Rewritable Optical Data Storage Research Articles

Multimode Dynamic Photoluminescence of Bi3+-Activated ZnGa2O4 for Optical Information Encryption.

Optical storage technology for information encryption is a popular means of safeguarding information. Herein, a Bi3+-activated ZnGa2O4 multimode dynamic photoluminescence (PL) material is developed. Upon being irradiated with an ultraviolet lamp at a fixed excitation wavelength of 254 nm, the ZnGa2O4: x% Bi3+ (x = 0.5-5.0) samples exhibit varying degrees of dynamic PL emission due to a distinct Bi3+ doping effect. The mechanism underlying the dynamic PL of ZnGa2O4: Bi3+ associated with Bi3+-activated trap concentration modulation is investigated using thermoluminescence spectra. Additionally, the ZnGa2O4: 5% Bi3+ sample shows a reversible thermally responsive dynamic PL with a color variation from blue to red upon heating from 283 to 393 K. Predesigned procedures based on single-wavelength-mediated photochromic and thermochromic dynamic PL emissions of ZnGa2O4: Bi3+ are designed for rewritable optical data storage and high-level information encryption. Also, an enhanced encryption scheme with a mask encoding technique applying a ZnGa2O4: Bi3+ hybridized polyvinylidene difluoride film is then proposed to increase the security level. Accordingly, this work provides a feasible way to rationally design dynamic PL material offering more creative designs for safeguarding information via encryption.

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.

A turn-on mechanochromic luminescent material serving as pressure sensor and rewritable optical data storage

Mechanochromic luminescent (ML) materials can be roughly classified into two categories: turn-off type and turn-on type. Clearly the latter is more reliable because it has higher sensitivity and lower false-positive signals or background noises. Herein, a turn-on type ML material, (E)-4-((2-hydroxy-5-methylbenzylidene)amino)-3-methylbenzoic acid (MMA), is presented, which is designed by integrating appropriate electron donor-acceptor (D-A) moieties into a twisted conjugation core. MMA displays both aggregation-induced emission (AIE) and intramolecular charge transfer (ICT) properties. In addition, it glows weakly but exhibits notable emission enhancement under mechanical stimuli, and the two emission states can be repeatedly switched by grinding-fumigation treatment. Unlike the ML materials based on crystalline-amorphous phase transition, the ML of MMA is ascribed to the coupling and decoupling of the D-A units, regardless of the phase state. And this new working mechanism is confirmed by crystallographic data and DFT calculations. A pressure sensor based on MMA was fabricated, affording a low detection limit down to 21.6 Mpa. MMA further facilitates the fabrication of a ML recording medium, which allows for piezo-writing and fumigation-erasing of data, demonstrating a reliable prototype of optical data storage.

Rewritable optical data storage based on mechanochromic fluorescence materials with aggregation-induced emission

In this work, the synthesis and aggregation-induced emission (AIE) properties of a new molecular system are reported. This system composes of three positional isomers, which are fabricated by attaching a methoxy group in different locations. The isomeric effects on the AIE properties as well as the intramolecular charge transfer (ICT) processes were studied. Additionally the new isomers exhibit mechanochromic fluorescence (MCF). Among the three isomers, (E)-2-(((6-chlorobenzo[d]thiazol-2-yl)imino)methyl)-5-methoxyphenol (CHM3) exhibits the most remarkable MCF performance with emission that decreases in intensity and red-shifts in wavelength under mechanical stimuli. The emission changes are caused by crystalline-amorphous transition as verified by Powder X-ray diffraction (PXRD) analysis. The mechano-induced state can be easily recovered through a recrystallization process, which can be activated by solvent fumigation or solvent immersion. Furthermore, the switching between two emission states shows nice reversibility. Based on the MCF material with high fatigue resistance, a new type of rewritable optical data storage (ODS) is developed. Binary data, comprised of “0” and “1” state, can be matched well with the two emission states, respectively. The data could thus be written or erased on the ODS device, demonstrating feasibility and practicability.

