Evaluation of partial-response maximum-likelihood detection for phase-change optical data storage

We describe the application of partial-response (PR) maximum-likelihood (ML) detection in rewritable phase-change optical data storage. The input to this detector, which is simulated in software, is the actual signal (without any equalization), reproduced from reading of the recorded sequence on an optical disk. The detection algorithm involves the extraction of the impulse response from the readout signal, PR equalization, the adjustment of gain and recovery of clock, ML sequence estimation with the Viterbi algorithm, and analysis of PRML performance. With a laser wavelength of 0.69 mum and an objective lens with a numerical aperture of 0.6, three linear densities are examined: 0.35 and 0.31 mum/bit without modulation code and 0.2 mum/bit with the (1, 7) modulation code. The equalized signal exhibits good eye patterns, especially at the densities of 0.35 and 0.31 mum/bit. Analyses of noise and bit-error rate indicate that jitter, rather than noise, is the main obstacle to realizing ultrahigh density in phase-change media with PRML detection. We also briefly discuss the problem of the inherent nonlinear effect in phase-change readout.     

Partial-response signaling for phase-change optical data storage without electronic equalization

We describe the application of partial-response (PR) signaling in rewritable phase-change optical data storage. No electronic filter is necessary to shape the readout signal to a certain PR target. A PR-like waveform at the output of the read channel is directly achieved by optical recording. A genetic algorithm is used to optimize the parameters for writing and therefore to minimize the difference between the actual readout signal and the ideal PR waveform. With a laser wavelength of 0.66 microm and an objective lens with a numerical aperture of 0.6, four linear densities were examined: 0.4, 0.3, 0.25, and 0.2 microm/bit (without modulation). Results showed that the linear density of 0.25 microm/bit can be realized on a rewritable digital-versatile disk.     

Optical characterization of multilayer stacks used as phase-change media of optical disk data storage

We report results of measurements of the optical constants of the dielectric layer (ZnS-SiO2), reflecting layer (aluminum-chromium alloy), and phase-change layer (GeSbTe, AgInSbTe) used as the media of phase-change optical recording. The refractive index n and the absorption coefficient k of these materials vary to some extent with the film thickness and with the film deposition environment. We report the observed variations of optical constants among samples of differing structure and among samples fabricated in different laboratories.     

Design rules for phase-change materials in data storage applications

Phase-change materials can rapidly and reversibly be switched between an amorphous and a crystalline phase. Since both phases are characterized by very different optical and electrical properties, these materials can be employed for rewritable optical and electrical data storage. Hence, there are considerable efforts to identify suitable materials, and to optimize them with respect to specific applications. Design rules that can explain why the materials identified so far enable phase-change based devices would hence be very beneficial. This article describes materials that have been successfully employed and dicusses common features regarding both typical structures and bonding mechanisms. It is shown that typical structural motifs and electronic properties can be found in the crystalline state that are indicative for resonant bonding, from which the employed contrast originates. The occurence of resonance is linked to the composition, thus providing a design rule for phase-change materials. This understanding helps to unravel characteristic properties such as electrical and thermal conductivity which are discussed in the subsequent section. Then, turning to the transition kinetics between the phases, the current understanding and modeling of the processes of amorphization and crystallization are discussed. Finally, present approaches for improved high-capacity optical discs and fast non-volatile electrical memories, that hold the potential to succeed present-day's Flash memory, are presented.     

The stability of thin tellurium and tellurium alloy films for optical data storage: II

The chemical and physical degradation and the effects of the substrate and a low surface energy subbing layer on the stability of thin tellurium and tellurium alloy films for optical data storage are reported. Chemically, thin tellurium films degrade mainly through the uniform oxidation of tellurium to TeO₂. In addition, localized degradation occurs at regions where initial defects on the substrate are present. Thin tellurium and tellurium alloy films can also degrade physically through crystal growth for an initially polycrystalline film and phase transformation for an initially amorphous film. While no significant difference in the uniform degradation rate is observed between films deposited onto poly(methyl methacrylate) (PMMA) and those deposited onto glass substrates, more localized degradation is detected for the films deposited onto PMMA substrates because of the higher initial defects of the substrate. The presence of a low surface energy subbing layer tends to degrade both the chemical and physical stability of these films. The degradation in the chemical stability is attributed to the change in the microstructure or thickness of the film due to the presence of the low surface energy subbing layer.     

