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1998 MRS Fall Meeting & Exhibit

November 30 - December 4, 1998 | Boston
Meeting Chairs:
 Clyde L. Briant, Eric H. Chason, Howard E. Katz, Yuh Shiohara

Symposium T—Recent Progress in Optical Data Storage and Processing



Hans Coufal, IBM Almaden Research Center
Glenn Sincerbox, Univ of Arizona

* Invited paper

Chair: Marc J. Cardillo 
Wednesday Morning, December 2, 1998 
Essex East (W)
8:30 AM *T1.1 
MATERIALS CHALLENGES AND OPPORTUNITIES FOR OPTICAL DISK STORAGE. Barry H. Schechtman , National Storage Industry Consortium, San Diego, CA. 

Developments over the last decade have driven the growth of the optical disk storage industry to its present size of about $8 billion worldwide drive revenue. Although a very great fraction of the industryís installed base involves read-only (ROM) devices, continued advances in optical disk products will be heavily dependent on use of recordable and rewritable media. Future media developments will involve a great many materials intensive challenges to address issues such as adequate signal-to-noise ratio, recording speed, cyclability and long-term stability in phase-change media; shifting of response to shorter wavelengths and detection of extremely small bits in magneto-optic media; development of graded response media for multi-level recording and development of adequately engineered optical stacks for multi-layer recording. In addition to these media challenges, additional materials issues will occur in advancing the other components of optical disk drives, including short wavelength lasers, compact optical heads with high bandwidth tracking servos, and near-field recording encompassing tribology issues for a new set of materials at a closely spaced head-disk interface. Many of these challenges are compounded by the desirability of preserving the removability feature of optical disk media. The drive configurations therefore need to be designed for tolerance to particulate and chemical contamination, and the media need to be robust despite being handled external to the drive. 

9:00 AM *T1.2 

Holographic memories [1] promise high storage density due to 3-D recording and fast transfer rate due to parallel access of data. Memories in general and holographic memories in particular can be divided in three broad categories: Read­only­memories (ROM), Write-Once-Read-Many (WORM), and rewritable memories. The majority of the early work in holographic memories was done with photorefractive crystals, a material that can be recorded and erased which makes it a good candidate for rewritable memories. Rewritable memories, however, are least likely to become commercially viable in the forseeable future because of material limitations (hologram decay when the memory is simply read-out or when one of the holograms in the memory is erased, fanning and low sensitivity), fierce competition from existing technologies (semiconductor and magnetic memories), and relatively high system complexity and cost. Recent progress in fixing mechanisms for photorefractives and the development of organic holographic materials have shifted the focus of attention towards WORM and ROM systems. In some respects ROM is the simplest type of system and hence the one that we might expect to reach the marketplace first. Typically, ROM systems are constructed in the disk configuration [2] where each side of the holographic disk is capable of storing 100 bits per squared micron, or 20 times the areal density of the recently introduced DVD optical disk system. Industrial developments in several industrial laboratories in the US and Japan are pursuing the development of such systems. The complexity and cost of a holographic system is not drastically different from a conventional optical disk system. Therefore, such systems can become a successor to the current optical ROM systems. WORM systems, are more complex than ROMs because they must include the capability to record in addition to reading the recorded holograms. This makes the optical system more complex and imposes more stringent requirements on the recording material. However, WORM systems have the very significant advantage of being easier to accept in the marketplace. This is because the user records the data (as in a ZIP drive for instance) whereas in a ROM system a third party must supply prerecorded disks. Typically, this requires the establishment of standard formats and agreements between a large number of parties, a lengthy process that can last years. Even though it is not yet clear whether ROM or WORM systems will be the first successful holographic memory products (assuming a holographic memory product will become a reality), it is clearer that organics are likely to be the recording materials in the products that might emerge in the forseeable future. 

