Rewritable Optical Storage Research Articles

Phase change memory materials: Why are alloys of Ge, Sb, and Te the materials of choice?

Rewritable optical storage is dominated by alloys of a small number of elements, particularly Ge, Sb, and Te, and Ge/Sb/Te alloys in the composition range (GeTe)1−x(Sb2Te3)x (0 ≤ x ≤ 1) have been the materials of choice: all have metastable rock salt structures that change little over decades at archival temperatures, and all contain vacancies (cavities). The special status arises from the close similarity of the valence orbitals of Ge, Sb, and Te, which arises from the irregular changes in atomic orbitals and properties as the atomic number increases (“secondary periodicity”). The instability of cubic (metallic) Bi to a (semimetallic) rhombohedral structure (H. Jones, 1934) can be adapted to Ge/Sb/Te alloys and explains the crucial metastable (rock salt) structures of these compounds. Vacancies almost always occur next to Te atoms, which form one sublattice of the rock salt structure.

Burn After Read: A Rewritable Multiplexing Optical Information Storage and Encryption Method

AbstractInformation storage and security are the building blocks of the information age. Optical information storage is a low‐cost, robust, and high‐capacity storage technique. Discovering new optical information storage media and techniques with beyond binary capacity and enhanced security is in demand. Here a beyond‐binary, “burn after read” and rewritable multiplexing optical information storage and encryption method is demonstrated via Ce:YAG trapping states. It is found that the thermo‐luminescence (TL) intensity of Ce:YAG trapping states is writing light power dependent, enabling multi‐gray‐level information storage and encoding.A new strategy to encode multiplexing information in a single pixel, increasing the storage capability is presented. Meanwhile, the burn after read feature protects the data from secondary leakage. This method has wide applications in data storage, anti‐counterfeiting, and encryption.

Sub-1-Volt Electrically Programmable Optical Modulator Based on Active Tamm Plasmon.

Reconfigurable optical devices hold great promise for advancing high-density optical interconnects, photonic switching, and memory applications. While many optical modulators based on active materials have been demonstrated, it is challenging to achieve a high modulation depth with a low operation voltage in the near-infrared (NIR) range, which is a highly sought-after wavelength window for free-space communication and imaging applications. Here, electrically switchable Tamm plasmon coupled with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is introduced. The device allows for a high modulation depth across the entire NIR range by fully absorbing incident light even under epsilon near zero conditions. Optical modulation exceeding 88% is achieved using a CMOS-compatible voltage of ±1V. This modulation is facilitated by precise electrical control of the charge carrier density through an electrochemical doping/dedoping process. Additionally, the potential applications of the device are extended for a non-volatile multi-memory state optical device, capable of rewritable optical memory storage and exhibiting long-term potentiation/depression properties with neuromorphic behavior.

Deep-trap persistent materials for future rewriteable optical information storage.

Deep-trap persistent luminescent (PersL) materials with enriched traps, which allow signals to quickly write-in and read-out with low-energy consumption, are one of the most promising materials for information storage. In this review, considering the demand for optical information storage, we provide comprehensive insights into the data storage mechanism of PersL materials. Particularly, we focus on various "trap-state tuning" strategies involving doping to design new deep-trap persistent phosphors with controlled carrier trapping-de-trapping for non-volatile and high-capacity information storage. Subsequently, various recent significant strategies, including wavelength-multiplexing, intensity-multiplexing, mechanical-multiplexing, and three-dimensional and multidimensional trap-multiplexing technologies for improving the information storage capacity of PersL phosphors are highlighted. Finally, the challenges and opportunities regarding optical information storage by PersL materials are discussed. We hope that this review will provide new insights for the future development of PersL materials in the field of optical data storage.

