Performance of a write-once multilayer optical disk that uses transparent recording material with an optical switching layer

A volumetric optical disk that has multiple transparent films with optical switching layers is used as a recording medium to increase the number of recording layers. In the disk the optical switching layer is adapted to reduce decay of laser energy and increase reading and recording sensitivity. Well-defined marks of approximately 100-nm depth can be placed precisely on the transparent films by a focused laser beam. Writing and reading of a four-layer recordable disk, fabricated by molding and spin bonding, have been demonstrated experimentally. The volumetric disk can achieve a high recording capacity with conventional optical pickups.     

超高密度磁存储的展望

磁性存储是最常用的海量存储技术,其记录密度越来越高,发展也越来越快。本文通过对信息记录、读出和存储三个过程的分析,对比了硬磁盘记录、垂直磁记录和磁光记录的优缺点,指出了采用垂直记录模式、非晶结构合金薄膜或铁氧体薄膜介质是实现超高密记录的方向,光辅助磁记录是很有希望的记录技术。同时,还指出量子磁盘技术是未来高密记录的方向。     

Local near-field enhancement of random Sb-SiN films

Local near-field enhancement of random Sb-SiN films has been studied. Specimens consisting of a random Sb-SiN film and an optical recording layer were prepared and exposed to a focused laser beam. Laser-induced ablation occurred on the recording layer adjacent to random Sb-SiN film much faster and at much lower power than on a single recording layer. These results indicate that an optical field can be enhanced by random Sb-SiN films. The enhanced field was subsequently investigated by scanning near-field optical microscopy, and the pictures revealed that the enhanced field was localized.     

Role of substrate temperature on the structural, morphological and optical properties of CuGaSe2 thin films grown by Pulsed Electron Deposition technique

CuGaSe₂ (CGS) thin films were grown on uncoated and Mo-coated soda lime glass by Pulsed Electron Deposition (PED) technique at substrate temperatures comprised between 25 °C and 475 °C. X-ray diffraction analysis reveals that CGS samples exhibit a noteworthy crystal quality even at low growth temperature, T_g = 100 °C, whereas the out-of-plane preferential orientation of CGS chalcopyrite phase switches from < 220 > to < 112 > by increasing the substrate temperature. Annealing treatments seem to enhance the crystallinity of the film and to release the residual strain energy. Visible/near-infrared absorbance spectra show a monotonic decrease of CGS optical bandgap (from 1.75 to 1.65 eV) by enhancing the substrate temperature. Yet the morphology of CGS films strongly depends on T_g, which promotes the formation of larger columnar grains perpendicular to the growth plane. Grain dimensions of ~ 2 μm are achieved when CGS films are grown at high temperature (> 400 °C) on Mo-coated glass. The results indicate that PED is a promising growth technique for achieving good-quality CGS that can be useful as absorber layers in thin film solar cells.     

Effect of Initialization Technical Parameters on Optical Absorption of Ge2Sb2Te5 Thin Films

The optical absorption properties of phase-change optical recording thin films subjected to various initialization conditions were investigated. The effects of initialization power and velocity on optical constants of the Ge2Sb2Te5 thin films were also studied. The energy gap of Ge2Sb2Te5 thin films subjected to various initialization conditions was also obtained. It was found that the optical energy gap of the Ge2Sb2Te5 thin films increased with either increasing initialization laser power or decreasing initialization velocity, with peak of 0.908 eV at laser power of 1000 mW or initialization velocity of 4.0 m/s, but the continued increasing initialization laser power or decreasing initialization velocity resulted in the decrease of the optical energy gap. The change of the optical energy gap was discussed on the basis of amorphous crystalline transformation.     

Thermodynamic simulation of magnetic-field-modulation methods for pulsed laser irradiation in magneto-optical disks

Direct overwrite magnetic-field modulation recording with dc and pulsed laser irradiation in a magneto-optical disk was simulated thermodynamically. Because the laser light heats the magnetic film, which allows the magnetization to be reversed, the waveform of the laser light strongly affects the shape of the magnetic domain. The dynamic temperature changes in magnetic thin films are simulated for dc and pulsed laser irradiations. With pulsed irradiation the heat is constrained within a limited area, which results in jitter-free magnetic domains, high optical efficiency, and high tracks-per-inch and bits-per-inch recordings.     

Polarization holographic high-density optical data storage in bacteriorhodopsin film

Optical films containing the genetic variant bacteriorhodopsin BR-D96N were experimentally studied in view of their properties as media for holographic storage. Different polarization recording schemes were tested and compared. The influence of the polarization states of the recording and readout waves on the retrieved diffractive image's intensity and its signal-to-noise ratio were analyzed. The experimental results showed that, compared with the other tested polarization relations during holographic recording, the discrimination between the polarization states of diffracted and scattered light is optimized with orthogonal circular polarization of the recording beams, and thus a high signal-to-noise ratio and a high diffraction efficiency are obtained. Using a He-Ne laser (633 nm, 3 mW) for recording and readout, a spatial light modulator as a data input element, and a 2D-CCD sensor for data capture in a Fourier transform holographic setup, a storage density of 2 x 10(8) bits/cm2 was obtained on a 60 x 42 microm2 area in the BR-D96N film. The readout of encoded binary data was possible with a zero-error rate at the tested storage density.     

