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Storage of Micro-Holograms in a Methyl Red Doped Polymer Dispersed Liquid Crystal Nathaniel P. Hermosa IIt and Marlon Rosendo H. Daza

National Institute of Physics, University of the Philippines-Diliman, Quezon City, Philippines IIOI Tel. No: +632 434 4243, Fax No: +632 434 4244, E-mail: tnhermosa@nip.upd.edu.ph

SUMMARY Optical memory is seen as a solution to the need for high storage capacities, fast data-transfer rate

and short access time [ 11. Recent developments in materials and CCD technologies have increased possibilities of using optical systems.

Two popular techniques in optical storage are being developed and used. These are multi-layered bit-recording [2-31 and volume holographic memories [4-lo]. Both recording methods have the potential for overcoming current optical storage systems’ recording density limitations. Bit recording includes planar (CD-ROM) and discrete multi-layer storage. On the other hand, volume holographic memories use several multiplexing techniques.

In a bit memory recording, a bit datum is created by focusing a laser beam on the material [3]. Hence, recording density is determined by the size of the focused beam spot on the recording medium. Most bit memory recording uses planar storage systems. Significant progress has been made in these formats to increase storage capacities. Recently, an extension &om the planar storage systems to discrete multi-layer bit recording has been demonstrated [2]. Recording was done via the refractive index change in photorefractive crystal and photopolymers. They have succeeded in recording up to thirty layers of data at axial-separation of 7 p and a dot separation of 2 pn for photopolymers and three layers of data at axial-separation of 24 pm and dot separation of 4 pm for Li:Nb03. Bits recorded are refractive index distributions and are viewed by the phase contrast method. The image produced by bit storage is 140 pm

Meanwhk, holography has been mainly used in displays and interferometric purposes. With the advent of new materials, improved technologies (SLM and CCD), holography as a way of data storage has been given renewed attention. Holographic memories appeal because of its high information density, high-speed retrieval and the access of information in parallel [ 11. High information density is realized by several multiplexing schemes. The schemes exploit the strict Bragg selectivity of hologram in a photorefractive crystal. Among the most widely used multiplexing schemes are; 1) shift-multiplexing [5- 61; 2) peristropic-multiplexing [7]; 3) angle-multiplexing [8-91; and 4) wavelength multiplexing [9-101. Combination of these multiplexing schemes enables recording of up to1000 holograms [8].

In this paper the author demonstrates the feasibility of a novel technique in storing data in a Methyl Red doped Polymer Dispersed Liquid Crystal (PDLC) by holography. Holographic gratings were recorded side by side and its effect on the diffraction efficiencies are noted.

in length.

Experimental Set-up Collimslcd Ar-ionSPm(514.5 m) The experimental set-up is shown in figure

1. An Ar+ laser beam is collimated and is split into \ I , . . , $ I

two. The beams are both p-polarized. Thd beams ’* I I 1

$ 1 are focused tightly. This limits the size of the holographic grating spot to be recorded. A PDLC ’ : ---_ +- & - ___ _ _ holder is placed on top of a translation stage (Melles-Griot, smallest travel = lop”, straightness : IM 8 , : !

:;;;I ; PDLC A i of travel = i2 p). This allows easy translation of

the PDLC. A : - Travel direction

:.----.--....-.I A holographic grating is Written near a

previously written set of grating to determine the feasibility of holographic point-by-point storage. The difiaction efficiencies of the gratings are

Figure I . Experimental Set-up used to record holograms in the Methyl Red Doped PDLCs.

0-7803-7379-0/02/517.0002002 IEEE 180

noted before and after recording of the second grating. The diffraction efficiency is calculated as the ratio between the first order diffraction, I, and total beam intensity, IT-.

I q = a & (1)

ITO,&

Images of holographic gratings are also taken with a polarizing microscope.

Results and Discussions A summary of the diffraction efficiencies before and after recording of another set of grating is

presented in Table 1. All the second gratings were witten 100 p from the first grating. The highest change observed was 26% while the lowest was 8.5%. The average change in diffraction efficiency is 17.25 %. The small change in the diffraction efficiency is due to the isolation attributed to the polymer matrix.

Table 1. Diffraction efficiencies of holographic gratings

Two holographic gratings written side-by-side are shown in figure 2a. The bigger grating spot is 533.4 p wide while the smaller grating spot has a diameter of 330 pm. Center to center distance is 508 Fm and the grating spot distance from their nearest tip is 72 p. It is interesting to note that there is a noticeable boundary between the holographic grating spots and the sample matrix.

Figure 2. Holographic gratings Mitten side-by-side. a) Boundary behveen holographic ga t ing spots and the sample matrix is noticeable. b) Gratings written with overlap. c) Smallest gratings d t t e n with the current set-up (diameter is 69.6 p). d) Small holograms written 139.5 pm away from each other (center to center).

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Another set of holographic grating is shown in figure 2b. The holographic gratings overlap and damage is seen in the encircled repion. However, outside the overlap, the distinctive gratings can still be seen. This would indicate that grating beyond the overlap is not damaged.

Figure 2c is a photomicrograph of the smallest diameter holographic grating spot possible with the current set-up. The diameter is 69.6 pn. Eighty-seven micrometers away (139.5pn, center to center) &om this grating spot is another grating spot whose diameter is 109 pm shown in figure 2d.

The implication of this is remarkable. If an alternating 70 pni diameter grating spot and 90 pn space is recorded, 62 holograms can be stored in a centimeter. In one square centimeter, more than three thousand holograms can be recorded.

Conclusion Holographic gratings are written side-by-side and the effect of the newly written grating on the

diffraction efficiency is presented. It was observed that an average of 17.25% of the grating efficiency was lost. A maximum and a minimum decrease of 26% and 8.5 % are reported.

The smallest possible holographic grating spot is 69.6 pn written 87 pm away &om another 109.4 pn holographic grating spot. Noticeable are the clear boundaries between the grating and the sample matrix.

The current set-up is not optimal for recording of smaller holographic grating. Parameters such as numerical aperture of the lens and the angle between the beams should be varied. A higher numerical a p e ” would give a smaller spot, however, this will be limited by the number of gratings it can accommodate within the spot. On the other hand, by varying the angle, the pitch of the grating can be controlled and a larger number of grating can be stored in the spot. These factors and their combined effects should be noted

Reference [l] Y . Ichioka, T. Iwaki and K. Matsuoka, “Optical Information Processing and Beyond,” Proc. of the

[2] H. Ueki, Y. Kawata and S. Kawata, “Three-dimensional optical bit-memw recording and reading with a photorefiactive crystal: analysis and experiment,” Applied Optics 35, 2457-2465, 1996.

[3] T. Tanaka and S. Kawata, “Comparison of recording densities in three-dimensional optical storage systems: multilayered bit redording versus angular multiplexed holographic grating,” J. Opt. Soc. Am. A 13,935-943, 1996.

[4] Adibi, K. Buse and D. Psaltis, “Multiplexing holograms in LiNbO3:Fe:Mn crystals,” Optics Letters 24,652-654, 1999.

[5] V. Markov, et.al., “Multilayer volume holographic memory,” Optics Letters 24, 265-267, 1999. [6] G. Barbastathis and D. Psaltis, “Shift-multiplexed holographic memory using the two-lambda

[7] K. Curtis, A. Pu and D. Psaltis, “Method for holographic storage using peristrophic multiplexing,”

[8] E. Chuang and D. Psaltis, “Storage of 1000 holograms with use of a dual-wavelength method,”

[9] S . Yu and D. Psaltis, “Three-dimensional holographic disks,” Applied Optics 33,3764-3774, 1994. [lo]

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