9:30 AM T1.3 
OPTIMIZATION AND CHARACTERIZATION OF ACRYLATE-OLIGOMER BASED PHOTOPOLYMERS FOR OPTICAL DATA STORAGE APPLICATIONS. Marcia L. Schilling , Vicki L. Colvin, Lisa Dhar, Howard E. Katz, Alex L. Harris, Frederic C. Schilling, Melinda L. Schnoes, Carol Boyd, Arturo Hale, Lee L. Blyler, Bell Laboratories, Lucent Technologies, Murray Hill, NJ. 

Photopolymers have a potential for both high sensitivity and high refractive index contrast in holographic data storage applications. Acrylate-oligomer based materials also provide the fast, complete reaction and high optical quality needed in these systems. Using co-reactivity ratios, reaction kinetics and component refractive indices as guidelines, high index organic monomers have been incorporated into acrylate formulations in order to achieve high contrast materials which can be used to multiplex >200 holograms. Complete chemical characterization of these materials will be described, as well as a protocol for the evaluation of other potential holographic media. 

9:45 AM T1.4 
EFFECTS OF INITIALIZATION ON THE MICROSTRUCTURES, WRITING AND ERASING PROPERTIES OF AGINSBTE PHASE-CHANGE OPTICAL DISKS. Lih-Hsin Chou , Yung-Chain Chai, Yem-Yeu Chang, National Tsing Hua Univ, Dept of Materials Science and Engineering, Hsinchu, TAIWAN; Shiunn-Yeong Wang, Lead Data Inc., Fu Kou Hsiang, Hsinchu, TAIWAN. 

Effects of initialization on the phase-change optical disks using a quaternary system of Ag-In-Sb-Te as the active layer are reported. The active layer showed different microstructures after applying different initialization power ranging from 150mW to 900mW. It was observed that with an initialization power lower than 400mW, the recording layer possessed an unevenly distributed crystallites and showed low crystallinity with the assistance of transmission electron microscopy. With an initialization power higher than 800mW, voids were observed and the grain sizes of the recording layer were smaller compared to those with proper initialization power. Only with proper initialization power, i.e. of around 550mW, the crystalline phases were uniformly distributed and the disk possessed the best carrier to noise ratio (CNR) and erasability. The writing and erasing character of the first cycle with three different initialization power: 350mW and 550mW and 850mW was also examined. By applying 3T writing pulses with 9mW writing power, the writing spots showed different amorphous microstructures associated with different initialization power. The erasing microstructures for samples with different initialization power will also be discussed. 

10:30 AM T1.5 
NaCa2Mn2V3O12 GARNET AS PHOTOSENSITIVE MATERIAL FOR OPTICAL RECORDING. A.P. Voronov, A.B. Levin, M.B. Kosmyna , B.P. Nazarenko, V.M. Puzikov, T.S. Teplitskaya, A.G. Fedorov, O.V. Shishkin, Institute for Single Crystals, NAS of Ukraine, Kharkov, UKRAINE; V.A. Bedarev, V.I. Gapon, S.L. Gnatchenko, I.S. Kachur, V.G. Piryatinskaya, Institute for Low Temperature Physics & Engineering, NAS of Ukraine, Kharkov, UKRAINE. 

Photosensitive garnets can be used as active media in devices for optical storage with high density and optical processing information. Therefore, the search and study of new photosensitive garnets is important for the applications. Up to now only two photosensitive crystals in large garnet family were known, namely ferrimagnetic Y3Fe5O12 and antiferromagnetic Ca3Mn2Ge3O12 garnets. In the present paper we report about the observation of change of optical properties of antiferromagnetic NaCa2Mn2V3O12 garnet under illumination. 
The single crystals of cubic NaCa2Mn2V3O12 garnet were grown in platinum crucible from solution-melts by the method of spontaneous crystallization at slow cooling and crystallization on a seed. Melted salts of potassium, sodium vanadates and their mixtures were used as solvents. The solubility of garnet­forming oxides in the melts of these salts was determined. It was 25-35 weight percents at the temperature 1100 C. The crystallization range of NaCa2Mn2V3O12 was 800-980 C for KVO3 solvent and 750 ­ 960 C in the case of NaVO3 one. NaCa2Mn2V3O12 single crystals with size of 3-8 mm and mainly faceted by the planes  have been grown. 
Effect of illumination of He-Ne laser with wavelength  nm and power of several milliwatts on optical properties of NaCa2Mn2V3O12 garnet was studied. It has been found that the linearly polarized light induces the linear birefringence  ( nm) in the crystal. The augmentation of optical absorption coefficient   nm) was also observed in this garnet. The change of optical absorption was induced as unpolarized light as linearly polarized one. Both effects were observed in the temperature range T<110 K. 
The found photoinduced effects can be used for optical recording in NaCa2Mn2V3O12 garnet, in particularly for recording phase and amplitude holograms. 