Magneto-optical recording on CoPtRe alloy films as a novel material

Magneto-optical (MO) disks using CoPtRe ternary alloy films have been investigated as a novel recording material for a blue laser recording. For various types of disk structure, the Kerr rotation, the Kerr ellipticity, and the figure of merit were calculated and an optimum structure was estimated. Magneto-optical recordings were made on a CoPtRe MO disk at wavelengths of 830 and 530 nm. The recording properties were almost equivalent to those for Co/Pt multilayers     

Characteristics of phase transitions in inhomogeneous silicon monoxide/chromium thin films exposed to pulsed laser irradiation

An investigation was made of the behavior of SiO/Cr cermet films of nonuniform composition exposed to pulsed laser irradiation. It is shown that radiation-stimulated surface segregation of chromium may be observed in these films and can be used for the optical recording of information. Pis’ma Zh. Tekh. Fiz. 25, 19–23 (February 26, 1999)     

Material science aspects of phase change optical recording

Chalcogenide thin films are used as the recording medium for phase change-type optical memory discs. The films are switched between amorphous and crystalline states using the heat of a focussed laser beam. Large reflectivity differences between amorphous and crystalline states are then used to store and retrieve the information. An active chalcogenide layer for this purpose should have a high optical absorption coefficient (α), and good structural and thermal stability. It should be possible to switch the chalcogenide layer between amorphous and crystalline states repeatedly within a short duration, the optical contrast should be high, and the material must have large cycling capability. Keeping the above requirements in mind, we have carried out systematic investigation of structural, optical and crystallization behaviour of thin films of various compositions of GaGeTe, Sb₂Te₃ and BiSe. These studies have shown that these materials can be good candidates for use as recording media in erasable phase-change optical recording.     

DFWM of focused laser beams in a-As2S3 and azobenzene oligomer films

Degenerate four-wave mixing (DFWM) has two advantages for the study of photoinduced processes when compared to usual holographic recording by two waves: (1) a signal beam shutter is not necessary; (2) a higher signal-to-noise ratio at the photodetector site is possible. Focusing of the recording beams makes it possible to significantly increase the light intensity thus widening the limits of experimental possibilities. In this paper DFWM by focused He–Ne laser 633 nm beams has been experimentally studied in a-As₂S₃ and azobenzene oligomer (ABO) [4-((4-nitrophenyl)diazenyl)-N,N-bis(2-(tetrahydro-2H-pyran-2-yloxy)ethyl)benzenamine] films in comparison with unfocused DFWM. Experiments show that growing of chromophore concentration increases the DFWM efficiency in ABO. Focusing significantly increases the DFWM efficiency in a-As₂S₃ but not in ABO. The DFWM efficiency increase in a-As₂S₃ films can be explained by photothermally stimulated relaxational structural changes. On the contrary, in ABO films the recording is based on the photoorientation of chromophores, and this process is, most probably, hindered by photoinduced heating. The possibility of a precise real-time DFWM with focused laser beams is discussed.     

Immersion holography based on amorphous chalcogenide films

A solid immersion holographic method for the recording of refractive index and surface-relief modulated gratings with a period of 0.1–1 μm in amorphous films of chalcogenide semiconductors As₂S₃ and As–S–Se has been developed and studied. The laser immersion interference lithography can be used as a low-cost method for the exposure of large surfaces with regular patterns like subwavelength-gratings and microsieves. The polarization sensitive properties of the subwavelength refractive-index modulated transmission gratings were studied. The possibility to use the amorphous chalcogenide films as a media for holographic recording and storage of information with high density is discussed.     

Effect of Cu excess on three-stage CuGaSe2 thin films using in-situ process controls

The objective of this work is to study the effect of Cu excess and compare the growth mechanism of CuGaSe₂ (CGS) films co-evaporated using a bilayer and a three-stage process and evaluate the consequences of the latter for CGS on transparent back contacts. CGS thin films are prepared by co-evaporation in a three-stage process onto Mo/soda-lime substrates and onto FTO. In contrast to the bilayer process, Cu-Se phases are only observed on the surface at the end of the second stage, e₂. This allows to work with a broader Cu-excess window. Atomic ratios (Cu/Ga)e₂ of around 1.3 at the end of deposition phase 2 in the three-stage process show the better device efficiencies due to a larger grain formation. Increasing the Cu content leads to a slight decrease of the grain size and voids are observed in the film, reducing the FF of the device. The CGS morphology and the solar cells efficiency are dominated by the Cu excess more than by the T_substrate between 535 °C and 500 °C. Similar results are obtained for CGS on FTO: (Cu/Ga)e₂ ∼ 1.3 as best composition at T_substrate = 500 °C.     

