1. Introduction

As the power of modern bit-oriented optical data storage systems continues to advance, the demands on its capacity and data transfer rate grow. There has been proposed various bit-oriented methods to boost the capacity, such as reducing data pit size [1, 2], multiplying recording polarizations [3, 4], and stacking recording layers [5, 6].

In these methods, however, the data transfer rate in recording and reading is limited by jitter, which is the deviation between a reconstructed bit sequence and a periodic synchronous idle from an electronic clocksignal generator. As the transfer rate increases jitter increases in general bit-oriented storage systems such as DVD and Blu-ray. As the jitter increases, bit error rate (BER) increases eventually. A further difficuly is that downsizing the data pit to boost the capacity gives rise to two problems: some overlap between read-out signals from adjacent pits; a resulting deterioration in signal-to-noise ratio (S/N). This is because the general storage systems only detect the reflectivity of the data’s bit sequence with a reflection-type optical pickup head. As a result, high jitter and low S/N reduce the overall effectiveness of existing mass storage system.

Our laboratory is currently investigating several techniques to perform jitter-free mass storage including the use of nanoparticles as recordable pit strings as well as the introduction of confocal microscopy for bit data reading. We have demonstrated a jitter-free single-layered storage disk system using 500-nm-diameter particles, which are arrayed against linear grooves [7]. The point we wish to emphasize is that the confocal microscope can detect two signals of: not only the reflectivity of data pits; but also the optical shape of particles. The shape signal produces a clocksignal, or a detection window, for data decoding. The particles storage disk technique has provided the bit-oriented storage system without jitter. Besides, photosensitive nanoparticles have attracted tremendous attention because of their photophysical properties which can be exploited in many optical applications [8].

As a next step, aiming at higher densification of the storage disk, we necessarily downsized the particles and adopt a closest-packed structure in their arrangement. In this case, however, the particle’s diameter being under both the axial and lateral spatial resolutions of the conventional confocal microscope, it cannot detect the data’s bit sequence recorded or the shape signal. To overcome the difficult research challenge, novel ways of bypassing the diffraction limits have to be incorporated into storage disk structure. In what follows, the current status of each of these on-going efforts is discussed.

In this paper, we propose a new design for a jitter-free mass storage disk in order to boost the capacity of the particles disk as well as decrease the BER even in the similar closest-packed structure. To this goal, we have adopted two approaches: one is buffer ring, which is non-photosensitive area around the photosensitive dye-doped particle; the other is multi-layer stacked three-dimensional particles, which are downsized to 300 nm in diameter. We have fabricated the buffer rings, or clearance gaps between adjacent particles, in the storage disk as a semi-hexagonal close-packed structure. The buffer ring enhanced the contrast of the shape signal even by the conventional confocal microscopy to allow measuring particles of size under its spatial resolutions. Combining the multi-layer stacked particles disk having the buffer rings with the confocal microscope, we have realized higher storage capacity and lower BER.

2. Experimental procedure

2.1 Sample preparation

Figure 1 illustrates a process to prepare the multi-layer stacked three-dimensional particles disk. First, in order to adjust substrate’s wettability,polymethyl-methacrylate (PMMA), 4.0 wt% solution in cyclohexanone, was spin-coated on a 26 mm x 38 mm glass substrate at a spin rate of 1st:500 rpm for 3sec, 2nd:3000 rpm for 10sec. Its film was baked for 2 hour at 60°C. Secondly, a 300-nm-diameter polystyrene particles solution was spin-coated on the thin PMMA film at a spin rate of 1st:500 rpm for 3sec, 2nd:2000 rpm for 10 sec. The spin rate was suitably adjusted to account for the surface tension between the thin PMMA film and the particles solution. The particles solution was composed of a particles suspension having a variance coefficient of equal to or less than 3%, in 10 wt% aqueous suspensions and a 10-mM sodium dodecyl sulfate (SDS) solution as a surface-active agent. Here, the particles suspension-to-SDS solution-mass ratio was 1:1. Since the polystyrene particle could easily dissolve under heat, it was dried naturally for about 1 hour. As a result a single-layer particles disk was fabricated with the buffer rings or clearance gaps between adjacent particles. Their width was controlled by changing the mass ratio of PMMA and SDS, the spin rate and the dry method. Finally, reiterating the process, we produced the multi-layer stacked three-dimensional particles disk. Here, a xanthene dye as the photosensitive material was pre-doped in the particle. A change in confocal reflectance due to saturable absorption and photobleach is used for short-term and long-term data storages, respectively.

 

Fig. 1 Process to prepare the nanoparticles disk with buffer rings.