Investigation of crystallization and amorphization dynamics of phase-change thin films by subnanosecond laser pulses

We report experimental results on amorphization and crystallization dynamics of reversible phase-change (PC) thin-film samples, GeSbTe and GeBiTe, for optical disk data storage. The investigation was conducted with subnanosecond laser pulses using a pump-and-probe configuration. Amorphization of the crystalline films could be achieved with a single subnanosecond laser pulse; the amorphization dynamics follow closely the temperature kinetics induced in the irradiated spot. As for crystallization of the samples initially in the amorphous state, a single subnanosecond pulse was found to be insufficient to fully crystallize the irradiated spot, but we could crystallize the PC film (in the area under the focused spot) by applying multiple short pulses. Our multipulse studies reveal that the GeSbTe crystallization is dominated by the growth of nuclei whose initial formation is slow but, once formed, their subsequent growth (under a sequence of subnanosecond pulses) happens quickly. In the case of GeBiTe samples, the crystalline nuclei appear to be present in the material initially, as they grow immediately upon illumination with laser pulses. Whereas our amorphous GeSbTe samples required approximately 200 pulses for full crystallization, for the GeBiTe samples approximately 15 pulses sufficed.     

Inorganic materials for optical data storage

Recyclable holographic (optical) storage in inorganic materials is nowadays possible due to the advent of laser. Various performance parameters of the state-of-the-art of optical storage are discussed in detail with reference to the well-established case of ferroelectric lithium niobate (LiNbO₃). Various physicochemical techniques are employed in understanding the microscopic mechanisms responsible for optical storage in LiNbO₃. A short summary of other inorganic materials capable of holographic storage is also presented.     

Preparation and optical properties of phase-change VO2 thin films

In this work, VO₂ thin films were prepared on three kinds of substrates by the sol-gel dipcoating method followed by heat treatment under vacuum. These thin films were analysed by x-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. The infrared and ultraviolet-visible spectra of the VO₂ thin films were also recorded during heating and cooling between room temperature and 100°C. The experimental results show that VO₂ thin films thus prepared exhibit thermally induced reversible phase transition, and the largest changes in transmittance and reflectivity are approximately 58 and 25%, respectively, in the case of vacuum heat treatment at 400°C and silica glass substrates. The refractive index (n) decreases and the absorption coefficient (k) increases when heating these thin films from room temperature to 100°C, and vice versa for cooling. The reasons why the optical constants and infrared absorption spectra change so remarkably are discussed.     

Biomolecular optical data storage and data encryption

The use of bacteriorhodopsin (BR) as an active layer in write-once-read-many optical storage is presented. This novel feature of BR materials may be used on a wide variety of substrates, among them transparent substrates but also paper and plastics. The physical basis of the recording process is polarization-sensitive two-photon absorption. As an example for this new BR application, an identification card equipped with an optical recording strip is presented, which has a capacity of about 1 MB of data. The recording density currently used is 125 kB/cm/sup 2/, which is far from the optical limits but allows operation with cheap terminals using plastic optics. In the examples given, data are stored in blocks of 10 kB each. A special optical encryption procedure allows the stored data to be protected from unauthorized reading. The molecular basis of this property is again the polarization-sensitive recording mechanism. The unique combination of optical storage, photochromism, and traceability of the BR material is combined on the single-molecule level. BR introduces a new quality of storage capability for applications with increased security and anticounterfeiting requirements.     

Image oversampling for page-oriented optical data storage

Page-oriented data storage systems incorporate optical detector arrays [such as complementary metal-oxide semiconductor (CMOS) arrays] in order to read data images. For laboratory demonstrations the detector array is typically pixel matched to the data image [Opt. Lett. 22, 1509 (1997)]. This approach requires exceedingly high-performance optics and mechanics for the simultaneous alignment of each data-bearing pixel image to a detector element to be achieved. Systems intended for commercialization are designed with detector arrays that spatially sample the image at or above the Nyquist rate in order to read poorly aligned and distorted images [S. Redfield, Holographic Data Storage (Springer-Verlag, 2000), pp. 347-349]. However, for data page sizes exceeding a megapixel this approach becomes prohibitive in terms of detector bandwidth, size, power, cost, and processing requirements. We have instead developed a sub-Nyquist oversampling methodology that can recover arbitrarily aligned and distorted megapixel data page images with pixel-matched fidelity by using fewer than double the number of detector pixels. Features required for practicable implementation are described, including fiducials for alignment determination.     

Dynamic theory of crystallization in Ge2Sb2.3Te5 phase-change optical recording media

We develop a theory of the crystallization dynamics of Ge(2)Sb(2.3)Te(5) thin films that shows good qualitative agreement with experimental reflectivity results from a two-laser static tester. The theory is adapted from the nucleation theory of liquid droplets from supersaturated vapor and elucidates the physics underlying the amorphous-to-crystalline phase transformation under short-pulse excitation. In particular, the theory provides a physical picture in which crystalline islands, or basic embryos, are thermally activated in the amorphous material and subsequently grow as stable nuclei are formed.     

