Optical Disk Data Research Articles

DFT computation, Hirshfeld surfaces analysis, non-linear optical and spectroscopic investigations of a novel non-centrosymmetric organic –inorganic hybrid material [(CH3CH2)4N]HSeO4(H2SeO4)2

The new hybrid compound, tetraethylammonium monohydrogenselenate bis selenic acid [(C2H5)4N]HSeO4(H2SeO4)2 is synthesized by slow evaporation at room temperature of a solution containing (C2H5)4NOH and H2SeO4. It is crystallized in the monoclinic space group Cc (Z ​= ​2), with the unit cell parameters: a ​= ​19.705 (1) Å, b ​= ​7.542 (3) Å, c ​= ​19.199 (1) Å, β ​= ​115°. The 3D Hirshfeld surface and 2D fingerprint plots analyses showed the predominance contribution of O–H⋯O bonding in the crystal structure crystal packing. The contributions importance of the different contacts in crystal packing of this compound are compared to that observed in the [(C3H7)4N]HSeO4(H2SeO4)2 crystal. The solid-state 13C NMR spectrum shows two signals corresponding to two different carbon environments in CH2 and CH3 groups. The IR and Raman spectra were recorded in the range 400–3600 ​cm−1 and 50-3500 ​cm−1, respectively. The observed bands are assigned and discussed based on the theoretical analyses and computational calculated frequencies. The theoretical bond lengths, angles, and frequencies computed from the optimized molecular geometry through DFT method using B3LYP function and the 6-31G(d) basis set, are discussed and compared to the experimental results, which reveals a good agreement between the observed and calculated parameters. The nonlinear optical properties of [(C2H5)4N]HSeO4(H2SeO4)2 were studied using the DFT method, the calculated values of the electric dipole μ, the polarizability α, and the hyperpolarizability β. The calculations showed that the selenate-based compound exhibits a non-zero b value, revealing NLO behavior which could be used in potential and functional applications including optical switching and optical disk data storage.

Error correction methodology based on redundant recovery code for optical storage system

In order to improve the reliability of optical storage data, this paper proposes an error correction methodology in optical storage system which is based on redundant recovery code technology, it relates to the field of optical disc data storage. The methodology consists of two opposite processes – recording and retrieving. While recording data, firstly, splitting user data into blocks and encoding it with redundancy recovery code; next, organizing data blocks as UDF (universal disc format) file system; finally, modulating and encoding UDF file system data as strip group according to the optical disc physical format and recording it into optical disc. In contrast, while retrieving data, demodulating and decoding data strip group which is stored in optical disc at first; later, following UDF file system format to resume user data; in the end, verifying user data with redundant recovery code and return it to user. The methodology is compatible with the standard optical disc file system, and improves the fault tolerance efficiently. Original blu-ray physical format signal error rate is 4.1×10-13, the signal error rate can be down to 7.4×10-24 after redundancy recovery code check.

Synthesis, Growth and Physicochemical Properties of Seminorganic NLO Crystal Bis(Thiourea) Ammonium Nitrate

Nowadays, NLO materials are of current research interest due to their variety of applications such as second harmonic generation, optical bi-stability, laser remote sensing, optical disk data storage, laser driver fusion, medical and spectroscopic laser, photonic integrated circuitry, optical parametric oscillations and THz wave generation. This work concentrated on (bis)thiourea ammonium nitrate (BTAN) crystal was grown from aqueous solution by slow evaporation technique. The grown crystal has been subjected to single crystal X-ray diffraction to determine the unit cell dimensions. The grown crystal was also characterized by recording the powder X-ray diffraction patterns and by identifying the diffracting planes. The Fourier Transform Infrared (FT-IR) spectra have been recorded in the range 400–4500 cm-1. Second harmonic generation (SHG) for the materials of this work was confirmed using Nd:YAG laser. The UV–visible spectra show that the grown crystals have wide optical transparency in the entire visible region. The Thermo gravimetric/Differential Thermal Analyses reveal that the materials have good thermal stability.

Optical and electrical properties of thermally evaporated Se90Sb10 thin film

Chalcogenide Se90Sb10 thin films are deposited by thermal evaporation from the bulk alloy. X-ray diffraction examination for the annealed films shows the amorphous-crystalline transformation. This is beneficial for optical disk data storage technology. The crystallinity is improved by increasing the annealing temperature. The films annealed at relatively low temperatures exhibit highly transparence reaching to about 90% at incident light of wavelength of 900 nm. The as-prepared and annealed Se90Sb10 films reveal an indirect allowed optical transition. The annealed film at 473 K has an optical band gap of 1.676 eV which is suitable value for solar cell as photovoltaic application. Both the indirect optical energy band gap (Eg) and the oscillator energy (Eo) decrease whereas the oscillator strength (Ed) increases with increasing the annealing temperature. The annealing increases the conductivity and decreases the activation energy for conduction resulting in enhancement of film properties for adapting to solar cells.