Active-Tuning and Polarization-Independent Absorber and Sensor in the Infrared Region Based on the Phase Change Material of Ge2Sb2Te5 (GST)

Phase-change materials (PCMs), possessing thermo-optic and thermo-electric properties, have constantly enabled the rewritable optical data storage and the commercialized phase-change memory devices. In particular, Ge2Sb2Te5 (GST) has been considered for configurable photonics applications, such as active dielectric metasurface. In this paper, we report an active absorber with metal-insulator-metal (MIM) scheme with GST in the infrared region. The absorber consists of Al disk and reflective Al film with a GST spacer layer. Extraordinary absorption with peaks of more than 90% can be achieved over a broad bandwidth, attributing to highly confined gap surface plasmon resonance. In addition, the absorption can be tuned via adjusting the proportion of GST crystallization, which is a unique advantage to design active device. Meanwhile, the absorption is polarization-independent owing to its structural symmetry. Furthermore, we introduce the designed absorber to the application of sensing. This nearly perfect absorbing strategy offers great potential in sensing applications due to its flexibility and polarization-independence.

Phase-change-driven dielectric-plasmonic transitions in chalcogenide metasurfaces

Chalcogenides—alloys based on group-16 ‘chalcogen’ elements (sulfur, selenium, and tellurium) covalently bound to ‘network formers’ such as arsenic, germanium, antimony, and gallium—have a variety of technologically useful properties, including infrared transparency, high optical nonlinearity, photorefractivity and readily induced, reversible, non-volatile structural phase switching. Such phase-change materials are of enormous interest in the fields of plasmonics and nanophotonics. However, in such applications, the fact that some chalcogenides accrue plasmonic properties in the transition from an amorphous to a crystalline state, i.e., the real part of their relative permittivity becomes negative, has gone somewhat unnoticed. Indeed, one of the most commercially important chalcogenide compounds, germanium antimony telluride (Ge2:Sb2:Te5 or GST), which is widely used in rewritable optical and electronic data storage technologies, presents this behavior at wavelengths in the near-ultraviolet to visible spectral range. In this work, we show that the phase transition-induced emergence of plasmonic properties in the crystalline state can markedly change the optical properties of sub-wavelength-thickness, nanostructured GST films, allowing for the realization of non-volatile, reconfigurable (e.g., color-tunable) chalcogenide metasurfaces operating at visible frequencies and creating opportunities for developments in non-volatile optical memory, solid state displays and all-optical switching devices.

Towards rewritable multilevel optical data storage in single nanocrystals

Novel approaches for digital data storage are imperative, as storage capacities are drastically being outpaced by the exponential growth in data generation. Optical data storage represents the most promising alternative to traditional magnetic and solid-state data storage. In this paper, a novel and energy efficient approach to optical data storage using rare-earth ion doped inorganic insulators is demonstrated. In particular, the nanocrystalline alkaline earth halide BaFCl:Sm is shown to provide great potential for multilevel optical data storage. Proof-of-concept demonstrations reveal for the first time that these phosphors could be used for rewritable, multilevel optical data storage on the physical dimensions of a single nanocrystal. Multilevel information storage is based on the very efficient and reversible conversion of Sm3+ to Sm2+ ions upon exposure to UV-C light. The stored information is then read-out using confocal optics by employing the photoluminescence of the Sm2+ ions in the nanocrystals, with the signal strength depending on the UV-C fluence used during the write step. The latter serves as the mechanism for multilevel data storage in the individual nanocrystals, as demonstrated in this paper. This data storage platform has the potential to be extended to 2D and 3D memory for storage densities that could potentially approach petabyte/cm3 levels.

