Rewritable Optical Disk Research Articles

Custom gold-patterned rewritable optical disc based plasmonic sensor for blood hemoglobin detection

In this paper, we have presented a novel plasmonic Refractive Index (RI) sensing scheme based on a rewritable optical disc structure with customized dimer-like nanopatterns. We have performed the finite-difference time-domain (FDTD) simulation for this work to exhibit the optical resonant response and sensing performance of our proposed sensing platform. We have also investigated how the changes in the different structural parameters affect the resonant wavelength. The high refractive index sensitivity of 768nm/RIU allowed our sensing system to achieve a resolution limit for blood Hemoglobin(Hb) detection superior to most Hb sensors previously reported. The use of the built-in grating structure of the optical disc, the ability to create customized patterns, and the high resolution achieved in blood Hb detection suggest the potential of our proposed RI sensor as a cost-efficient, reliable sensing technology for medical applications.

Nonthermal Transport of Energy Driven by Photoexcited Carriers in Switchable Solid States of GeTe

Phase change alloys have seen widespread use from rewritable optical discs to the present day interest in their use in emerging neuromorphic computing architectures. In spite of this enormous commercial interest, the physics of carriers in these materials is still not fully understood. Here, we describe the time and space dependence of the coupling between photoexcited carriers and the lattice in both the amorphous and crystalline states of one phase change material, GeTe. We study this using a time-resolved optical technique called picosecond acoustic method to investigate the \textit{in situ} thermally assisted amorphous to crystalline phase transformation in GeTe. Our work reveals a clear evolution of the electron-phonon coupling during the phase transformation as the spectra of photoexcited acoustic phonons in the amorphous ($a$-GeTe) and crystalline ($\alpha$-GeTe) phases are different. In particular and surprisingly, our analysis of the photoinduced acoustic pulse duration in crystalline GeTe suggests that a part of the energy deposited during the photoexcitation process takes place over a distance that clearly exceeds that defined by the pump light skin depth. In the opposite, the lattice photoexcitation process remains localized within that skin depth in the amorphous state. We then demonstrate that this is due to supersonic diffusion of photoexcited electron-hole plasma in the crystalline state. Consequently these findings prove the existence of a non-thermal transport of energy which is much faster than lattice heat diffusion.

Unraveling the optical contrast in Sb2Te and AgInSbTe phase-change materials

Chalcogenide phase-change materials (PCMs) show a significant contrast in optical reflectivity and electrical resistivity upon crystallization from the amorphous phase and are leading candidates for non-volatile photonic and electronic applications. In addition to the flagship Ge2Sb2Te5 phase-change alloy, doped Sb2Te alloys, in particular AgInSbTe used in rewritable optical discs, have been widely investigated for decades, and nevertheless the theoretical insights on the optical properties of this important family of PCMs are scarce. Here, we carry out thorough ab initio simulations to gain an atomistic understanding of the optical properties of Sb2Te and AgInSbTe. We show that the large optical contrast between the amorphous and crystalline phase stems from the change in bond type in the parent compound Sb2Te. Ag and In impurities serve mostly the purpose of stabilization of the amorphous phase, and have marginal impact on the large variation in the dielectric function upon the phase transitions.

A Nonvolatile Phase‐Change Metamaterial Color Display

AbstractChalcogenide phase‐change materials, which exhibit a marked difference in their electrical and optical properties when in their amorphous and crystalline phases and can be switched between these phases quickly and repeatedly, are traditionally exploited to deliver nonvolatile data storage in the form of rewritable optical disks and electrical phase‐change memories. However, exciting new potential applications are now emerging in areas such as integrated phase‐change photonics, phase‐change optical metamaterials/metasurfaces, and optoelectronic displays. Here, ideas from these last two fields are fused together to deliver a novel concept, namely a switchable phase‐change metamaterial/metasurface resonant absorber having nonvolatile color generating capabilities. With the phase‐change layer, here GeTe, in the crystalline phase, the resonant absorber can be tuned to selectively absorb the red, green, and blue spectral bands of the visible spectrum, so generating vivid cyan, magenta, and yellow pixels. When the phase‐change layer is switched into the amorphous phase, the resonant absorption is suppressed and a flat, pseudowhite reflectance results. Thus, a route to the potential development is opened‐up of nonvolatile, phase‐change metamaterial color displays and color electronic signage.

Engineering Interface-Dependent Photoconductivity in Ge2Sb2Te5 Nanoscale Devices.