10:45 AM T1.6 
CONFINING METALLIC GALLIUM - NEW MATERIAL STRUCTURE FOR OPTICAL DATA PROCESSING AT MILLIWATT POWER LEVEL. Vassilios Albanis, Periklis Petropoulos, Suki Dhanjal, David Richardson and Nikolay Zheludev , University of Southampton, UNITED KINGDOM. 

We report the application of a recently discovered huge optical nonlinearity of gallium/dielectric interface in controlling light with light. The nonlinearity is associated with a light-induced, reversible and very fast structural phase transition between solid alpha-gallium and a metastable phase of much higher reflectivity. The material is fully compatible with existing telecom fiber technology as the gallium structure can be manufactured on the tip of an optical fiber or can form a wall of a planar optical waveguide. It and can be used with laser operating anywhere from 400nm to 1.7mkm. 
We have demonstrated a fully fiberized high contrast all-optical gate based on this material which operates at only A few milliwatts of light power. The gate performance has been characterized with diode lasers operating at telecom wavelengths of 1.55mkm, 1.3mkm and 670nm. The gate is suitable for analog and digital applications and shows useful modulation at frequencies up to 1MHz. The gate can also handle optical pulses as short as 10ns. Applications of the new material structure for optical routing, in particular in the WDM technology, optical bistability, optical data storage, power limiting and controllable power attenuation are discussed. 
Our demonstrations have shown that photonic devices based on gallium interfaces can be comparable to conventional silicon transistors in terms of speed versus energy consumption. 

11:00 AM T1.7 
PERSISTENT SPECTRAL HOLE BURNING GLASSES AS HIGH DENSITY MEMORY DEVICES. Masayuki Nogami , Nagoya Institute of Technology, Dept. of Materials Science and Engineering, Nagoya, JAPAN. 

Persistent spectral hole burning (PSHB) phenomenon is one of the most significant optical properties for use in high-density frequency-domain optical data memories. For practical use in high-density memories, high temperature PSHB is required. Eu3+ and Sm2+-doped silica and silicate glasses, exhibiting PSHB up to 200 and 300 K, respectively, were successfully prepared using a sol-gel method of metal alkoxides. The depth of hole burned in the 7F05D0 transition band of the Sm2+and Eu3+ ions increased with increasing the number of OH bonds surrounding the rare earth ions. The holes burned at 77 K were thermally refilled with increasing temperature, and the holes in the Eu3+-doped glasses disappeared above 220 K, whereas the holes in the Sm2+-doped glasses were still observed at room temperature. The burnt-holes were thermally filled and the average thermal barrier heights for the hole filling were 0.1 to 0.5 eV, which were higher for the glass heated at high temperature. In addition to the photoionization of Sm2+ into Sm3+ ions, we proposed that holes were burned by the rearrangement of OH bonds surrounding the rare earth ions upon the laser irradiation. This finding should be instructive for developing the glasses exhibiting PSHB up to high temperature. 

11:15 AM T1.8 
THREE-DIMENSIONAL OPTICAL STORAGE IN VITREOUS SILICA: VOXELS FABRICATION AND LUMINESCENCE. Mitsuru Watanabe, Shigeki Matsuo, Hiroaki Misawa, Tokushima Univ, Dept of Ecosystem Engineering, Tokushima, JAPAN; Hongbo Sun, Saulius Juodkazis , Tokushima Univ, Satellite Venture Business Laboratory, Tokushima, JAPAN. 