Impact of Various Dopants on Thermoelectric Transport Properties of Polycrystalline GeSb2Te4

GeSb2Te4 (GST124), one of the well‐known phase‐change materials for nonvolatile memory and rewritable optical storage, has been recently found to be promising thermoelectric materials with low lattice thermal conductivity and high electrical conductivity. However, its thermoelectric performance is greatly restricted by the excessively high hole concentration. Herein, the impact of a series of group IIIA (Al, Ga, In) and group VIA (S, Se) dopants on the electrical transport properties of polycrystalline GST124 has been studied. It is found that element sulfur (S) has the best doping efficiency because the GeS bonds are very strong and ionic that are beneficial for suppressing Ge vacancies to reduce the carrier concentration. Meanwhile, element indium (In) also shows decent doping efficiency because its ionic radius is close to the Ge ion and the InTe bonds have moderate bonding strength. Moreover, In doping introduces a resonant level in the valence band, leading to enhanced Seebeck coefficient and power factor. A high figure of merit (zT)of 0.73 at 700 K and an average zT of 0.48 over 300–750 K are obtained in Ge0.92In0.08Sb2Te4, which are 26% and 66% higher than pristine GST124. This study will advance the understanding and development of high‐performance GeSbTe‐based thermoelectric materials.

Investigating the Polarity Dependence of Multilevel Cell Operation in Conventional Mushroom Phase‐Change Memory Cells

Phase‐change materials have long been employed for rewriteable optical data storage at the industrial scale and are hailed as one of the most mature technologies for their applications in emerging nonvolatile memories. Memristors based on these materials have the potential to circumvent the long‐standing von Neumann bottleneck and offer significant computational advantage through neuromorphic computing. A very core fundamental concept that lies at the crux for such realization is their ability to offer multilevel cell (MLC) storage, in contrast to the binary counterpart. Yet, numerous challenges still remain to be tackled for their successful implementation in this regard. This work is a finite element analysis that particularly reports the polarity dependence of such MLC operation in the conventional mushroom geometry of devices employing this technology. The mechanism lying underneath is discussed and a perspective combining thermoelectric effects with the energy band diagrams resulting from metal–semiconductor contact formation is additionally put forth.

Hierarchical Surface Structures and Large-Area Nanoscale Gratings in As2S3 and As2Se3 Films Irradiated with Femtosecond Laser Pulses.

Chalcogenide vitreous semiconductors (ChVSs) find application in rewritable optical memory storage and optically switchable infrared photonic devices due to the possibility of fast and reversible phase transitions, as well as high refractive index and transmission in the near- and mid-infrared spectral range. Formed on such materials, laser-induced periodic surface structures (LIPSSs), open wide prospects for increasing information storage capacity and create polarization-sensitive optical elements of infrared photonics. In the present work, a possibility to produce LIPSSs under femtosecond laser irradiation (pulse duration 300 fs, wavelength 515 nm, repetition rate up to 2 kHz, pulse energy ranged 0.03 to 0.5 μJ) is demonstrated on a large (up to 5 × 5 mm2) area of arsenic sulfide (As2S3) and arsenic selenide (As2Se3) ChVS films. Scanning electron and atomic force microscopy revealed that LIPSSs with various periods (170-490 nm) and orientations can coexist within the same irradiated region as a hierarchical structure, resulting from the interference of various plasmon polariton modes generated under intense photoexcitation of nonequilibrium carriers within the film. The depth of the structures varied from 30 to 100 nm. The periods and orientations of the formed LIPSSs were numerically simulated using the Sipe-Drude approach. A good agreement of the calculations with the experimental data was achieved.

Deep-level trap formation in Si-substituted Sr2SnO4:Sm3+ for rewritable optical information storage

Persistent luminescent materials containing deep-level traps have attracted extensive research interest in the field of optical information storage due to their unique features of long-lasting emission, energy storage, and controllable photon release upon external stimulations. However, the lack of suitable luminescent materials with deep-level traps is still the bottleneck of such an optical storage application. Herein, we report a series of persistent luminescent materials with reddish-orange emissions and controllable photon release upon thermal stimulation. Through substitution of Sn by Si, more and deeper trap levels are achieved in Sr2SnO4:Sm3+,Si4+ phosphors. Moreover, both ultraviolet light and high-energy X-ray irradiations can induce reddish-orange emitting persistent luminescence from the as-synthesized samples. Rewritable optical information storage and readout based on photon trapping and de-trapping processes are demonstrated. Optical information can be handily encoded using a commercially available 365 nm light-emitting diode, and decoded upon thermal stimulation. The Sr2SnO4:Sm3+,Si4+ phosphors as presented in this work show a great promise for rewritable optical data storage and information encryption.