Phase-change materials are increasingly being explored for photonics applications, ranging from high-resolution displays to artificial retinas. Surprisingly, our understanding of the underlying mechanism of light-matter interaction in these materials has been limited to photothermal crystallization because of its relevance in applications such as rewritable optical discs. Here, we report a photoconductivity study of nanoscale thin films of phase-change materials. We identify strong photoconductive behavior in phase-change materials, which we show to be a complex interplay of three independent mechanisms: photoconductive, photoinduced crystallization, and photoinduced thermoelectric effects. We find that these effects also congruously contribute to a substantial photovoltaic effect, even in notionally symmetric devices. Notably, we show that device engineering plays a decisive role in determining the dominant mechanism; the contribution of the photothermal effects to the extractable photocurrent can be reduced to <0.4% by varying the electrodes and device geometry. We then show that the contribution of these individual effects to the photoresponse is phase-dependent with the amorphous state being more photoactive than the crystalline state and that a reversible change occurs in the charge transport from thermionic to tunnelling during phase transformation. Finally, we demonstrate photodetectors with an order of magnitude tunability in photodetection responsivity and bandwidth using these materials. Our results provide insights to the photophysics of phase-change materials and highlight their potential in future optoelectronics.

Re-amorphization of GeSbTe alloys not through a melt-quenching process

Ge–Sb–Te (GST) alloys are one of the most successful chalcogenides used in rewritable optical discs and electric non-volatile memories. In these materials the structural phase transition between amorphous and crystalline states is used for recording. Here, we report that a melt-quenching method is not necessarily the only way to form a high-resistance phase. We found that amorphous-like grains with high resistance are formed in the GST crystal film by holding it at above the crystallization temperature under an external magnetic field, followed by cooling accompanied with pulse current injection. The treatment may open new route for ultra-low current switching phase change memory.

Optical bistability in shape-memory nanowire metamaterial array

Non-volatile temperature-induced structural phase transitions such as those found in chalcogenide glasses are known to lead to strong changes in optical properties and are widely used in rewritable optical disk technology. Herein, we demonstrate that thermally activated optical memory can be achieved via the nanostructural reconfiguration of a metallic nanowire metamaterial array made from a shape-memory alloy: A nickel-titanium film of nanoscale thickness structured on the subwavelength scale exhibits bistability of its optical properties upon temperature cycling between 30 °C and 210 °C. The structure, comprising an array of NiTi nanowires coated with a thin film of gold to enhance its plasmonic properties, can exist in two non-volatile states presenting an optical reflectivity differential of 12% via nanoscale mutual displacements of alternating nanowires in the structure. Such all-metal shape-memory photonic gratings and metamaterials may find applications in bistable optical devices.

Resolving Crystallization Kinetics of GeTe Phase-Change Nanoparticles by Ultrafast Calorimetry.

Chalcogenide-based phase change materials (PCMs) are promising candidates for the active element in novel electrical nonvolatile memories and have been applied successfully in rewritable optical disks. Nanostructured PCMs are considered as the next generation building blocks for their low power consumption, high storage density, and fast switching speed. Yet their crystallization kinetics at high temperature, the rate-limiting property upon switching, faces great challenges due to the short time and length scales involved. Here we present a facile method to synthesize highly controlled, ligand-free GeTe nanoparticles, an important PCM, with an average diameter under 10 nm. Subsequent crystallization by slow and ultrafast rates allows unravelling of the crystallization kinetics, demonstrating the breakdown of Arrhenius behavior for the crystallization rate and a fragile-to-strong transition in the viscosity as well as the overall crystal growth rate for the as-deposited GeTe nanoparticles. The obtained results pave the way for further development of phase-change memory based on GeTe with sub-lithographic sizes.

Технології створення оптичних носіїв для систем довготермінового зберігання даних

It was shown that long-term preservation of digital information is an important issue in modern world and there are a number of engineering tasks and applications that need long-term storage of information. The variety of technical solutions were proposed for technology of long term data storage which is based on using highly stable materials for both substrate and recording medium. The concept of storing data optically in the bulk of non-photosensitive transparent materials which is renowned for its high chemical stability and resistance via femtosecond-laser allows for high-capacity optical recording, It was noticed that in the process of creation of WORM optical discs the main attention was also paid to the choice of materials for recording media capable to provide long-term storage of recorded data, though, modern WORM-media as well as rewritable optical discs do not meet requirements and standards of the long-term recording. It was analyzed M-DISC's design which was intended to provide greater archival media longevity. M-DISC's technology asserts that the data layer is a glassy carbon and that the material is substantially inert to oxidation and has a high melting point. It was concluded that information structure of long-term optical discs should bring possibility to provide readout process by different methods: with modern optical disc drive, optical microscopy and ultra-sonic readout.

Some issues of long-term storage of electronic documents

This paper considers basic aspects of the preservation of digital content using optical discs and special storage systems. Features of write-once and rewritable optical discs are shown. The EMC Centera datastorage system is discussed. The experience of work on the formation and storage of information arrays on the example of Digital Library Scientific Heritage of Russia and projects of the Russian State Archive of Scientific and Technical Documentation is analyzed.

