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A new diarylethene doped with poly(methyl methacrylate) film is developed and its characteristics of volume holographic recording are investigated. The maximum diffraction efficiency of the 10µm thick film is 1.2%, and the rewritable hologram recording exhibits its high resolution, fatigue resistance, negligible shrinkage, and long lifetime, which are critical to apply this material to high-density rewritable holographic data storage.
©2005 Optical Society of America
1. Introduction
Holographic data storage has attracted considerable interest in recent years for its advantages of large capacity and high data transfer rate. To a great extent, the performance of holographic system depends on the properties of the recording media [1]. Extensive research works have been performed to synthesize and utilize new materials for volume holographic storage [2–6].
Photochromism is defined as a reversible transformation of a single chemical species between two states that have different absorption spectra and refractive index. The reversible change is achieved by the action of light with different wavelengths. This characteristic of photochromic compounds makes them available in the field of rewritable holographic storage.
Photochromic diarylethene, as a good candidate for rewritable holographic recording material, has attracted much attention from many researchers because of its high resolution, fatigue resistance, thermal irreversibility, and self-development properties [7]. In this paper, the diarylethene based on a novel synthetic method doped PMMA thin film is introduced for rewritable holographic storage and its volume holographic recording characteristics are investigated. In the diarylethene doped PMMA film of 10µm thickness, the diffraction efficiency of the grating is measured to be as high as 1.2% and the recorded hologram demonstrates its high resolution, fatigue resistance, and negligible shrinkage. Low cost red laser for recording and readout makes the rewritable holographic storage in the diarylethene doped PMMA film attractive.
2. Experiment and discussion
2.1 Materials and sample preparation
The diarylethene 1a was synthesized [8] from starting material 2-methylthiophene; M.p. 200–201 °C; 1H-NMR (CDCl3) δ(ppm): 9.75(s, 2H, CHO), 7.46(s, 2H, thiophene-H), 4.10(s, 4H, CH2-S), 2.10(s, 6H, CH3). MS (m/z): 334 (M+100).
As shown in Scheme 1, the diarylethene has two isomers, one with a less extended π-electron conjugation (open state) and the other with a more π-electron conjugation (closed state). Irradiation with UV light (λ1) or visible light (λ2>450nm) can switch the compound between colourless open-ring form 1a and blue closed-ring form 1b.
The diarylethene 1a (20mg) was dissolved in PMMA-cyclohexanone solution (10%, w/w, 5ml). The film was obtained by spreading several drops of the diarylethene-PMMA solution on a quartz glass substrate (25mm×25mm×0.8mm) and dried in air. The thickness of the film is about 10µm. Figure 1 shows the absorption spectral changes of the film. The absorption peak of the closed-ring form of the sample is at 582nm, OD=1.23. The open-ring form has the maximum absorption at 320nm. Before the holographic recording, the diarylethene molecules 1a are initiated with ultraviolet light of intensity 2.6mW/cm2 at λ1=325nm and the molecules are all in closed-ring form with concentration u 0.
Fig. 1. Absorption of the diarylethene doped PMMA film at open-ring form 1a (solid line) and closed-ring form1b (dashed line).
2.2. Holographic grating recording
When 1b is illuminated with two coherent plane beams (polarized normal to the plane of incidence) at λ2 of constant intensities I 1 and I 2, a spatial modulation of concentration ub(x) will be formed and recorded as the holographic grating. The sinusoidal intensity modulation is I(x)=(I 1+I 2)[1+mcos(2πx/Λ)], where m=2(I 1 I 2)1/2/(I 1+I 2) is the intensity contrast and Λ is the grating period. Assuming a two-state model for the molecules ub(x, t)+ua(x, t)=u 0 with a uniform distribution in the film, ub(x, 0)=u 0, and ub(x, ∞)=0, we obtain ub(x, t) [9]
where τ=Nahc/[2303(I 1+I 2)εbϕλ 2] is the recording time constant. Na represents the Avogadro constant; h is Planck’s constant; c is the speed of light; εb is the molar absorption coefficient of molecules 1b at λ 2; ϕ is the quantum yield of the cycloreversion reaction.
When m=1, the theoretical simulation of the normalized concentration of molecules in form 1b is shown in Fig. 2. At the beginning (T 0), the film was just initialized, and all the molecules were in form 1b. Then during (T 1,T 2), the grating changes linearly with the distribution of light, whose profile is sinusoidal. Later (i.e., T 3), the sinusoidal profile is disturbed because of the saturation effects. After very large exposures the grating will turn into a δ-function distribution [10], and the recorded grating will be almost erased.
Fig. 2. Theoretical simulation of normalized concentration of molecules 1b for T 0<T 1<T 2<T 3 under illumination. The grating form changes from a sinusoidal shape at low exposures (linear part) to a “rectangular” shape at high exposures (nonlinear part)
Retaining the zero and the first harmonic of the binomial series expansion of exp[-mcos(2πx/Λ)t/τ], ub(x, t) in Eq. (1) is obtained as
where u b0(t) and u b1(t) are the average and the peak of the (time-dependent) spatial modulation, respectively. The diffraction efficiency η in a first approximation is proportional to the square of the u b1(t) [9,11,12], as follows:
where k is a constant related to properties of the diarylethene and recording geometry.