Three dimensional (3D) optical data storage is expected for various applications such as script, sound and image recordings due to the capability of its higher recording density than conventional 2D optical memory as compact disc (CD) with 67 Mbits/cm2. We demonstrate feasibility to use dry vitreous silica (concentration of OH < 10 ppm) as 3D Read-Only-Memory material with writing densities per surface area up to 2.7 Gbits/cm2 (corresponding volume density of 134 Gbits/cm3), on silica plates of 200 m thickness. CD's recording density was achieved for in-plane voxels fabrication by single ps- and fs-pulses. Intra-plane distance was 5 m for 400 nm wavelength fs-pulse fabrication. The recording up to 40 planes was achieved with high optical contrast of 60% when 0.45 W pulse energy was employed, what allows to employ conventional optical microscope for read-out focal plane images. As recorded contrast is sustained after 1150C 3 h annealing. Even higher, crosstalk free voxel planes stacking densities were found for 532 nm, 30 ps pulse irradiation with inter-plane distances of 3.5 m. Also, we report the observation of three distinct photoluminescence (PL) bands at 283 nm, 468 nm, and 558 nm originated from optically damaged area created by near-infrared and visible irradiation of ps- and fs-duration. All bands exhibit similar excitation spectra peaked at 250 nm. The power laws of PL bands intensity vs. pulse energy employed for voxel creation were found to be approximately linear with the slope 1, 0.7 and 1 for corresponding band (log-log presentation). The origin of observed PL bands is explained by following transitions: conductive band electron to hole trapped on oxygen vacancy for 283 nm, electron trapped on E' center to molecular and atomic oxygen interstitials for 468nm and 558 nm, respectively. The possibility of voxel image read-out by their PL is discussed. 

11:30 AM *T1.9 
PHOTOADDRESSABLE POLYMERS FOR OPTICAL DATA STORAGE. Johannes Eickmans, Thomas Bieringer , Bayer AG, Central Research Department, Physics, Leverkusen, GERMANY. 

A new class of polymers for optical data storage is shown. By irradiation with light a birefringence is created due to photochemical induced cooperative molecular reorientation. This new molecular configuration can be used for digital as well as for holographic data storage purposes. 
Chair: Glenn T. Sincerbox 
Wednesday Afternoon, December 2, 1998 
Essex East (W)
1:30 PM *T2.1 
MATERIALS ISSUES FOR HOLOGRAPHIC DATA STORAGE. R.M. Shelby , M.-P. Bernal, G.W. Burr, H. Coufal, H. Günther, J.A. Hoffnagle, C.M. Jefferson and R.M. Macfarlane, IBM Almaden Research Center, San Jose, CA. 

Holographic storage records data as images consisting of two-dimensional arrays or ``pages'' of as many as 106 pixels which encode the digital data. Holography is a volumetric storage technique, which stores a number of data pages in a common volume of the storage material and selectively accesses them using a technique such as Bragg angle multiplexing. With such methods, volumetric densities of 1011 bits per cm3 should be achievable. The data is read out onto a two-dimensional detector, such as a CCD array; the parallel nature of the readout suggests that high data rates, of the order of 1 Gbit/sec, are possible. 
However, in practice, the data density that can be achieved at a given bit-error rate (BER) is limited by the material response, its so-called ``dynamic range.'' A large dynamic range implies that a large number of holograms can be recorded, each with enough efficiency to produce a signal over the short detection time needed to give a high data rate, such that the BER is within acceptable limits (< 10-5 - 10-3). In addition to having a high dynamic range, the material should respond very sensitively during the writing process, yet the written data must be stable in the dark and during irradiation by the readout beam. Furthermore, the need to image the reconstructed hologram, pixel-for-pixel on a detector array puts stringent requirements on the optical quality of the medium as does the need to maintain low scattered light noise. 
To date, no known material meets all these requirements well enough to be the basis of a practical storage technology. Measurements of materials properties relevant for holographic data storage and progress toward realizing a useful material will be reviewed in this presentation. 