Chalcogenide glasses as prospective materials for optical memories and optical data storage

This paper first reviews recent progress in the understanding of the nanometer-scale mechanism of reversible photostructural changes in chalcogenide glasses, and its relevance to various photo-induced phenomena. Then the principles of phase-change optical recording are described. Finally, a novel technique for overcoming the diffraction limit in optical recording is considered, namely the super-resolution near-field structure. This technique, used in combination with multicomponent Te-based chalcogenides as a recording medium, is believed to be a prototype for future optical-storage devices.     

Al-Si合金的储热性能

对Si含量为10％-13％的Al—Si合金进行了加速氧化、热循环和掺Fe实验,研究了其在不同热循环条件下的相变储热性能和可靠性．结果表明,在空气中经几百h的高温氧化后,氧化率小于0．01％,其影响可以忽略不计．经过0,4,23,60,100,200,300,400,500,600,700次熔化-凝固循环后,相变温度的变化为3．8-11．8℃,相变潜热从484．86kJ／kg下降到432．62kJ／kg．当Al—Si合金的掺铁量为0．5％时,相变潜热下降6．5％;对于缓冷的储能过程,偏析较小并在循环多次后趋于缓和和稳定．Al—Si合金成份和结构的变化对材料的储热性能影响较小,在长期的热循环过程中有良好和稳定的储热性能．     

Reactive DC magnetron deposition of copper nitride films for write-once optical recording

Copper nitride (Cu_xN) films were prepared on glass slides by reactive direct current (DC) magnetron sputtering under different nitrogen flow rates and substrate temperature, respectively. X-ray diffraction and reflectance spectra were employed to characterize the films. The Cu_xN films can completely decompose through heat treatment at temperature as low as 160 °C. The reflectance spectra of the as-deposited films are quite different from that of the decomposed films in a wide wavelength range from UV to infrared. Its relatively low decomposition temperature and large reflectance change before and after decomposition makes it a potential write-once optical recording medium.     

Three-dimensional optical data storage in a fluorescent dye-doped photopolymer

We propose a new, to our knowledge, monolithic multilayer optical storage medium in which data may be stored through the diffusional redistribution of fluorescent molecules within a polymer host. The active portion of the medium consists of a photopolymer doped with a fluorescent dye that is polymerized at the focal point of a high-numerical-aperture lens. We believe that as fluorescent molecules bond to the polymer matrix they become more highly concentrated in the polymerized regions, resulting in the modulated data pattern. Since data readout is based on detection of fluorescence rather than index modulation as in other photopolymer-based memories, the problems of media shrinkage and optical scatter are of less concern. An intensity threshold observed in the recording response of this material due to the presence of inhibitor molecules in the photopolymer allows for the three-dimensional confinement of recorded bits and therefore multilayer recording. The nonlinear recording characteristics of this material were investigated through a simple model of photopolymerization and diffusion and verified experimentally. Both single-layer and multilayer recordings were demonstrated.     

Phase-to-amplitude data page conversion for holographic storage and optical encryption

A new phase-to-amplitude data page conversion method is proposed for efficient recovery of the data encoded in phase-modulated data pages used in holographic storage and optical encryption. The method is based on the interference between the data page and its copy shifted by an integral number of pixels. Key properties such as Fourier plane homogeneity, bit error rate, and positioning tolerances are investigated by computer modeling, and a comparison is provided with amplitude-modulated data page holographic storage with and without static phase masks. The feasibility and the basic properties of the proposed method are experimentally demonstrated. The results show that phase-modulated data pages can be used efficiently with reduced system complexity.     

Optimization of physicomechanical properties of photopolymeric materials for optical data storage by treatment with high-frequency currents

We study the effect of the duration of ultraviolet irradiation of D-1 and D-2 photopolymerizable compositions on some physicomechanical properties of a photopolymeric material obtained in the process of photopolymerization. It is shown that these properties become most pronounced for a certain fixed duration of ultraviolet treatment and, hence, continuation of this process becomes unreasonable. The degree of conversion of photopolymeric materials does not exceed 70%. Strength and some other characteristics of photopolymers can be improved by additional treatment of the material with high-frequency currents. This kind of treatment is characterized by lower power consumption and can be performed much faster than routine thermal treatment (by about two orders of magnitude).I. Fedorov Ukrainian Polygraphic Institute, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 6, pp. 74–77, November – December, 1994.