Optical Data Storage and Multicolor Emission Readout on Flexible Films Using Deep‐Trap Persistent Luminescence Materials

AbstractThe fast‐growing amount of data that is produced every year creates an urgent need for ultracapacity storage media. However, 2D spatial resolution in the conventional optical data storage media has almost reached the limit. Further enlargement of storage capacity may rely on the development of the next‐generation data storage materials containing multiplexed information dimensions. Herein, a series of novel deep‐trap persistent luminescence materials (Sr1‐xBax)Si2O2N2:Eu/Yb,Dy with multicolor emissions in the whole visible region is developed and demonstrated a bit‐by‐bit optical data storage and readout strategy based on photon trapping and detrapping processes in these materials. Optical data can be handily encoded on a flexible film by a commercially available 405 nm laser and decoded by heating or by 980 nm laser scanning. The decoded information contains tunable spectral characteristics, which allows for the emission–intensity–multiplexing or emission–wavelength–multiplexing. The storage and readout strategy not only shows a great promise in the application of multidimensional rewritable optical data storage, but also opens new opportunities for advanced display technology and information security system.

Both Dielectrics and Conductance Anomalies in an Open-Framework Cobalt Phosphate.

Switchable conducting or dielectric materials, as the key component, show important technological applications in modern electrical and electronic devices, including data communication, phase shifters, varactors, and rewritable optical data storage. To explore new types of switchable conducting or dielectric materials could significantly accelerate the development of efficient electrical and electronic devices. Herein we present the first example of switchable conducting and dielectric material, which is based on an open-framework phosphate, (C2N2H10)0.5CoPO4. A reversible isostructural phase transition occurs at ∼348 K in this open-framework phosphate, to give both dielectrics and conductance anomaly around the critical temperature of phase transition. This study will provide a roadmap for searching new switchable conducting or dielectric materials as well as new applications of open-framework phosphates.

Metasurfaces Based on Phase-Change Material as a Reconfigurable Platform for Multifunctional Devices.

Integration of phase-change materials (PCMs) into electrical/optical circuits has initiated extensive innovation for applications of metamaterials (MMs) including rewritable optical data storage, metasurfaces, and optoelectronic devices. PCMs have been studied deeply due to their reversible phase transition, high endurance, switching speed, and data retention. Germanium-antimony-tellurium (GST) is a PCM that has amorphous and crystalline phases with distinct properties, is bistable and nonvolatile, and undergoes a reliable and reproducible phase transition in response to an optical or electrical stimulus; GST may therefore have applications in tunable photonic devices and optoelectronic circuits. In this progress article, we outline recent studies of GST and discuss its advantages and possible applications in reconfigurable metadevices. We also discuss outlooks for integration of GST in active nanophotonic metadevices.

AIE-active and reversible mechanochromic tetraphenylethene-tetradiphenylacrylonitrile hybrid luminogens with re-writable optical data storage application

Two new tetraphenylethene-based diphenylacrylonitrile derivatives (TPEDPAN1 and TPEDPAN2) were synthesized in high yields and their reversible mechanochromic luminescence and aggregation-induced emission characteristics are reported. These luminogen crystals exhibiting green fluorescence could be transformed into powders emitting an orange-yellow color upon grinding, and the green emission could be restored when the ground powders were fumed with dichloromethane. To reveal the mechanochromic mechanism, a series of tests including powder XRD, single-crystal X-ray, DSC, and solid state UV–visible absorbance were carried out. These studies suggested that the reversible mechanochromism originated from the transformation of a crystalline state into an amorphous state and vice versa. The single crystal structure of TPEDPAN1 revealed that the poor intermolecular interactions of C-N (3.087 Å) with in the dimer, would give loose packing of the molecular conformation, thus yielding mechanochromic behavior in the crystals upon grinding.

Thermally Tunable Ultrasensitive Infrared Absorption Spectroscopy Platforms Based on Thin Phase-Change Films

The thermal tunability of the optical and electrical properties of phase-change materials has enabled the decades-old rewritable optical data storage and the recently commercialized phase-change memory devices. Recently, phase-change materials, in particular, Ge2Sb2Te5 (GST), have been considered for other thermally configurable photonics applications, such as active plasmonic surfaces. Here, we focus on nonplasmonic field enhancement and demonstrate the use of the phase-change materials in ultrasensitive infrared absorption spectroscopy platforms employing interference-based uniform field enhancement. The studied structures consist of patternless thin GST and metal films, enabling simple and large-area fabrication on rigid and flexible substrates. Crystallization of the as-fabricated amorphous GST layer by annealing tunes (redshifts) the field-enhancement wavelength range. The surfaces are tested with ultrathin chemical and biological probe materials. The measured absorption signals are found to be compa...

