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
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  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  or holography  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 . 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 .
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.
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 × 10−2 difference in the refractive index is sufficient to achieve 100% diffraction efficiency with a 1mm thick volume grating . 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 . 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 .
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 .
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 , 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 × 10−6. 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 . Furthermore, the angle deviation of 0.025° at an asymmetric angle of 20° is smaller than that of a previously reported blue-sensitized photopolymer , 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 . Table 1 lists experiment result of holographic recording of photopolymer.
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).
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 . 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.
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 . 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.
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.
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.
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