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Abstract
Hologram memory is a strong candidate for optical storage due to its high recording density and high data transfer rate. We have studied and engineered a magnetic hologram memory medium using a stable magnetic garnet as recording material. To record a deep and clear magnetic hologram, it is important to control the heat diffusion generated during recording. Numerical simulation suggested that a multilayer structure with transparent heat-dissipation layers would be effective to address this. We fabricated a multilayer magnetic medium for a collinear magnetic hologram. This medium exhibited a diffraction efficiency as high as that of the single layered one, and errorless recording and reconstruction was achieved with the magnetic assist technique.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Hologram memory is a promising data storage technology with high recording density and fast data transfer rate [1–11]. Photopolymers have been widely used as hologram media because of their high diffraction efficiency and transparency. The disadvantage of photopolymer-based media is that it is a write-once media. Magnetic holograms, on the other hand, are a candidate for rewritable holograms; they also display long-term stability. In magnetic holograms, the interference pattern of light can be observed as differences in the direction of magnetization in magneto-optical recording materials [12–15]. We have studied a magnetic hologram media using polycrystalline magnetic garnet films, which has transparency and long-term stability [16–23]. Volumetric magnetic holograms are recorded on magnetic films with a high resolution, because polycrystalline garnet films can have small magnetic domains, originating from crystal grains by weak magnetic coupling between grains [24]. In a magnetic hologram, a light interference pattern is recorded onto a magnetic film as magnetic fringes by the thermomagnetic method [12,25]. Figure 1 schematically shows the recording process of the magnetic hologram. When signal and reference beams are radiated in a vertically magnetized film, a temperature distribution corresponding to interference fringes is formed by the absorption of light in the magnetic film. Magnetization of the region, where the temperature is raised higher than the Curie temperature, is lost, and the magnetization of the region is reversed by stray magnetic fields from the surrounding region and/or external magnetic field in the subsequent cooling process. Finally, the magnetic hologram is recorded as magnetization orientations corresponding to the interference fringes. A written hologram can be reconstructed by a magneto-optical effect such as the Faraday effect [12–15], and a large Faraday rotation angle results in a bright reconstruction image. We have succeeded in recording and reconstructing magnetic hologram without error [22]; however, the diffraction efficiency of magnetic garnet films, such as bismuth substituted rare earth iron garnet (Bi:RIG), would not be sufficient to apply to actual storage devices.
Fig. 1. Principle of formation of a magnetic hologram. (a) Perpendicularly magnetized film, (b) the magnetization of certain regions is lost due to an increase in temperature above the Curie temperature; this is caused by the constructive interference between the signal and reference beams, (c) a stray magnetic field from the surrounding areas having the opposite direction in the non-magnetized regions, and (d) a magnetization distribution corresponding to the interference fringes formed after cooling.
An increase in the Faraday rotation angle θF is an effective solution to enhance the diffraction efficiency which is an index of brightness; this can be obtained by using a large Faraday rotation materials and also by forming deep magnetic fringes, dw. The depth of the magnetic fringe can be enlarged by increasing the recording energy density. In practice, however, the diffraction efficiency is saturated because of the merging of adjacent magnetic fringes near the surface, due to the thermal diffusion of excessive heat at the high recording energy conditions, which limits the effective depth of the magnetic fringes to about 1.5∼1.7 µm [17]. Numerical simulations suggested that the insertion of heat dissipation layers (HDLs), which do not absorb the light energy, would be effective in suppressing the merging of the fringes [21]. However, it is not clear whether an accurate reconstruction image can be obtained from a hologram recorded in discrete recording layers separated by HDLs. In this study, a multilayer medium using SiO2 as HDLs was fabricated in order to determine the necessary conditions for recording and reproduction of magnetic holograms, and it was verified whether the multilayer film could be used as the recording medium for magnetic hologram by evaluating the diffraction efficiency and error ratio.