Highly efficient valence state switching of samarium in BaFCl:Sm nanocrystals in the deep UV for multilevel optical data storage

We report on the highly efficient generation of stable Sm2+ in nanocrystalline BaFCl:Sm3+ by exposure in the deep UV (UV-C) below 250 nm. The generated Sm2+ can be read out via its distinct fluorescence signature which is efficiently excited around 425 nm by the relatively strong f-d transitions with e ≈400 (l mol−1 cm−1). The generation of Sm2+ can also be reversed, erasing the fluorescence signal via two-photon ionization by increasing the power at 425 nm from ≤ 1 mW/cm2 to ~100 mW/cm2. It follows that the switching mechanism is based on oxygen impurities that are in close proximity to the Sm3+ ions. The photoionization kinetics indicate that the average Sm3+- oxide impurity separation is a few interionic spacings. The level of Sm3+- Sm2+ conversion in BaFCl is shown to be tunable over a large dynamic range, and therefore could serve as a platform for rewritable ultra-high density multi-level optical data storage. The present study also sheds light on BaFCl:Sm3+ as a photoluminescent X-ray storage phosphor.

One-pot synthesis of a mechanochromic AIE luminogen: implication for rewritable optical data storage

A new prototype of a rewritable optical data storage device with high fatigue resistance is designed based on a mechanochromic AIE luminogen.

Direct Writing of Three-Dimensional Macroporous Photonic Crystals on Pressure-Responsive Shape Memory Polymers.

Here we report a single-step direct writing technology for making three-dimensional (3D) macroporous photonic crystal patterns on a new type of pressure-responsive shape memory polymer (SMP). This approach integrates two disparate fields that do not typically intersect: the well-established templating nanofabrication and shape memory materials. Periodic arrays of polymer macropores templated from self-assembled colloidal crystals are squeezed into disordered arrays in an unusual shape memory "cold" programming process. The recovery of the original macroporous photonic crystal lattices can be triggered by direct writing at ambient conditions using both macroscopic and nanoscopic tools, like a pencil or a nanoindenter. Interestingly, this shape memory disorder-order transition is reversible and the photonic crystal patterns can be erased and regenerated hundreds of times, promising the making of reconfigurable/rewritable nanooptical devices. Quantitative insights into the shape memory recovery of collapsed macropores induced by the lateral shear stresses in direct writing are gained through fundamental investigations on important process parameters, including the tip material, the critical pressure and writing speed for triggering the recovery of the deformed macropores, and the minimal feature size that can be directly written on the SMP membranes. Besides straightforward applications in photonic crystal devices, these smart mechanochromic SMPs that are sensitive to various mechanical stresses could render important technological applications ranging from chromogenic stress and impact sensors to rewritable high-density optical data storage media.

An optoelectronic framework enabled by low-dimensional phase-change films.

The development of materials whose refractive index can be optically transformed as desired, such as chalcogenide-based phase-change materials, has revolutionized the media and data storage industries by providing inexpensive, high-speed, portable and reliable platforms able to store vast quantities of data. Phase-change materials switch between two solid states--amorphous and crystalline--in response to a stimulus, such as heat, with an associated change in the physical properties of the material, including optical absorption, electrical conductance and Young's modulus. The initial applications of these materials (particularly the germanium antimony tellurium alloy Ge2Sb2Te5) exploited the reversible change in their optical properties in rewritable optical data storage technologies. More recently, the change in their electrical conductivity has also been extensively studied in the development of non-volatile phase-change memories. Here we show that by combining the optical and electronic property modulation of such materials, display and data visualization applications that go beyond data storage can be created. Using extremely thin phase-change materials and transparent conductors, we demonstrate electrically induced stable colour changes in both reflective and semi-transparent modes. Further, we show how a pixelated approach can be used in displays on both rigid and flexible films. This optoelectronic framework using low-dimensional phase-change materials has many likely applications, such as ultrafast, entirely solid-state displays with nanometre-scale pixels, semi-transparent 'smart' glasses, 'smart' contact lenses and artificial retina devices.