2:00 PM *T2.2 
HOLOGRAPHIC DIGITAL DATA STORAGE IN PHOTOPOLYMER SYSTEMS. Lisa Dhar , Marcia Schilling, Melinda Schnoes, Arturo Hale, Howard Katz, Alex Harris, Carol Boyd, Scott Campbell, Nicholas Levinos, Keven Curtis, William Wilson, Michael Tackitt and Adrian Hill, Bell Laboratories, Lucent Technologies, Murray Hill, NJ. 

We will discuss holographic digital data storage in photopolymer systems. Photopolymer materials are attractive candidates for storage media for holography because they are low cost and easily processed and they can be designed to have large dynamic range and high photosensitivity. We will describe holographic recording and recovery of high capacity (480 kbit) data pages in thick (500 m) photopolymer media. The advances in photopolymer materials that have enabled digital data storage, the design of systems with high index contrast and high dimensional stability and the ability to fabricate high optical quality media, will be emphasized. 

2:30 PM T2.3 
TECHNIQUES FOR CHARACTERIZATION OF MEDIA FOR HOLOGRAPHIC DATA STORAGE. Melinda Schnoes , Marcia Schilling, Lisa Dhar, Arturo Hale, Howard Katz, Alex Harris, Carol Boyd, Scott Campbell, Nicholas Levinos, Kevin Curtis, William Wilson, Michael Tackitt, and Adrian Hill, Bell Laboratories, Lucent Technologies, Murray Hill, NJ. 

We will discuss methods of optical characterization of holographic materials. Measurements of dynamic range, dimensional stability, temperature sensitivity, and archival life are critical to the development of new materials for data storage applications. Examples will be drawn from experiments in photopolymer systems where we have been able to fabricate high response materials that exhibit little recording induced dimensional change. 

2:45 PM T2.4 
SOME NOVEL SYSTEMS FOR HOLOGRAPHIC OPTICAL STORAGE. M. Hosse, I. Lieker , A. Stracke, J. H. Wendorff, Philipps-Univ Marburg, Dept of Physical Chemistry and Center of Material Sciences, Marburg, GERMANY; Th. Fuhrmann, Kyushu Univ, Graduate School of Engineering Sciences, Dept of Materials Science, Fukuoka, JAPAN. 

Azo dye containing side chain polymers can be used for optical storage as demonstrated within the last decade. Calamitic liquid crystalline systems have the advantage of strong changes of the refractive index originating from the light induced reorientation of the director but the disadvantage of difficult preparation of homogenious films. We have investigated two concepts to overcome this problem. The first concept consisted of the use of frustrated side chain systems. The basic idea was to start from liquid crystalline side chain systems and to introduce frustration effects via the introduction of non mesogenic side groups. The aim was to destruct the formation of liquid crystalline phases while keeping the intermolecular orientational forces characteristic for liquid crystals. In fact, Monte Carlo simulations have predicted the effectiveness of this approach. Experimental studies involving Kerr-effect studies, dielectrical relaxation spectroscopy as well as calorimetric investigations showed the presence of frustration effects, yet the optical response was week. A second approach involved the use of discotic materials consisting of electron rich multialkine systems connected via an azobenzene unit to a strong electron acceptor. The formation of homogenious films is easy for such systems. The findings are that gratings can be stored in such materials by illumination at room temperature. An interesting finding is that the diffraction efficiency can be strongly enhanced by annealing at elevated temperature in the absence of light (gain effect). The character of the grating changed strongly during annealing. We finally analysed the variations induced by replacing the azobenzene units by stilbene units which do not show absorption in the visible range and we studied the use of such materials for holographic optical elements. 

3:30 PM T2.5 
OPTICAL MEMORY USING BIPHOTONIC STIMULATION IN RARE-EARTH-DOPED GLASS. B. Lee , C.J. Chi and A.J. Steckl University of Cincinnati, Cincinnati, OH. 