Methylene blue sensitized poly(methyl methacrylate) matrix: a novel holographic material

Methylene blue sensitized poly(methyl methacrylate) is shown to be an efficient medium for recording three-dimensional holographic gratings. Phase and/or amplitude holograms can be written in the methylene blue sensitized films of poly(methyl methacrylate) with a conventional source of light, a He-Ne laser operating at 632.8 nm. Diffraction efficiencies of 60% were found for thick holograms. Multiple holograms were recorded in the described system, and an optical erasing of holograms was achieved. Hologram recording speed was found to increase with temperature.     

Thermal properties and crystallization dynamics of a phase-change alloy for write-once optical data storage

Using a two-laser static tester, we measured the crystallization temperature and the thermal conductivity of a phase-change alloy thin film used in write-once-read-many media of optical data storage. The experimental technique, in general, and the calibration procedures, in particular, are described. The measurement results are used as entry points into numerical calculations that ultimately yield estimates of the material parameters. Valuable information about the dynamics of mark formation (i.e., localized crystallization) in amorphous phase-change alloy films is obtained from the observed variations of the sample reflectance under short-pulse and long-pulse recording conditions. The dependence of these reflectance variations on the laser pulse power has also been investigated.     

Nanoscale bit formation in highly (111)-oriented ferroelectric thin films deposited on glass substrates for high-density storage media

PbTiO(3) (PTO) ferroelectric films on Pt(111) bottom electrode layers covering Ta/glass were prepared using pulsed laser deposition. X-ray diffraction patterns revealed that the PTO films were preferentially (111)-oriented. The films were highly crystalline and had a smooth surface with root mean square (RMS) roughness of 1.5 nm. Ferroelectric properties of the PTO films were characterized using piezoresponse force microscopy (PFM). PFM techniques achieved ferroelectric polarization bits with a minimum width of 22 nm, which corresponds to a potential recording density of 1.3 Tbit/in(2) in ferroelectric storage devices.     

Effect of inert gas on pulsed laser deposition of rare earth doped fluoride thin films

The realization of crystalline sub micrometric films of Nd³⁺:LiYF₄ fluorides on pure LiYF₄ substrates by pulsed laser deposition is reported. The films were obtained by laser ablation with 355 nm photons of a bulk LiYF₄ crystal doped with Nd³⁺ ions at 1.5% atomic concentration. Both low and high laser ablation fluency regimes were checked. The fundamental contribution of He inside a vacuum chamber to obtain crystalline Nd³⁺:LiYF₄ films was demonstrated. The optical characteristics of the films were analyzed via recording the emission spectra following IR excitation, polarized both with E || and E ⊥ to the substrate c-axis. Lifetime measurements of the fundamental Nd³⁺ ion transition in the films were also performed. All the results were compared with those obtained in the Nd³⁺:LiYF₄ bulk crystal under the same experimental conditions. The scanning electron microscope pictures of the depositions gave indications about the film surface morphology.     

Surface morphology of AlN films synthesized by pulsed laser deposition

Surface morphology of AlN films, synthesized on Si substrates by pulsed laser deposition, has been examined by recording atomic-force-microscopy (AFM) images. The influence of N₂ ambient pressure, ranging from 5 × 10⁻⁴ Pa to 10 Pa, is reflected well in the alteration of the surface roughness and size of crystallites of the AlN films. A tendency of a decrease in the surface roughness with increasing N₂ pressure was observed, which also correlates with the polycrystalline structure of the films. Deposition in vacuum resulted in the highest surface roughness due to the large size of crystallites emerging from the surface, while increasing the nitrogen pressure yielded smaller crystallites and a smoother film surface. The presented results could be useful for applications of pulsed laser deposited AlN in different optical and acoustic devices, where the crystalline quality of the AlN films and the surface is very important.     

TeOx Thin Films for Write-Once Optical Recording Media

TeOx thin films were prepared by vacuum evaporation of TeO2 powder. Structural characteristic and surface mor-phology of the as-deposited films was analyzed by using X-ray photoelectron spectroscopy, transmission electron microscopy, X-ray diffractometer and atomic force microscopy. It was found that the films represented a two component system comprising Te particies dispersed in an amorphous TeO2 matrix. The dispersed Te particies were in a crystalline state. The TeOx films showed a finely granular structure and a rough surface. Results of the statie recording test showed that the TeOx films had good writing sensitivity for short-wavelength laser beam (514.5 nm). Primary results of the dynamic test at 514.5 nm were also reported. The TeOx films were suitable for using as a blue-green wavelength high density optical storage medium.     

Littrow-type external-cavity blue laser for holographic data storage

An external-cavity laser with a wavelength of 405 nm and an output of 80 mW has been developed for holographic data storage. The laser has three states: the first is a perfect single mode, whose coherent length is 14 m; the second is a three-mode state with a coherent length of 3 mm; and the third is a six-mode state with a coherent length of 0.3 mm. The first and second states are available for angular-multiplexing recording; all states are available for coaxial multiplexing recording. Due to its short wavelength, the recording density is higher than that of a 532 nm laser.