquired typical FT

hologram. We are convinced of well-formed volumetric grating by strong diffracted rainbow

color under white light illumination. The retrieved image in Fig. 5(b) also confirmed quite a

good recording feasibility without undesirable blur or shift of each pixel. Although white

pixel intensity seems to be lower than that of the source, it is enough to recognize on/off

information. It is noted that signal to noise ratio could be enhanced by means of converting

the retrieved image from gray to binary scale with proper threshold as it is shown in Fig. 5(c).

Fig. 5. CCD captured image of page-wise pixels (top) and its signal intensity profile of the 16

upper pixels (bottom) (a) transmitted (b) retrieved gray image (insect: photograph of hologram

recorded photopolymer) (c) binary-converted retrieved image through photopolymer,

respectively.

3.3 Long-term stability of photopolymer

In order to evaluate long-term archival feasibility, diffraction efficiency was examined for 55

days with 17 samples, of which 8 specimens were fixed with UV irradiation and the others

were not subjected to any further processes. For the samples prepared without UV fixation, a


24% decrease in the diffraction efficiency with respect to the initial value occurred within the

first day immediately after recording, and eventually a total 30% drop was observed after 2

months, as seen in Fig. 6(a). In line with this phenomenon, Sheridan et al. reported the unique

characteristic of photopolymers that the diffraction efficiency additionally increased for a few

seconds after interruption of illumination but thereafter decreased [16,17]. It is anticipated

that polymerization of the monomer lasts for a very short time as long as live radicals remain,

which gives rise to an increase in the diffraction efficiency [18]. In addition, the diffraction

efficiency is thought to decrease due to diffusion of the monomer upon a concentration

gradient even after the polymerization reaction is terminated, as represented in Fig. 6(b iii).

To verify this, we measured the residual amount of monomers after holographic irradiation by

use of FT-Raman spectroscopy. Indeed, 56% of the residual monomer remained, indicating

that, theoretically, at least, half of the remaining monomer, i.e., approximately 28%, can

diffuse across the constructive region. Eventually, this would lead to a significant decrease in

the diffraction efficiency for a short period of time after the writing. From a long-term

stability point of view, in contrast to this, it is suggested that a reduction in modulation of the

refractive index profiles could also occur by chain relaxation of the grating polymer,

originating from a spontaneous process to minimize total entropy. This would account for the

degradation of the diffraction efficiency to 30% after 2 months.

Fig. 6. (a) Long-term archival feasibility of the samples with/without post UV treatment for 55

days: all diffraction efficiency values were normalized by the initial value. (b) schematic

representation of change in refractive index difference during post UV treatment

On the other hand, it is noteworthy that a 10% increase in diffraction efficiency was

observed for the samples with UV post-fixing. This can possibly be attributed to two factors:

(i) an absence of undesirable diffusion of residual monomer to the constructive region after

exposure; and (ii) a reduction in electron density of the monomers and initiators remaining in

the destructive region [19]. In order to test this hypothesis, the final conversion of monomer

was measured after strong UV illumination with 56 mW/cm2 for 5 minutes and it was found

that only 8% of the monomer remained. Moreover, the average molecular weight of the

oligomers formed by UV induced reaction was only 854 (less than 5 monomers were

polymerized). This is not sufficient to additionally enhance effective modulation of the

refractive index, but it could contribute to restricting dark diffusion transient, as illustrated in

Fig. 6(b iv). Also, the reduction in the refractive index of the destructive region, which is

caused by a decrease of the electron density from cleavage of the initiator and double bonds

of the monomer, increases the difference in the refractive index.


4. Conclusions

In this report, we presented blue-sensitized photopolymers based on an epoxy-resin of a two

chemistry system and the holographic recording properties of the photopolymers were

investigated. Our blue-sensitized photopolymer showed high optical transparency of more

than 95% for 1mm thick film and limited volume shrinkage even in the wide recording angles

for multiplexing as well as high diffraction efficiency exceeding 80%. We also demonstrated

recording/retrieval of 961 bits/page of a digital page with amplitude SLM. The archival

stability was maintained for 2 months after UV post-fixing, which was attributed to monomer

dark diffusion and electron density change. We believe that our photopolymer is a promising

candidate recording medium for high-density optical data storage devices.

Acknowledgements

This research was supported by a grant from the 'Center for Nanostructured Materials

Technology' (code#: 2010K000360) under the '21st Century Frontier R&D Programs' of the

Ministry of Education, Science and Technology, Korea.


blue laser-sensitized photopolymer for a holographic high density data storage...

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