Order-disorder structural transition in Pr3+-doped Ba3Ga2O6 for rewritable and write-once-read-many optical data storage

The emerging challenges of the big data era, both in storage density and security, require the development of the next-generation optical data storage materials. Here, we report for the first time a photo-stimulated luminescence (PSL) material, Ba3Ga2O6: Pr3+, for rewritable optical storage and write-once-read-many data preservation. Ba3Ga2O6: Pr3+, with an isolated deep trap depth in the range 1.26–1.53eV, has been used for data encoding/decoding under the excitation of 254 nm UV light and by the simulation of an 808 nm NIR laser. Meanwhile, the phosphor allows for high-security write-once-read-many optical memory by taking advantage of the irreversible change of the photoluminescence (PL) color from blue to green (a binary blue ‘0’ and green ‘1’ code) irradiated by 365 nm UV light. The comprehensive investigations indicate that the irreversible PL switching is as a result of the order-disorder structural transition by thermal treatment. The new persistent luminescence material not only exhibits promising applications in sustainable rewritable data storage, but also paves the way for write-once-read-many optical information storage with a high level of security.

Phosphor SrZrO3:Sm3+ with fluorescence modulation and photochromic characteristics for erasable optical storage

At present, for inorganic photochromic materials, the single function limits their application level in the field of optical storage and anti-counterfeiting. Generally, the reversible modulation of photoluminescence is achieved through a photochromic process, thereby improving the ability of the photochromic material to store information. Here, we designed a Sm 3+ doped perovskite oxide (SrZrO 3 :Sm 3+ ) inorganic photochromic material. Combined with its red photoluminescence and photochromic properties, it can realize dual information storage and recognition functions. The fluorescence regulation function is realized by the doping of Sm 3+ , which is conducive to the application in the direction of optical switch. The mechanism of photoluminescence and photochromism was studied. Finally, a flexible film with SrZrO 3 :Sm 3+ material was fabricated to demonstrate rewritable optical recording and storage and reading of dual optical information.

Designing Photochromic Materials with Large Luminescence Modulation and Strong Photochromic Efficiency for Dual‐Mode Rewritable Optical Storage

AbstractInorganic materials combining photochromism and luminescence modulation characteristics have great potential in dual‐mode rewritable optical storage due to their unique optical features and excellent thermal stability. However, the failure of achieving a large luminescence modulation and a strong photochromic efficiency in photostimulated inorganic photochromic materials limits their applications. Herein, a new strategy for realizing an overlap between the photochromic absorption peak and the photoluminescent emission/excitation peak is proposed for designing high‐performance photochromic materials. The obtained BaMgSiO4: M (M = Ce3+, Mn2+, or Nd3+) ceramics exhibit a reversible white‐pink color change upon alternate 310 nm and 590 nm illumination (or thermal stimulus) accompanied by a high photochromic efficiency (>50%). Benefiting from a perfectly matched photochromic absorption peak and Mn2+ emission peak, a record luminescence modulation of 96.3% with excellent fatigue resistance is obtained in BaMgSiO4: Mn2+ ceramics. These properties are superior to all photochromic materials reported to date, demonstrating great potential in optical information storage applications. The trap‐related photochromic and regulated luminescence behavior is investigated together with a prototype of a dual‐mode information display. This work is expected to promote the practical application of photochromic materials in various optical devices and provides an effective strategy to develop other photochromic materials.

Photon energy conversion and management in SrAl12O19: Mn2+, Gd3+ for rewritable optical information storage

• Optical stimulated luminescence is observed from SrAl 12 O 19 : Mn 2+ , Gd 3+ . • The detrapping of electrons is operable by controlling the IR laser. • An optical information storage that integrate a writing-storing-readout-erasing-rewriting is realized. In the current big data era, state-of-the-art optical data storage has become a front-runner in competing data storage technologies. In this work, we propose an information storage prototype using optical stimulated luminescent phosphor of SrAl 12 O 19 : Mn 2+ , Gd 3+ , which converts the photon into trapped electrons and partially releases them in a form of luminescence when irradiated by infrared irradiation, rendering an optical information storage ability. Information storage can be realized in both graphical and digital formats. In this way, graphical patterns can be stored and rejuvenated by infrared irradiation, and a set of digital data is recorded step by step by a home-make decoding system, completing an optical data reading process. This work provides a practical example for future application of writing-storing-readout-erasing-rewriting integrated optical information storage.