Optical and electrical properties of Sn-Sb-Se chalcogenide thin films

The optical and electrical properties of the as-prepared and annealed Sn x Sb 20 Se (where x = 8, 10, 12, 13.5, 15, 16.5 and 18 at.%) thin films were studied. X-ray diffraction showed that all the as-prepared Sn-Sb-Se films were amorphous. Annealing the films at 473 K or above crystallized the films and the degree of crystallinity depended on the Sn content. The optical transmittance and reflectance were measured in the wavelength rang 200–2500 nm. The estimated optical band gap was found to decrease with increasing Sn content. A great difference in the optical constants values due to transformation from amorphous to crystalline phase structure of the films were found after annealing. This is advantageous for optical disk data storage applications. It was found that the resistivity decreases with increasing temperature for all the compositions indicating that these films have a semiconducting behavior with thermally activated conduction. The conduction in these films was suggested to be thermally assisted charge carrier movement in the extended states. Annealing the films caused a reduction in the room temperature resistivity by six order of magnitude. This was ascribed to the amorphous-crystalline transformation.

Three-dimensional optical disk data storage via the localized alteration of a format hologram

Three-dimensional optical data storage is demonstrated in an initially homogenous volume by first recording a reflection grating in a holographic photopolymer. This causes the entire volume to be weakly reflecting to a confocal read/write head. Superposition of two or three such gratings with slightly different k-vectors creates a track and layer structure that specialized servo detection optics can use to lock the focus to these deeply-buried tracks. Writing is accomplished by locally modifying the reflectivity of the preexisting hologram. This modification can take the form of ablation, inelastic deformation via heating at the focus, or erasure via linear or two-photon continued polymerization in the previously unexposed fringes of the hologram. Storage by each method is demonstrated with up to eight data layers separated by as little as 12 microns.

Using ellipsometry methods for depth analyzing the optical disc data layer relief structures

We studied the relief depth of the data layer formed in a glass disk by ion beam etching process with using classical ellipsometry at the constant wavelength 632.8 nm for different angles of incidence. It was found that for 0° and 90° azimuth angles, a pair of ellipsometric parameters Ψ and ∆ is sufficient to characterize the changes in light reflection for various structure depths. The depth of optical disc data layer relief structures was estimated via experimental dependences of ellipsometric parameters. The estimated data layer depths were found to be in good agreement with independent tunnelling electron microscopy measurements.

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.

Acousto-optic parallel read/write head for optical disk data storage

Parallel read and write of optical disks has traditionally used a static grating for read or a linear array of independent lasers for read and write. Depending on the implementation, these systems suffer from coherent cross talk, excessive space between spots, and an inability to independently track. We show that a dynamic acousto-optic grating can generate multiple parallel read/write spots on the disk, each of which can be independently modulated and tracked and all of which are incoherent in less that a bit period. The resulting disk pickup can potentially reach gigabit per second transfer rates with only a modest increase in the drive complexity.

Computing in optics - Simulating near-field effects in high-density optical-disk data storage

Plane-wave expansion methods for classical diffraction theory equations, coupled with the direct numerical solution of Maxwell's equations using local grid refinement and the finite-difference time-domain method, help us reliably simulate an optical system's macroscopic and subwavelength components.

Optical Properties of Multi-layer Chalcogenide Thin Film for Optical Recording Media

ABSTRACTWe present the results of optical properties of multi layer thin films as in the media of phase change optical disk data storage. Reflectance and optical contrast of multi layer thin films increased rapidly between 100 °C and 150 °C. Moreover, optical contrasts at different wavelength were also studied. The refractive index and the optical band gap decreased, while the extinction coefficient increases as the crystallization occurs. The Egopt of crystalline thin film was ∼0.6 eV lower than that of amorphous thin film. Egopt decreased as the number of stacked layer increased.

Crystallization behavior of Ge-doped eutectic Sb70Te30 films in optical disks.

We report laser-induced crystallization behavior of binary Sb-Te and ternary Ge-doped eutectic Sb70Te30 thin film samples in a typical quadrilayer stack as used in phase-change optical disk data storage. Several experiments have been conducted on a two-laser static tester in which one laser operating in pulse mode writes crystalline marks on amorphous film or amorphous marks on crystalline film, while the second laser operating at low-power cw mode simultaneously monitors the progress of the crystalline or amorphous mark formation in real time in terms of the reflectivity variation. The results of this study show that the crystallization kinetics of this class of film is strongly growth dominant, which is significantly different from the crystallization kinetics of stochiometric Ge-Sb-Te compositions. In Sb-Te and Ge-doped eutectic Sb70Te30 thin-film samples, the crystallization behavior of the two forms of amorphous states, namely, as-deposited amorphous state and melt-quenched amorphous state, remains approximately same. We have also presented experiments showing the effect of the variation of the Sb/Te ratio and Ge doping on the crystallization behavior of these films.

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.