Two-beam interference technique is used to examine the film characteristics of holographic recording. In this experiment, A He-Ne laser at the wavelength of 632.8nm is used to generate laser beam for holographic recording. The split laser beams in the diarylethene doped PMMA film intersect with a beam to beam angle of 60° in air. The beam diameters are 0.4cm and the power of light in each split beam is 0.4mW. We sample the diffraction power at the frequency of 1Hz (to weaken erasure of recorded grating, the sampling duration time is set by 10ms). The diffraction efficiency η=Id/(Id+It) is determined by measuring the intensities of diffracted beam Id and transmitted beam It. The dotted curve in Fig. 3 displays the change of diffraction efficiency with recording time. The maximum diffraction efficiency of 1.2% is achieved after having recorded for 750 seconds. We substitute the two values into Eq. (3) and plot the theoretical curve (blue solid line). The agreement between theoretical simulation and experiment validates our dynamic model. The small deviation of experimental curve from the theoretical one is mainly due to the small fluctuation of laser power during the exposure.
As shown in Fig. 2 and Fig. 3, before the grating gets saturated, the holographic grating shape is a sinusoidal shape, and the diffraction efficiency rises with the increase of the exposure energy; while after saturation, the holographic grating shape evolves from the sinusoidal shape to the “rectangular” shape, and η slowly drops to zero if the exposure continues.
When the intensity of light in each split beam is adjusted to 6.4mW/cm2 and 8.9mW/cm2 under the same recording geometry of above, the saturation time is reduced to 500 seconds and 268 seconds in the experiment, respectively. This indicates the saturation time is inversely proportional to irradiation intensity, which can be also concluded from the recording time constant equation above.
2.3. Hologram recording
The experimental setup is designed for holographic recording and readout (Fig. 4). The linear polarized light generated from a He-Ne laser passes through a half-wave plate HP1 and a polarized beam splitter (PBS) after filtering and collimating, the light is divided into two parts as a reference beam and a signal beam. A proper power distribution between the reference beam and the signal beam can be implemented by rotating the plate HP1. The half-wave plate HP2 is used to adjust the signal beam polarization. The shutters, S1 and S2, aim to control the time of recording and readout. In this experiment, the optical powers of the reference beam and signal beam are 1.4mW and 0.5mW, respectively. In order to investigate the material’s shrinkage effect [13], the unsymmetrical recording structure is adopted and the incident angles of the reference beam and signal beam are -42° and 18°, respectively, relative to the normal of the film.
Fig. 4. Experimental setup for holographic recording and readout. HP1 and HP2: half-wave plates, S1 and S2: shutters, M1 and M2: reflecting mirrors, PBS: polarized beam-splitting prism, EL: beam-expanding lens, FL1 and FL2: Fourier lens, RC: resolution chart.
Figure 5(a) shows the direct image of the resolution chart with the minimum line width of 13.3µm from the signal beam with no film placed in the beam path. Such image is attenuated as expected due to the film absorption at this wavelength when the sample film is in the beam path. Despite this attenuation, the see-through image, shown in Fig. 5(b), retains clear fidelity similar to the original image indicating negligible light scattering in the film. It demonstrates that our current material preparation and film formation procedures can produce homogeneous and uniform photopolymer films for holographic data storage. Figure 5(c) presents the image of the resolution chart reconstructed by the reference beam. It can be seen that the reconstructed image has almost the same fidelity as the original transmitted image. The whole page of the image of high resolution can be also reconstructed completely when it was recorded. This implies that the shrinkage effect is almost negligible in the sample [13–14]. The reason may be that the formation of gratings depends only on the change of the absorption and refractive index between two molecular states during the recording, and the effect is purely local [15]. This is different from photopolymer materials in which the grating is formed by diffusion and polymerization of monomer [16]. Figure 5(d) shows the reconstructed image after 100 write/erase cycles, which indicates no degradation in quality after 100 decoloration/coloration cycles. In fact, the material could have been written for 1000 cycles or more without degradation. Here, the limitation of our experiment condition made us finish writing/erasing for 100 cycles only.
To investigate the lifetime of the closed-ring isomers of the diarylethene in PMMA films, a sample kept for ten months in darkness at room temperature is used to verify its characteristic of holographic recording. The reconstructed hologram is shown in Fig. 6. The reconstruction of the recorded hologram in Fig. 6 indicates no degradation in quality. In fact, the stability of the colored closed-ring isomers is dependent on the type of aryl groups. Because the aryl group of the sample is thiophene, which has low aromatic stabilization energies, the closed-ring isomers are thermally stable and do not return to the open-ring form isomers even at 80°C [7]. This characteristic of the diarylethene allows a wide range of operating temperatures and long lifetime in darkness.
Fig. 5. (a) Original resolution chart image without the sample film in the beam path; (b) attenuated see-through resolution chart image with the sample film in the beam path; (c) resolution chart image reconstructed with the reference beam (d) resolution chart image reconstructed with the reference beam after 100 write/erase cycles.
Fig. 6. The reconstruction of the recorded hologram after the sample had been kept for ten months in darkness at room temperature.
3. Conclusion
The diarylethene has been developed by a novel synthetic route, and films of which doped PMMA were applied to rewritable holographic storage successfully. Its good characteristics such as high resolution, fatigue resistance, negligible shrinkage, and long lifetime have been validated in this paper. Future work based on these results is in progress and will explore a non-destructive method of reading for the diarylethene doped PMMA films.
Besides rewritable holographic recording, this material can be applied to other fields such as micro-optic element fabrication, and optical information processing. Additionally, the material is promising in optical interconnects and optical switches because of its non-absorption at the long wavelength (>700nm).
Acknowledgments
This work was supported by National Natural Science Foundation of China (No. 60277011) and National Research Fund for Fundamental Key Projects NO.973 (G19990330).
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