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2.2 Experimental setup

Figure 2 presents a schematic of the optical setup for the jitter-free nanoparticles optical storage. The data recording and read-out apparatus with the reflection-type confocal scanning microscope has laser light sources: a Nd:YAG SHG laser (λ = 532 nm) and a He-Ne laser (λ = 633 nm), respectively. The He-Ne laser beam (λ = 633 nm) is expanded 5 times with a beam expander. The expanded beam reflected by BS1 passes through the two beam splitters and objective. The expanded beam is focused onto the disk sample surface through an objective lens (NA = 0.75). The confocal reflection signal from the surface is measured through a pinhole (ϕ = 0.5μm) with a photo detector. The disk sample is attached to a 3D auto stage controlled with a computer. An approximate particles position is checked with a transmission-type Koehler-illumination and a CCD image sensor.

 

Fig. 2 Optical setup for reflection type confocal laser scanning microscopy: BS1, BS2, BS3, beam splitters; L1, L2 spherical lenses; PD, photo detector.

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The axial spatial resolution of the above confocal microscope was pre-measured by scanning a mirror instead of the disk sample. Figure 3 shows a profile of the normalized confocal signal along an optical axis. The microscope’s resolution was defined as the Full Width, at Half Maximum, (FWHM) of the profile. The axial resolution measured about 1.25 μm; it was much larger than the diameter (300 nm) of the particles. What we should note, however, is that the conventional confocal microscope is expected to image the arranged particles. This is because the buffer ring will help to improve the S/N of the reflected signal and the resulting contrast in the confocal image. To this end, we shall now move towards further investigations in the following.

 

Fig. 3 Profile of normalized confocal reflection signal along an optical axis.

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3. Experimental results

In the first place, two single-layer particles-disk samples without and with the buffer rings were observed with an Atomic Force Microscope (AFM). On the one hand, Fig. 4(a) shows a profile of the hexagonal close-packed particles-disk sample; which has no clearance gap around the particles. On the other hand, Fig. 4(b) shows that of the semi-hexagonal close-packed particles-disk sample; Fig. 5 displays a bird’s-eye view of the figure. A closer look at Fig. 4(b) and Fig. 5 reveals that the particles successfully have a small clearance gap (≈30 nm) around themselves. When the confocal microscope reconstructs the data’s bit sequence of particles, the clearance gap works as the buffer ring for enhancing the image contrast.

 

Fig. 4 AFM micrographs of nanoparticle’s disk: (a) with no buffer ring, (b) with buffer rings.

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Fig. 5 Bird’s-eye view of AFM micrograph of the nanoparticle’s disk with the buffer rings.

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In the second place, a multi-layer stacked particles-disk sample with the buffer ring was laterally visualized with the confocal microscope which focused on the top of the particles; the first layer or the deepest layer of the sample was exemplified in an image pickup operation (see Fig. 6 ). This figure demonstrates that the particles involve the buffer rings. The point we wish to stress is that the microscope with a lateral resolution down to about 400 nm imaged the particles with a diameter of 300 nm as well as the buffer rings with a gap width of 30 nm. The buffer ring made it possible to identify the particles positioned in the deepest layer with the conventional confocal microscope.

 

Fig. 6 Cofocal image of the first disk layer.

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In the third place, a clocksignal as a detection window, Fig. 7(a) , was extracted from the confocal image, in which a shape signal of the arranged particles was the confocal signal scanned at the area surrounded by the dotted line in Fig. 6. Figure 7(b) shows that the shape signal produces the non-periodic binary clocksignal, even in the multi-layer stacked particles-disk sample. As a result, no jitter occurs on the ground that the shape signal itself generates the detection window for data decoding; no fluctuation of the particle’s position becomes a major problem in the data reconstruction. The BER, which depends not on the jitter but on only the reflectivity of the dye-doped particles, is necessarily lowerd.

 

Fig. 7 (a) Confocal reflection signal, or particle’s shape signal. (b) non-periodic binary clocksignal from the shape signal.

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Furthermore, the confocal microscope provided a cross-sectional view of the multi-layer stacked particles-disk sample with the buffer rings (see Fig. 8 ). Nanoparticles were three-dimensionally arranged with the buffer rings, though there were some structural defects; the confocal image contrast of the buffer ring sample increased compared with that of the non-buffer ring sample. The results presented that the improvement of the image contrast led to the confocal image reconstruction in the multi-layer stacked particles disk. The important point to note is that the microscope with an axial resolution of 1.25 μm resolved the deep particles with a diameter of 300 nm.

 

Fig. 8 Cross-sectional confocal image of the multi-layer stacked nanoparticles disk with the buffer rings.

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5. Conclusions

In this study, a multi-layered nanoparticles optical disk with buffer rings has been developed for a jitter-free high-density data storage system. The disk has nano structures composed of 300-nm-diameter photosensitive particles and 30-nm-width non-photosensitive buffer rings around them. With the buffer ring around the particles, the conventional confocal microscope with low NA objectives will realize a cost-effective optical pickup head simplified even in higher density three-dimensional data storage.