2. Experimental
A multilayer sample was fabricated using the ion beam sputtering method. The Bi:RIG layers were deposited from a target with the composition ratio of Bi:Dy:Y:Al:Fe = 1.5:1.0:1.0:1.2:3.8, and SiO2 layers were deposited as HDLs. The deposition conditions are shown in Table 1. A simple multilayer structure with nine pairs of a 242 nm thick Bi:RIG layer and a 358 nm thick SiO2 layer, was deposited on a substituted gadolinium gallium garnet (SGGG) substrate. The sample had a combined total of approximately 2.2 µm thick layer of Bi:RIG. The thickness of the uppermost SiO2 layer was set to 170 nm to decrease the reflection on the surface. Since the deposited Bi:RIG is not crystallized, a rapid heat treatment at 750 °C for 15 min in air was performed to crystallize the Bi:RIG. This was done each time 2 pairs of Bi:RIG layers were deposited. However, the thermal diffusion performance of this sample is unknown because thermal analysis was not performed. Figure 2 shows the designed structure of the Bi:RIG/SiO2 multilayer medium and the cross sectional SEM image of the fabricated sample. For comparison, a 2.7 µm thick Bi:RIG single layer medium was also fabricated.
Table 1. Deposition Conditions
The diffraction efficiencies of the fabricated media were evaluated with a 2-beam interferometer. The experimental setup used in this work is the same as that used in an earlier study [23]. A pulsed laser with a wavelength of 532 nm and a pulse width of 50 ps was used for recording a magnetic hologram. The diffraction efficiency, η, is evaluated by
where I0 is the intensity of zero-order transparent light, and I1 is that of the first-order diffracted light. The period of interference fringe was set to be 1500 line pair/mm. Furthermore, the collinear interference system was used to record and reconstruct magnetic holograms in the samples. The same pulsed laser was divided into two beams (a signal beam and a reference beam) using a digital mirror device [22,23]. Interference patterns were recorded by the magnetic assist (MA) recording method under magnetic fields between 0 Oe and 160 Oe. We used 48 × 48 pixels with 3:16 encoding methods for the signal [22]. The reconstructed images were obtained by radiating only the reference image; and the pixel error ratio (PER) was evaluated using the following equation: where Npage is the number of pixels in the signal region, and Nerror is the number of error pixels.3. Results and discussion
The transmittance of the single layer and Bi:RIG/SiO2 multilayer media can be seen in Fig. 3(a). As shown in this figure, the multilayer medium exhibits a photonic band gap, due to periodic structure, at a wavelength of around 680 nm, while the transmittance at a wavelength of 532 nm, which is the wavelength used in this study, is about 27%. The Faraday loop at the wavelength of 532 nm can be seen in Fig. 3(b). The Faraday rotation coefficient of the multilayer medium was 2.70 deg./µm, and that of the single layer film was 2.43 deg./µm. This means that the layered deposition process with SiO2 did not degrade the Faraday rotation of the Bi:RIG in the multilayer medium. In addition, the coercivity of the multilayer medium was found to be larger than that of the single layer medium, possibly due to the thermal stress induced by the heat treatment of the Bi:RIG layers. Since the thermal expansion coefficient of Bi:RIG is larger than that of the SGGG substrate and SiO2, tensile strain was applied to the thin Bi:RIG layer during the cooling process, and the perpendicular magnetic anisotropy was considered to be enhanced by the inverse magnetostriction effect.
Fig. 3. Optical properties of the single layer and HDL multilayer media: (a) transmissivity and (b) Faraday loop.
