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NatureInterface > No.07 > P044-048 [Japanese]

Natureinterface Frontier - For High-Density Optical Storage -- Toshifumi Okubo









For High-Density Optical Storage

TOSHIFUMI OHKUBO

Professor, Department of Mechanical Engineering, Toyo University


Research for light-density optical memory utilizing the optical near-field principle, which does not include propagating into space, is now proceeding. Herein, backgrounds and basic principles of this research are overviewed, and the frontier of this technology is introduced, in which a sub-micron-size bit is detected with the use of a compact head assembly almost applicable for practical use.

Rapid advance of storage density performance

Explosive development of the information society has increased personal opportunity to deal with large-capacity and highly defined digital data much superior to that one decade ago, such as pictures, images, videos, and sounds. These advances of digital information processing are due to the performance improvement of personal computers and network systems. And, rapid advances of both large capacity and high data transfer rate for storage system are also important. Figure 1(a) shows the recording density trend of a hard disk drive (HDD), which is representative storage device. The recording density has increased at a surprising rate of 30 percent per year even ten years ago. Recently, the rate increases up to 100 ~ 120 percent, and the recording density is doubled year by year. It looks like as if the development would last forever. Specifically, as shown in Fig. 1(b), the highest density was about 150 Mbits per square inches (almost equivalent to one side of five hundred Japanese yen) in 1992. The information capacity was same as only 16 floppy disks. Now, it has increased more than 200 times over last ten years so that it contains large capacity equivalent to 3500 floppy disks. This means not only that the information cabinet becomes larger but also that a person can easily treat a large amount of information which was not considered in previous times and distribute the information. This is a good example that rapid advance of the equipment performance greatly changes and creates the application itself.

Although the HDD has been taken as an example of a representative storage device from the beginning, there is another important storage system, the optical disk drive. While the HDD attaches importance to high data transfer rates and direct data processing as a computer desktop file, the optical disk drive has an advantage of back-up application and medium compatibility due to the standardization. Although networks have been developing today, a transfer of large-capacity information over a few tens to a few hundreds mega bytes is still difficult without an exclusive line. The optical disk device is just the only one storage system which easily delivers large volume of digital data. The optical disk drive has taken the advantage over the HDD, since a micron-sized laser spot could have been applied for read-out from the beginning of its practical use. However, the recent performance of the HDD has been greatly developed, as the result, the optical disk drive has been left behind a little. This has been caused by the tight restriction of the compatibility due to the standardization, and moreover, the fundamental limit described later has influenced greatly.

Further miniaturization of a focused optical spot

The property of the recording density of an optical recording system greatly depends on the miniaturization of a focused optical spot by a lens (minute optical brush) as shown in Fig. 2(a). Since the minuter beam spot can read out the smaller pattern, much effort has been focused on the miniaturization of °»optical brush°…. However, the parameters that determine an optical spot size are not so much. These are utilizing a blue laser with a short wavelength (¶ň) and applying a lens with a large numerical aperture (NA). The refractive index of a space between a lens and an optical spot (n; usually air) also attributes to the miniaturization of an optical spot size. For example, insertion of immersion oil between an objective lens and a sample improves the spatial resolution of an optical microscope. This is because the immersion oil increases the refractive index of the space (or, this is considered to be due to the shortening of the wavelength which is equivalent to the reduction of the light speed propagating in the space with a high refractive index).

An optical spot can be miniaturized with the design of these three parameters in the conventional optical system in which a laser beam from far field is focused by a lens. However, the great performance improvement has been a significant challenge since the blue laser has invented. Therefore, the technology to break through this difficult situation has been proposed in 1990°«s; this is the innovative optical head that mounts a lens on a flying head slider which is used in HDD, as shown in Fig. 2(b). Now, the flying head slider flies above a magnetic medium rotating at high speed with the minimum separation of 12 to 15 nm, which is much shorter than the molecular mean free path of the air (the average distance that a molecule moves from colliding with one molecule to another molecule: 64 nm at the atmospheric pressure). Applying this mechanism to °»optics°… makes it possible that focused optical spot is formed just under the slider and the focal point from the lens is ultimately shortened. In other words, this means that the space between the lens and the focused optical spot is filled with a material of the slider core, and greatly increases the refractive index. This method is called, Solid Immersion Lens (SIL), and is considered to be a powerful card for improving the recording density of the optical storage system. However, this method does not overcome the principle of the miniaturization of the optical spot, and is restricted by the applied material and the optical system.