Femtosecond structural transformation of phase-change materials far from equilibrium monitored by coherent phonons.

Multicomponent chalcogenides, such as quasi-binary GeTe–Sb2Te3 alloys, are widely used in optical data storage media in the form of rewritable optical discs. Ge2Sb2Te5 (GST) in particular has proven to be one of the best-performing materials, whose reliability allows more than 106 write–erase cycles. Despite these industrial applications, the fundamental kinetics of rapid phase change in GST remain controversial, and active debate continues over the ultimate speed limit. Here we explore ultrafast structural transformation in a photoexcited GST superlattice, where GeTe and Sb2Te3 are spatially separated, using coherent phonon spectroscopy with pump–pump–probe sequences. By analysing the coherent phonon spectra in different time regions, complex structural dynamics upon excitation are observed in the GST superlattice (but not in GST alloys), which can be described as the mixing of Ge sites from two different coordination environments. Our results suggest the possible applicability of GST superlattices for ultrafast switching devices.

Fast Crystallization of the Phase Change Compound GeTe by Large-Scale Molecular Dynamics Simulations.

Phase change materials are of great interest as active layers in rewritable optical disks and novel electronic nonvolatile memories. These applications rest on a fast and reversible transformation between the amorphous and crystalline phases upon heating, taking place on the nanosecond time scale. In this work, we investigate the microscopic origin of the fast crystallization process by means of large-scale molecular dynamics simulations of the phase change compound GeTe. To this end, we use an interatomic potential generated from a Neural Network fitting of a large database of ab initio energies. We demonstrate that in the temperature range of the programming protocols of the electronic memories (500-700 K), nucleation of the crystal in the supercooled liquid is not rate-limiting. In this temperature range, the growth of supercritical nuclei is very fast because of a large atomic mobility, which is, in turn, the consequence of the high fragility of the supercooled liquid and the associated breakdown of the Stokes-Einstein relation between viscosity and diffusivity.

Germanium antimony lateral nanowire phase change memory by chemical vapor deposition

Abstractmagnified imageThe phase change technology behind the current rewritable optical disks and the latest generation of electronic memories has provided clear commercial and technological advances for the field of data storage; by virtue of the many key attributes chalcogenide materials offer. In this work, germanium antimony (Ge–Sb) lateral nanowire phase change memory devices have been fabricated from thin films deposited by chemical vapor deposition (CVD). Deposition takes place at atmospheric pressure using metal chloride precursors at reaction temperatures between 750 and 875 °C. The fabricated devices have been characterized electrically demonstrating reversible phase change, while a lowering in power consumption in these memory cells is observed with scaling of the geometry of the nanowire cells. The results are investigated by electrothermal modeling to understand the temperature of the devices during operation. These prototype CVD‐grown Ge–Sb lateral nanowire devices show promise for applications such as phase‐change memory and optical, electronic, and plasmonic switching.magnified image Artistic impression of Ge–Sb nanowire memory device.

One-Head Near-Field Writing/Erasing on a Rewritable Dual-Layer Optical Disk Having High-Index Cover and Separation Layers

Near-field writing and erasing on a rewritable dual-layer optical disk with a single solid immersion lens (SIL) optical head is demonstrated for the first time. A novel beam expander enabled access to both information layers of the disk without exchanging two individual optical heads. A rewritable dual-layer disk having high-index (n=1.8) cover and separation layers was newly developed. By using the SIL optical system and the disk, an effective numerical aperture (NA) of 1.42 is achieved and a recording capacity of 180 Gbyte for the dual-layer rewritable disk is expected.

One-Head Near-Field Writing/Erasing on a Rewritable Dual-Layer Optical Disk Having High-Index Cover and Separation Layers

One-Head Near-Field Writing/Erasing on a Rewritable Dual-Layer Optical Disk Having High-Index Cover and Separation Layers