The diffraction efficiency of the multilayer and single layer media can be seen in Fig. 4(a). As shown in this figure, the diffraction efficiency of the multilayer medium was smaller than that of the single layer film, contrary to the fact that the numerical simulation suggested that the HDL medium would show a better diffraction efficiency [21]. This low diffraction efficiency of the multilayer medium is attributed to small stray magnetic fields due to the thin Bi:RIG layers in the HDL medium [23]. The stray magnetic field in the magnetic fringes was calculated using the simulation model shown in Fig. 5(a) with various Bi:RIG thicknesses. Figure 5(b) shows the calculation results of average stray magnetic fields along the center of a non-magnetized region, and the stray magnetic field distributions in a non-magnetized region at several aspect ratios are shown in the inset. The numerical simulation suggested that the strength of the stray magnetic field would decrease rapidly as the thickness of the magnetic layer decreased under the same fringe period; the stray magnetic field at the thickness of 242 nm was about 57% of that of around 1.6 µm, which is a possible fringe depth in single layer medium [17]. To compensate for the reduction of the stray magnetic field, MA recording could be used [23]. Therefore, MA recording was performed with an assist magnetic field strength of up to 160 Oe applied in the direction opposite that of the initial magnetization. The resultant maximum diffraction efficiencies of the multilayer and single layer media are shown in Fig. 4(b). The diffraction efficiency of the multilayer medium reached as high as that of the single layer medium as shown in this figure. This means that the reduction of stray magnetic field was compensated by MA recording. However, the improvement of the diffraction efficiency due to the insertion of SiO2 heat dissipation layers was not observed. This may be partially explained by the merging of fringes near the surface, because this multilayer structure is not adequately designed for thermal diffusion, and the resultant thickness of the magnetic fringe is similar to that of the single layer film. In fact, the numerical simulation suggested that the magnetic fringes merged in the top two layers. However, since this structure is considered to record magnetic fringes with the same depth as that of a single layer film, collinear data recording was performed using this multilayer film.
Fig. 4. Diffraction efficiency of the single layer and multilayer media (a) without and (b) with MA recording.
Fig. 5. Calculated stray magnetic field in the magnetic fringe: (a) calculation model and (b) the calculated average stray magnetic field along the center of a non-magnetized region and the stray magnetic field distribution in a non-magnetized region at several aspect ratios (inset). Negative values mean the direction of the magnetic field is opposite to the initial direction.
Two-dimensional data with 48 × 48 pixels were recorded on the multilayer medium using a collinear interference system with MA [22,23]. The signal pattern can be seen in Fig. 6(a), and Fig. 6(b) shows the image reconstructed from the hologram recorded on the HDL medium with a recording energy of 70 µJ and assist magnetic field of 160 Oe. As shown in this figure, the data were successfully reconstructed even though the recording layers discretely existed in the medium. Moreover, Fig. 6(c) shows the evaluated error ratios with various recording energies using MA. The error ratio decreased with increasing recording energy and essentially reached zero for recording energies over 80 µJ without MA. On the other hand, the error ratios recorded with MA were smaller than that without MA, and the error ratio became substantially zero at a low recording energy of about 50 µJ under 160 Oe. This means that multilayered structures in which the recording magnetic layers are not continuous, but exist discretely, can be used as recording media for magnetic hologram memory with MA.
Fig. 6. (a) Recording pattern of the collinear hologram. (b) the reconstruction of the signal image with MA recording, and (c) the error ratio of the reconstructed image from HDL medium with various recording energies and assisted magnetic fields. The error ratio decreased with increasing recording energy and assist magnetic field.
4. Conclusions
We investigated the applicability of HDL media for magnetic holograms. The multilayer medium, in which the recording magnetic layers discretely exist in the medium, showed diffraction efficiencies comparable to the single layer medium, and errorless reconstruction of the data was achieved using a collinear system with MA. This means that the multilayered HDL medium can be used as the recording medium for magnetic hologram memory with MA. In this work, although the diffraction efficiency of the multilayer medium was not improved, it may be achieved by implementing an appropriate design for heat diffusion. The HDL medium, capable of recording deep magnetic holograms, is believed to be effective as a volumetric magnetic hologram-recording medium.
Funding
Japan Society for the Promotion of Science (A 15H02240, S 26220902).
Acknowledgments
We gratefully acknowledge Mr. Kodai Kawazu and Mr. Zen Shirakashi for their experimental support.
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