1. Introduction

Optical data storage is a technology in which information is recorded in a bitwise manner by changing the physical state of a recording material in response to a laser beam focused on the surface of the recording material. This technique was developed in the first generation as the Compact Disc, in the second generation as the Digital Versatile Disc, and in the third-generation as the Blu-ray Disc. In each generation, the recording density was improved by reducing the size of the recording spot through improvements in the numerical aperture of the objective lens and by shortening the wavelength of the semiconductor laser. However, the high absorption of recording media at the shorter wavelengths of ultraviolet radiation and the difficulties in increasing the numerical aperture of the lens make it difficult to achieve further improvements in performance. As a result, holographic memory is attracting attention as a possible technique next to Blu-ray as a data-storage technology.

To record data on holographic memory, signal and reference lights are irradiated to medium, and the interference fringes are recorded in the recording material. To read out the data, only the reference light is irradiated to the interference fringes to reconstruct the signal image [1–4]. In holographic memory, multiple signals of two-dimensional page data can be recorded and read selectively from a single position [2–7]. These characteristics permit the high recording densities and high data transfer rates. The Holographic Versatile Disk is an international standard for holographic memory and employs a collinear holographic system that can write and read data using a single optical axis with a spatial light modulator (SLM) as the key device [8–10].

Holographic recording media can be roughly divided into phase-type and amplitude-type media; the latter can be subdivided into refractive-index-modulation type and uneven-modulation-type media. Most current systems for holographic memory use photopolymers of the refractive-index-modulation type. However, photopolymers require light shielding and are not possible to rewrite data onto them; rewritability as well as long-term storage stability is desirable for practical application. Thin films of amorphous rare earth–transition metal alloys might satisfy these demands. Magnetic holograms have been recorded on thin films of MnBi and TbFe alloys as magneto-optical recording materials [11–14]. The basic principle of writing a magnetic hologram on thin-film media is called “thermomagnetic recording” or “magneto-optical recording”; a perpendicularly magnetized film is locally heated to Curie temperature or higher by irradiating a focused laser beam, so that the magnetization of the heated region is reversed by stray magnetic fields during cooling. By using this mechanism, the interference pattern of light can be recorded as differences in the direction of magnetization in the magnetic hologram. To achieve a temperature distribution corresponding to the interference fringe, the irradiation time of laser beam needs to be sufficiently short to suppress thermal diffusion during heating. A pulse laser with a short pulse width is therefore used for recording.

The resulting magnetic hologram can be considered as a magnetic binary grating. When a plane wave that is linearly polarized in the x-direction, as shown in Fig. 1, is incident perpendicular to the magnetic film, the polarization planes of the light that passes through the magnetized regions + M and –M are rotated in the opposite direction for example + θ_F and -θ_F, respectively. As a result, the x-component of the transmitted light passing through the + M domain has the same amplitude and phase as that passing through the –M domain. On the other hand, although the y-component of the light passing through the + M domains has the same amplitude as that passing through the –M domains, its phase is shifted by 180°. Therefore, because the phase of the y-component of linearly polarized light is modulated by the magnetic grating, diffraction and interference occur. The first-order diffracted light becomes the amplitude component in the x-direction, and the polarization plane is rotated by 90° to that of the incident light. Because the light intensity of the reconstructed image of magnetic hologram depends on the Faraday rotation angle, it is desirable for the recording material to have a large Faraday rotation angle, perpendicular magnetization, and a high transmissivity with small losses. Because of these requirements, especially that of transmissivity, thin films of amorphous rare earth–transition metal alloy are unsuitable. In this work, therefore, we demonstrate the performance of magnetic volumetric holograms using a highly transparent ferromagnetic garnet film [15] as a recording medium.

 

Fig. 1 The principle of the reconstruction of a magnetic hologram.

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2. Experimental

Figure 2 is a schematic illustrating the collinear optical setup used in our experiments. Because in magnetic holography the plane of polarization of the diffracted light is rotated by 90° from that of the incident light, a polarizer was placed in front of the CCD camera so that only the diffracted image from the magnetic hologram was recorded. A pulsed YIG laser with a wavelength of 532 nm and a pulse width of 25 ns was used for recording and reconstructed the hologram. In addition, a focus position was adjusted by the sharing interference method using He-Ne laser. Ferromagnetic garnet films with the composition Bi_1.5Dy_1.0Y_1.0Fe_3.8Al_1.2O_x, deposited on nonmagnetic substituted gadolinium gallium garnet (SGGG) substrate by radiofrequency magnetron sputtering, were used as magnetic recording media. The thickness of the films was 3.3–4.0 µm. The garnet films were crystallized by rapid thermal annealing at 750 °C for 10 min in air, because the as-deposited films were in an amorphous state. A Helmholtz coil was used to apply a magnetic field to fix the direction of magnetization of the recording medium and to erase the hologram.

 

Fig. 2 Schematic illustration of optical setup for collinear holography.