Is there another approach for further miniaturization of the optical spot instead of this technology?

One possible approach is to focus the optical spot compulsorily into a much smaller size with a metal aperture mask (pinhole). Usually, the light converges again from this pinhole due to the diffraction phenomena, and the minute optical brush cannot be formed. Here, if the pinhole gradually becomes smaller than the wavelength, the light that passes through the pinhole and propagates from other side gradually reduces. On the other hand, a non-propagating optical field is formed in the proximity of the pinhole. This phenomenon is like the water flow from a faucet as shown in Fig. 2(c). The propagating light from a large pinhole is like the water flow flashing from a large faucet; that from a minute pinhole smaller than the wavelength is like a drip of water overhanging from a faucet due to the surface tension. Because the overhanging water drip is as large as the size of the faucet, the field of the non-propagating light becomes as large as the pinhole. This field does not literally propagate, so that this cannot be observed as it is. A scattering material like a recording medium, however, interferes this field and changes it a propagating light. Then, a detector placed far away from the back surface of the medium can catch the field. The approach applying the optical field property of the optical field has been studied for achieving much higher recording density than the conventional system which is restricted by °»the limit of the focused the optical spot°…. (Repeated speaking, the optical field formed in the proximity of the pinhole can not be observed directly. Numerical simulation, however, is able to verify that °»a drip of water°… overhangs from an edge of the minute pinhole which is smaller than the wavelength (Fig. 3). )

For realizing a near-field optical recording system

The next step is to detect minute recording bits smaller than sub-micron size at practical data transfer rates, which corresponds more than 1 MHz in frequency, based on this principle. Since the size of °»water drip°… is almost as same as the pinhole, as described above, the pinhole must be moved (scanned with) a few tens of nm distance from the medium surface at the relative velocity of several meters per second. Figure 4 shows an experimental setup practically verifying this method. For high speed scanning of the pinhole, the mechanism of the flying head slider described above was used and a special slider, made of transparent sapphire, was newly fabricated. A laser beam, which was focused to 1.5 micrometer diameter with a high-resolution optical microscope, illuminated the back surface of the pinhole to form the near-field light around the pinhole. A striped metal pattern with a minimum width of 200 nm was carefully fabricated on the surface of the quartz disk as a ROM pattern. The intensity change of the scattered light was detected with ultra high sensitivity when the near-field light around the pinhole traversed this pattern. The signal corresponding to the 200-nm-wide pattern has been detected at high frequencies of 5 to 7 MHz, and the fundamental potential of the near-field system has been verified.

There remain so many problems in realizing this optical storage system. One of them is an optical system with a large lens placed at the back of the slider (pinhole) shown in Fig. 4. It is necessary to remove this and realize the focusing function within the compact head assembly (Fig. 5). The key is to guide the light into the pinhole efficiently, not hindering the flying head slider function of accurately following the medium surface as a cruise missile. From this viewpoint, a resinous waveguide is introduced. It is usually used in an optical circuit, etc. and delivers the light between elements. It includes a core of 10 um square which is optically molded and delivers the light, and a clad which covers the core and has a refractive index slightly different form that of the core. This waveguide can realize the flexibility suitable for a slider suspension and efficient by transfer of the light. The edge of the waveguide is ground to 45 degrees inclined surface and is used as an extremely small total reflective mirror. The photolithography technique can fabricate a focusing lens of several hundred micrometer diameter at the back of the slider. Although assembling the several parts in the composition of Fig. 5 is still needed, this surely becomes the same order of the magnetic head as a small and compact optical head. The result of high seed read-out of 200-nm-wide pattern is shown in fig. 6. This system, which is on the way to the final version as shown in fig. 5, is achieved by the assembly system that delivers the light with the silicon mirror and the optical fiber. This certainly advances the miniaturization of the optical head.

Conclusion

The demands for the higher storage performance show no sign of slowing down. As described in this paper, the demands and the technology for realization are in complemented and synergistic state each other. The technology endlessly generates a new request and demand, and then a new necessity generates the technological innovation. Although the termination of the demands for the ultra high density and speed is beyond our imagination, there are many margins and inversely many difficulties until the demands are satisfied. The author hopes that this report evokes your interest and concern.

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