Rewritable Triple-Layer Phase-Change Optical Disk Providing 100 Gbyte Capacity

A rewritable phase-change optical disk providing a large capacity of 100 Gbyte on a 120 mm disk was first demonstrated using the multilayer Blu-ray DiscTM (BD-XL) format. The doubled capacity of this optical disk compared with that of a conventional dual-layer disk was achieved firstly by stacking triple recording layers and secondly by increasing the recording capacity per layer from 25 to 33.4 Gbyte at 33.6%. The high transmittances of 50% (middle layer) and 60% (front layer) were achieved by thinning a Ge–Sb–Te phase-change film to 7.5 and 6 nm and also by thinning a Ag-alloy film to 9 and 7 nm, respectively. An additional TiO2-based film formed on the Ag-alloy film was effective in improving the transmittance at 3%, compared with the structure using a conventional TiO2 film. Furthermore, a transmittance-balanced structure was adopted for these layers in order to stabilize the recording-reading properties. To improve cyclability, ZrO2–Cr2O3-based interface films were provided on both sides of the phase-change film for the middle and front layers. The increase in recording capacity per layer was achieved by reducing the minimum mark length from 0.149 to 0.112 µm. Since the optical changes degrade with the reductions in the mark lengths and thicknesses of the Ge–Sb–Te and Ag-alloy films, a phase-change material with a GeTe-rich composition on a GeTe–Sb2Te3 pseudo-binary line was adopted for every layer to compensate it. It was confirmed that the sample disk successfully satisfies all the requirements of the BD-XL format.

Rewritable Triple-Layer Phase-Change Optical Disk Providing 100 Gbyte Capacity

A rewritable phase-change optical disk providing a large capacity of 100 Gbyte on a 120 mm disk was first demonstrated using the multilayer Blu-ray DiscTM (BD-XL) format. The doubled capacity of this optical disk compared with that of a conventional dual-layer disk was achieved firstly by stacking triple recording layers and secondly by increasing the recording capacity per layer from 25 to 33.4 Gbyte at 33.6%. The high transmittances of 50% (middle layer) and 60% (front layer) were achieved by thinning a Ge–Sb–Te phase-change film to 7.5 and 6 nm and also by thinning a Ag-alloy film to 9 and 7 nm, respectively. An additional TiO2-based film formed on the Ag-alloy film was effective in improving the transmittance at 3%, compared with the structure using a conventional TiO2 film. Furthermore, a transmittance-balanced structure was adopted for these layers in order to stabilize the recording-reading properties. To improve cyclability, ZrO2–Cr2O3-based interface films were provided on both sides of the phase-change film for the middle and front layers. The increase in recording capacity per layer was achieved by reducing the minimum mark length from 0.149 to 0.112 µm. Since the optical changes degrade with the reductions in the mark lengths and thicknesses of the Ge–Sb–Te and Ag-alloy films, a phase-change material with a GeTe-rich composition on a GeTe–Sb2Te3 pseudo-binary line was adopted for every layer to compensate it. It was confirmed that the sample disk successfully satisfies all the requirements of the BD-XL format.

Thermal and elastic properties of Ge-Sb-Te based phase-change materials

ABSTRACTPhase-change materials undergo a change in bonding mechanism upon crystallization, which leads to pronounced modifications of the optical properties and is accompanied by an increase in average bond lengths as seen by extended x-ray absorption fine structure (EXAFS), neutron and x-ray diffraction. The reversible transition between a crystalline and an amorphous phase and its related property contrast are already employed in non-volatile data storage devices, such as rewritable optical discs and electronic memories. The crystalline phase of the prototypical material GeSb2Te4 is characterized by resonant bonding and pronounced disorder, which help to understand their optical and electrical properties, respectively. A change in bonding, however, should also affect the thermal properties, which will be addressed in this study. Based on EXAFS data analyses it will be shown that the thermal and static atomic displacements are larger in the meta-stable crystalline state. This indicates that the bonds become softer in the crystalline phase. At the same time, the bulk modulus increases upon crystallization. These observations are confirmed by the measured densities of phonon states (DPS), which reveal a vibrational softening of the optical modes upon crystallization. This demonstrates that the change of bonding upon crystallization in phase-change materials also has a profound impact on the lattice dynamics and the resulting thermal properties.

Chalcogenide glasses in active plasmonics

AbstractThe phase‐change technology behind rewritable optical disks and the latest generation of electronic memories can also offer high contrast plasmonic switching functionality. Numerical simulations illustrate the extent of this potential while preliminary experiments show that a silver/gallium‐lanthanum‐sulphide interface can support surface plasmon‐polaritons and demonstrate the principle of plasmonic modulation through reversible photo‐induced changes in the chalcogenide. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

33.4 Gbyte/Layer Recording with Adaptive Write Strategy for 100 Gbyte Rewritable Triple-Layer Disc

For multilayer rewritable optical discs, we developed a new adaptive write strategy named the S-22 (sequential 2T–2T) write pulse control method, which controls the laser pulse of the 2T-mark according to the combination of both the preceding and succeeding space lengths. By applying this write strategy, a rewritable triple-layer disc based on Blu-ray disc optical systems with a capacity of 100 Gbyte was realized. In particular, the advantage of this write strategy is significant for a transparent information layer. The bit shift of sequential 2T–2T patterns, which is a dominant error in 100 Gbyte rewritable discs, was well compensated. We experimentally confirmed that adopting the S-22 write pulse control method is effective to improve symbol error rate.