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3. Results and discussion

Figure 3(a) shows a cross-sectional view of the garnet film used in our experiments. This shows that the garnet polycrystalline film consisted of grains measuring 30–50 nm. Because the magnetic coupling between these grains is thought to be weak [5], the magnetization of each particle can be individually reversed. As a result, the interference pattern of the collinear hologram can be recorded with a sufficient resolution by these nanoscale grains. Figure 3(b) schematically shows the image of interference fringes of a magnetic hologram; the gray domains correspond to regions where the temperature rises to the Curie temperature or above as a result of laser irradiation, whereas the white regions correspond to regions where the temperature remains below the Curie temperature. As a result of the presence of stray magnetic fields, the magnetization in the gray domains should be in the opposite direction to that in the white domains. Since the interval between successive stripes in a collinear hologram is about 1 µm, it is difficult to record this complex interference pattern by reversing normal magnetic domains. However, magnetic garnet films containing grains that are weakly magnetically coupled were expected to able to record the interference fringes, as shown in Fig. 3(b), because each individual grain can maintain its magnetization independently of other grains [16]. In this case, the limit of resolution depends on the grain size. Because the grain size is about 50 nm or less, 20 or more grains can exist in a linear segment measuring 1 µm. As a result, the fabricated garnet film is capable of recording the interference fringes of a collinear hologram.

 

Fig. 3 The microstructure of recording magnetic garnet film and the image of recording. (a) Cross-sectional image of a typical garnet film used in this study. (b) Schematic showing an interference fringe of a collinear hologram and a model of the magnetic cluster consisting of fine grains that is needed to record the interference fringe.

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The pattern shown in Fig. 4(a) was recorded in this garnet film by using a collinear system. To read the recorded image, the reference beam modulated as the pattern shown in Fig. 4(b) displayed on the SLM was shone onto the garnet film, and the reconstructed image shown in Fig. 4(c) was obtained. This reconstructed image could be erased by applying an external magnetic field, as shown in Fig. 4(d), confirming that it was magnetically written in the garnet film and that the film was rewritable. This is the first report of the recording of a magnetic hologram in a magnetic garnet film by thermomagnetic imaging and its subsequent reconstruction. Furthermore, when we compare Figs. 4(c) and 4(d), it can be seen that the background noise around the reconstructed image disappeared on applying the external magnetic field. This means that this noise was formed by magnetic hologram recording in a manner similar to that of the reconstructed image.

 

Fig. 4 The signal image of magnetic hologram: (a) Recording pattern of the collinear hologram and (b) reconstruction of the signal image, and the reconstructed images of the magnetic collinear hologram: reconstructed signal image (c) before and (d) after applying magnetic field. The recorded signal image was erased, and background noise decreased by applying the magnetic field. These results proved that the signal image was magnetically written in the garnet film and that the film was rewritable.

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To verify the multiplicity of the magnetic hologram, four spots of circular signal light were recorded with the shift pitch of 150 µm, as shown in Fig. 5(a). Because the diameter of each spot was 330 µm, the second and third spots were completely overlapped by their neighboring spots. Figure 5(b) show the state of the reconstructed signal lights and the relationship between the intensity of the reconstructed signal light and the displacement of the sample when the intensity was measured at 10 µm intervals. In the detected signals, each reproduction signal light could be separately distinguished, regardless of whether the recorded spots overlapped, and the detected spots could be reconstructed separately. Although the background noise was about 35 a.u., crosstalk was not detected. To maintain information on recordings when the corresponding spots on the recording media overlap, the interference pattern has to be recorded volumetrically as a magnetization distribution or a reflective index distribution. Therefore, our results confirm that the interference pattern was recorded volumetrically in the polycrystalline magnetic film.

 

Fig. 5 Shift multiplex recording of a magnetic collinear hologram. (a) Schematic showing the shift multiplex recording. (b) The intensity of the signal light and the corresponding reconstructed image plotted against the position in the medium.

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4. Summary

We demonstrated the recording and reconstruction of a magnetic hologram in an optically transparent magnetic garnet film, with the aim of producing a holographic memory that is rewritable and has long-term stability. We showed, for the first time, that a magnetic hologram can be recorded in a garnet film by thermomagnetic writing and that the recorded hologram can be successfully reconstructed. We also examined the multiple recording properties of the medium and we showed that individual holograms could be reconstructed even though the locations of their recordings overlapped. Because items of information could be retained in the magnetic garnet film regardless of the overlap of their recording locations, the corresponding interference fringes must have been recorded volumetrically. We believe that the directions of magnetization of individual grains in the magnetic garnet films that we used in this experiment can change independently of one another because of the polycrystalline structure of the material, which results in weak magnetic coupling between grains. This permits the recording in the garnet film of the complicated interference pattern of the collinear hologram.

Unfortunately, the ratio of the intensity of the background noise to that of the signal was high in this work; this might be improved by recording clearer interference fringes through control of the thermomagnetic writing conditions and by suppressing light scattering by reducing the size of the crystal grains of the magnetic garnet films.