A rewritable optical storage medium of silk proteins using near-field nano-optics.

Nanoscale lithography and information storage in biocompatible materials offer possibilities for applications such as bioelectronics and degradable electronics for which traditional semiconductor fabrication techniques cannot be used. Silk fibroin, a natural protein renowned for its strength and biocompatibility, has been widely studied in this context. Here, we present the use of silk film as a biofunctional medium for nanolithography and data storage. Using tip-enhanced near-field infrared nanolithography, we demonstrate versatile manipulation and characterize the topography and conformation of the silk in situ. In particular, we fabricate greyscale and dual-tone nanopatterns with full-width at half-maximum resolutions of ~35 nm, creating an erasable 'silk drive' that digital data can be written to or read from. As an optical storage medium, the silk drive can store digital and biological information with a capacity of ~64 GB inch-2 and exhibits long-term stability under various harsh conditions. As a proof-of-principle demonstration, we show that this silk drive can be biofunctionalized to exhibit chromogenic reactions, resistance to bacterial infection and heat-triggered, enzyme-assisted decomposition.

Phase change memory materials: Rationalizing the dominance of Ge/Sb/Te alloys

Rewritable optical storage is dominated by alloys of a small number of elements, overwhelmingly Ge, Sb, and Te. For over 30 years, Ge/Sb/Te alloys in the composition range ${(\mathrm{GeTe})}_{1\ensuremath{-}x}{({\mathrm{Sb}}_{2}{\mathrm{Te}}_{3})}_{x}\phantom{\rule{0.28em}{0ex}}(0\ensuremath{\le}x\ensuremath{\le}1)$ have been the materials of choice in commercial devices: all have metastable rock-salt structures that change little over decades at archival temperatures, and all contain vacancies (cavities). The special status of Ge/Sb/Te alloys arises from the close similarity of their valence orbitals as measured by the orbital radial moments, so that bonds are stronger than in other combinations of elements of groups 14--16 with appropriate valences. The orbital similarity arises from the irregular changes in atomic orbitals and properties as the atomic number increases (``secondary periodicity''). Jones showed [P. R. Soc. London, Ser. A 147, 396 (1934)] that the simple cubic structure of (metallic) Bi (valence configuration $6{s}^{2}6{p}^{3}$) is unstable to a distortion to a (semimetallic) rhombohedral structure. This picture can be adapted to Ge/Sb/Te alloys to explain the metastable structure of the above family of compounds where vacancies almost always occur next to Te atoms, which form one sublattice of the rock-salt structure. The disorder in the Ge/Sb/vacancy sublattice is not random.

Reversible single-crystal-to-single-crystal conversion of a photoreactive coordination network for rewritable optical memory storage.

Reversible single-crystal-to-single-crystal photocyclization-cycloreversion reaction of a stilbene-based coordination network exhibits a conspicuous fluorescence change. The controllable fluorescence and high fatigue resistance feature of this bistable material make it a single-crystalline device for applications in rewritable optical memory storage systems.

A systematic evolution of optical band gap and local ordering in Ge1Sb2Te4 and Ge2Sb2Te5 materials revealed by in situ optical spectroscopy

A unique amalgam of fascinating potentials enabled the usage of phase change materials in rewritable optical and electrical storage technology. Furthermore, the identification of stoichiometric tuning of disorder in these materials opened up new pathways for the selection of materials for multi-bit data storage. Despite a reasonable understanding of structure and properties, an in situ study could aid in unravelling vital insights. Hence, we report on an analogy between the degree of disorder in Ge1Sb2Te4 and Ge2Sb2Te5 materials by employing novel temperature-dependent correlations of optical band gap (Eg), Tauc parameter (measure of disorder, B1/2 slope) and the change in band tail. Eg decreases from 0.8 eV (90 K) to 0.34 eV (480 K) for Ge1Sb2Te4 and from 0.82 eV (90 K) to 0.36 eV (480 K) for Ge2Sb2Te5. This trend reveals the semiconducting nature of both amorphous and crystalline phases. The change in disorder as exemplified by an increase in B1/2 of 2.7% and 14.9% during the phase transition of Ge1Sb2Te4 and Ge2Sb2Te5 respectively reveals a stronger degree of disorder in the cubic phase of Ge1Sb2Te4 as compared to the cubic phase of Ge2Sb2Te5. These findings are strongly supported by an unambiguous change in the width of the band tail. This confirms the origin of a pronounced disorder-induced localization in Ge1Sb2Te4 compared to the Ge2Sb2Te5 material.