Superresolution near-field readout in phase-change optical disk data storage

Readout of a phase-change optical disk with a superresolution (SR) near-field structure (Super-RENS) is theoretically examined on the basis of three-dimensional, full-wave vector diffraction theory. Calculations have demonstrated that Super-RENS has a high spatial resolution beyond the diffraction limit in readout. The read signal is dependent on the nature of SR, the layer structure of the disk, and the state of polarization of the incident laser beam. For the Super-RENS in which antimony is used for SR readout, the readout signal is quite small, and the estimated carrier-to-noise ratio (CNR) is only ~30 dB for marks of 300 nm. For the Super-RENS in which a metallic region is formed during readout, the read signal is large, and the CNR can be as high as 50 dB in reading 300-nm marks.

Thermal cross-track cross talk in phase-change optical disk data storage

We have investigated the temperature distribution in land/groove phase-change optical disks. The incident beam is linearly polarized either parallel to track (E∥ polarization) or tangential to track (E⊥ polarization). Calculations have shown that temperature profiles in the medium are dependent on the wavelength of light, the state of polarization, the geometry of the grooved structure, and the multilayer stack. The temperature profiles are quite different between the land track and the groove track. Thermal cross-track cross talk from a land track to its neighboring groove tracks is higher than that from a groove track to its neighboring land tracks. The interaction between the E⊥ electric field and the grooved structure is mainly responsible for thermal cross-track cross talk.

Crystallization and amorphization studies of a Ge_2Sb_23Te_5 thin-film sample under pulsed laser irradiation

We present the results of crystallization and amorphization studies on a thin-film sample of Ge(2)Sb(2.3)Te(5), encapsulated in a quadrilayer stack as in the media of phase-change optical disk data storage. The study was conducted on a two-laser static tester in which one laser, operating in pulsed mode, writes either amorphous marks on a crystalline film or crystalline marks on an amorphous film. The second laser, operating at low power in the cw mode, simultaneously monitors the progress of mark formation in terms of the variations of reflectivity both during the write pulse and in the subsequent cooling period. In addition to investigating some of the expected features associated with crystallization and amorphization, we noted certain curious phenomena during the mark-formation process. For example, at low-power pulsed illumination, which is insufficient to trigger the phase transition, there is a slight change in the reflectivity of the sample. This is believed to be caused by a reversible change in the complex refractive index of the Ge(2)Sb(2.3)Te(5) film in the course of heating above the ambient temperature. We also observed that the mark-formation process may continue for as long as 1 mus beyond the end of the write laser pulse. This effect is especially pronounced during amorphous mark formation under high-power, long-pulse illumination.

Effects of an asymmetrically molded plastic objective lens on the push–pull tracking-error signal in an optical disk drive

The effects of a plastic objective lens's astigmatism on the push-pull tracking-error signal (TES) of an optical disk data storage system were investigated theoretically and experimentally. Astigmatism of plastic objective lenses arises commonly from the asymmetric deviation from their designed shape during the molding process. By carefully studying the aberration characteristics of the objective lens and including the astigmatism of the laser diode in the analysis, we can calculate the combined effects of astigmatism of these two components on the push-pull TES. It is found, from both the simulations and the experiments, that, by rotation of the objective lens about the optical axis, the peak-to-peak value of the push-pull TES varies with the lens's rotation angle, and a change as great as 340% in its value was observed in a given optical pickup.

Some considerations of organic materials for high density optical disk data storage

The application possibilities of organic materials for high density optical disk data storage are discussed. Several points, such as physical and chemical stabilities, wavelength match and reversible property changes, which should be taken into consideration, are presented.

Measurements and simulations of differential phase-tracking signals in optical disk data storage

The tracking-error signal generated in differential phase detection (DPD) is theoretically analyzed and numerically simulated. Experimental measurements of the DPD signal versus the tracking offset obtained on compact read-only and phase-change disks are also reported. The signal is sensitive to the geometry of the marks, intersymbol interference along the track, and cross-track cross talk. A characteristic parameter is introduced to relate the DPD signal to the reflectivities of the mark and the spacer. For read-only disks such as CD-ROM and DVD-ROM, the magnitude of the DPD signal does not seem to depend on the reflectivity of the disks, nor does it depend on the pit depth. As for the influence of the various aberrations on the DPD signal, coma in the cross-track direction is shown to give rise to significant tracking offset, whereas defocus and spherical aberrations reduce the magnitude of the DPD signal appreciably.

Characterization of erasable inorganic photochromic media for optical disk data storage

Optical recording characteristics of an ion-exchanged inorganic photochromic media is investigated. The experiments show that the ion-exchanged recording layer can support high data recording density with excellent optical readout contrast. Its recording speed at available red diode laser power can be comparable and even faster than most CD-erasable drives. A focused UV light or electron beams can erase the recorded data on the ion-exchanged layer and facilitate further data rewriting by focused laser beams. The recording medium has excellent thermal stability and supports nondestructive data readout for erasable optical data storage applications.