Electrical conduction in chalcogenide glasses of phase change memory

Amorphous chalcogenides have been extensively studied over the last half century due to their application in rewritable optical data storage and in non-volatile phase change memory devices. Yet, the nature of the observed non-ohmic conduction in these glasses is still under debate. In this review, we consolidate and expand the current state of knowledge related to dc conduction in these materials. An overview of the pertinent experimental data is followed by a review of the physics of localized states that are peculiar to chalcogenide glasses. We then describe and evaluate twelve relevant transport mechanisms with conductivities that depend exponentially on the electric field. The discussed mechanisms include various forms of Poole-Frenkel ionization, Schottky emission, hopping conduction, field-induced delocalization of tail states, space-charge-limited current, field emission, percolation band conduction, and transport through crystalline inclusions. Most of the candidates provide more or less satisfactory fits of the observed non-linear IV data. Our analysis calls upon additional studies that would enable one to discriminate between the various alternative models.

Observing the amorphous‐to‐crystalline phase transition in Ge2Sb2Te5 non‐volatile memory materials from ab initio molecular‐dynamics simulations

AbstractPhase‐change memory is a promising candidate for the next generation of non‐volatile memory devices. This technology utilizes reversible phase transitions between amorphous and crystalline phases of a recording material, and has been successfully used in rewritable optical data storage, revealing its feasibility. In spite of the importance of understanding the nucleation and growth processes that play a critical role in the phase transition, this understanding is still incomplete. Here, we present observations of the early stages of crystallization in Ge2Sb2Te5 materials through ab initio molecular‐dynamics simulations. Planar structures, including fourfold rings and planes, play an important role in the formation and growth of crystalline clusters in the amorphous matrix. At the same time, vacancies facilitate crystallization by providing space at the glass–crystalline interface for atomic diffusion, which results in fast crystal growth, as observed in simulations and experiments. The microscopic mechanism of crystallization presented here may deepen our understanding of the phase transition occurring in real devices, providing an opportunity to optimize the memory performance of phase‐change materials.

Casimir Force Contrast Between Amorphous and Crystalline Phases of AIST

AbstractPhase change materials (PCMs) can be rapidly and reversibly switched between the amorphous and crystalline state. The structural transformation is accompanied by a significant change of optical and electronic properties rendering PCMs suitable for rewritable optical data storage and non‐volatile electronic memories. The phase transformation is also accompanied by an increase of the Casimir force of 20 to 25% between gold and AIST (Ag5In5Sb60Te30) upon crystallization. Here the focus is on reproducing and understanding the observed change in Casimir force, which is shown to be related to a change of the dielectric function upon crystallization. The dielectric function changes in two separate frequency ranges: the increase of absorption in the visible range is due to resonance bonding, which is unique for the crystalline phase, while free carrier absorption is responsible for changes in the infrared regime. It is shown that free carriers contribute ≈50% to the force contrast, while the other half comes from resonance bonding. This helps to identify PCMs that maximize force contrast. Finally it is shown that if this concept of force control is to be employed in microelectromechanical devices, then protective capping layers of PCMs must be only a few nanometers thick to minimize reduction of the force contrast.

Schottky barrier formation at amorphous-crystalline interfaces of GeSb phase change materials

The electrical properties of amorphous-crystalline interfaces in phase change materials, which are important for rewritable optical data storage and for random access memory devices, have been investigated by surface scanning potential microscopy. Analysis of GeSb systems indicates that the surface potential of the crystalline phase is ∼30–60 mV higher than that of the amorphous phase. This potential asymmetry is explained qualitatively by the presence of a Schottky barrier at the amorphous-crystalline interface and supported also by quantitative Schottky model calculations.