Tailoring Trap Depth and Emission Wavelength in Y3Al5- xGa xO12:Ce3+,V3+ Phosphor-in-Glass Films for Optical Information Storage.

Deep-trap persistent luminescent materials, due to their exceptional ability of energy storage and controllable photon release under external stimulation, have attracted considerable attention in the field of optical information storage. Currently, the lack of suitable materials is still the bottleneck that restrains their practical applications. Herein, we successfully synthesized a series of deep-trap persistent luminescent materials Y3Al5- xGa xO12:Ce3+,V3+ ( x = 0-3) with a garnet structure and developed novel phosphor-in-glass (PiG) films containing these phosphors. The synthesized PiG films exhibited sufficiently deep traps, narrow trap depth distributions, high trap density, high quantum efficiency, and excellent chemical stability, which solved the problem of chemical stability at high temperatures in the reported phosphor-in-silicone films. Moreover, the trap depth in the phosphors and PiG films could be tailored from 1.2 to 1.6 eV, thanks to the bandgap engineering effect, and the emission color was simultaneously changed from green to yellow due to the variation of crystal field strength. Image information was recorded on the PiG films by using a 450 nm blue-light laser in a laser direct writing mode and the recorded information was retrieved under high-temperature thermal stimulation or photostimulation. The Y3Al5- xGa xO12:Ce3+,V3+ PiG films as presented in this work are very promising in the applications of multidimensional and rewritable optical information storage.

ELECTROTHERMAL CHARACTERIZATION OF PHASE-CHANGE FILMS AND DEVICES

The reversible changes in the optical properties of the phase-change materials have made the rewritable optical storage possible which has revolutionized the dissemination of data since 1990s. For the last two decades, the phase-change materials have been studied extensively for its applications as nonvolatile memory elements (phase-change memory (PCM) devices). While the PCM devices were initially considered as replacements for the flash memory, today they promise a universal memory acting as the main memory and the storage unit. Here we demonstrate a simple alternative to study phase-change films and devices for further fundamental studies. The films are deposited using a single sputtering target and the devices are formed using single lithography, deposition and liftoff steps. The electrical resistivity of the films and devices are characterized in a temperature range varying from room temperature to 250 °C. Finally, microscale GST wires are amorphized by melting using self-heating and quenching.

Visible-Light-Responsive Azopolymers with Inhibited π–π Stacking Enable Fully Reversible Photopatterning

Photoswitchable polymers are promising candidates for information storage. However, two general problems for photoswitchable polymers used in rewritable optical storage are photobleaching and inefficient switching processes in solid state. To overcome both of these obstacles, we demonstrate the synthesis of a new visible-light-switchable azobenzene-containing polymer (azopolymer) with nonstackable azo chromophores for reversible and stable information storage. The new azopolymer (PmAzo) contains ortho-methoxy-substituted azobenzene (mAzo) groups on the polymer side chains and shows reversible trans-to-cis or cis-to-trans isomerization by using distinct wavelengths of visible light. PmAzo is better suited for reversible optical storage than conventional UV-responsive azopolymers because visible light avoids the photodamage caused by UV light. Additionally, mAzo groups do not π–π stack in solid state, making photopatterning of PmAzo fully reversible. Moreover, photoinduced patterns on PmAzo can be stored fo...

Rewritable Optical Storage with a Spiropyran Doped Liquid Crystal Polymer Film.

Rewritable optical storage has been obtained in a spiropyran doped liquid crystal polymer films. Pictures can be recorded on films upon irradiation with UV light passing through a grayscale mask and they can be rapidly erased using visible light. Films present improved photosensitivity and optical contrast, good resistance to photofatigue, and high spatial resolution. These photochromic films work as a multifunctional, dynamic photosensitive material with a real-time image recording feature.