This paper discusses an ultra-high density optical memory device using biphotonic emission from rare-earth-doped semiconductors and insulators. The basic mechanism involves a two-step, two-wavelength excitation of the RE ions through an intermediate energy level with a relatively long lifetime. Because of this long lifetime, this ``two-color'' process is much more efficient than conventional up-conversion which utilizes the absorption of two photons of the same energy through a virtual state which has a very short lifetime. 
This biphotonic mechanism can be used to ``read'' information stored in various locations in the optical memory host. This is accomplished by using two interrogating pump lasers which are scanned to intersect at various memory locations. The information is ``written'' in the optical memory by implanting a closely-spaced pattern of RE ions into an optically transparent medium, either an insulator or a wide band-gap semiconductor. This could be accomplished by focused ion beam (FIB) implantation with a spacing as small as 10 to 50 nm (depending on species, energy, substrate). Theoretically, this could result in an area bit density of 1010 to 1012 /cm2. 
The first implementation of this optical memory in Pr-doped ZBLAN glass is discussed. Localized biphotonic emission was obtained using excitation with two orthogonal laser beams: (a) 1 = 1014 nm, causing a transition from the ground state (3H4) to the first excited state (1G4); (b) 2 = 840 nm, causing a subsequent transition from the first to the second excited state (1I6). Excited electrons then return to the ground state through multi-phonon emission and visible light emission at 605 and 636 nm. The photoemission intensity and linewidth have been investigated as a function of laser pump wavelength and power. 

3:45 PM T2.6 
HOLOGRAPHIC RECORDING WITH USE OF METASTABLE CENTERS IN SEMICONDUCTORS. A. Suchocki , B. Koziarska-Glinka and J.M. Langer, Institute of Physics, Polish Academy of Sciences, Warsaw, POLAND. 

Metastability of defects in semiconductors such as the EL2 in GaAs or the DX-centers in III-V and II-VI semiconductors is associated with large lattice relaxation (LLR) accompanying their phototransformation from the localised state to the metastable shallow state. Local polarizability of the deep and shallow states are different due to the LLR effect. Therefore phototransformation of the defect centers induces changes of the refractive index of the crystal. Large concentrations of the metastable centers can be obtained. The oscillator strength of the optical transitions leading to their phototransformation is usually very high due to their photoionization character. This result in large dynamic range of induced changes of the refractive index (in the range  10-3) and high photorefractive sensitivity of materials containing metastable centers. This makes the metastable centers in semiconductors a very promising candidate for holographic recording both in static and dynamic regimes. Unfortunately, low metastability temperatures, usually below 150 K, hinder their practical application. In this work we report efficient recording of holograms in CdMnTe and CdF2 crystals doped with Indium and Gallium. Metastability temperatures of both dopants in CdMnTe crystals are below 150 as in the other II-VI and III-V semiconductors. In contrast to that the CdF2 crystals doped with Gallium exhibit metastability temperature about 250 K, much higher than in the other materials. The metastable centers in CdF2 have very similar properties to well-known DX centers in III-V semiconductors. The details of microscopic mechanism leading to the metastability of some dopants in CdF2will be discussed on the base of holographic, SQUID and STM dilatometric experiments. A comparison of parameters important for holographic recording of CdF2:In and CdF2:Ga crystals with the appropriate data for LiNbO3and for other metastable centers in III-V and II-VI semiconductors testify that the metastable centers in CdF2 are suitable for holography. 

4:00 PM T2.7 
TERM HOLOGRAPHIC RECORDING IN Mn DOPED ALUMINATES. N. Noginova, M.A. Noginov, W. Lindsay, G.B. Loutts, Center for Materials Research, Norfolk State University, Norfolk, VA. 

The photoinduced coloration and associated with it refraction index change in Mn:YAlO3 have been reported in our recent publications [1,2]. It has been shown that high diffraction efficiency, nonvolatile reading at 600 nm, long storage-time at room temperature (more than one year), and fast reversible erasing of the photocoloration at 250oC make Mn:YAlO3promising material for holographic optical data storage. In the present work we investigate the photocoloration and holographic recording in the wide range of relevant Mn doped materials, GdAlO3, Gd1-xLaxAl_3(x=0.27, 0.33, and 0.5), YbAlO_3, and CaYAlO_4and compare them with Mn:YAlO_3