Optical Memory Materials Research Articles

Controlling crystallization morphology of semi-crystalline block polymer by photo-triggered immiscible phase aggregation

This work presents a novel strategy of performing a reversible microcrystalline-spherulite morphology transition based on photo-triggered immiscible phase aggregation in semi-crystalline block polymer. We propose that by assembling or disintegrating immiscible photo-active aggregates during melt crystallization under illuminations, end-to-end distance R of the photo-inactive semi-crystalline polymer chains can be adjusted to generate high-entropy state, R< R 0 or low-entropy state, R > R 0. Thus, the corresponding entropic effect influences chain nucleation and can be used to control crystallization morphologies. Besides, photo-tunable mechanical and electrical properties can be also simultaneously endowed. Promising potential applications of this study for optical patterning, memory materials and solid-state electrochemistry can be anticipated.

Samarium Monosulfide (SmS): Reviewing Properties and Applications.

In this review, we give an overview of the properties and applications of samarium monosulfide, SmS, which has gained considerable interest as a switchable material. It shows a pressure-induced phase transition from the semiconducting to the metallic state by polishing, and it switches back to the semiconducting state by heating. The material also shows a magnetic transition, from the paramagnetic state to an antiferromagnetically ordered state. The switching behavior between the semiconducting and metallic states could be exploited in several applications, such as high density optical storage and memory materials, thermovoltaic devices, infrared sensors and more. We discuss the electronic, optical and magnetic properties of SmS, its switching behavior, as well as the thin film deposition techniques which have been used, such as e-beam evaporation and sputtering. Moreover, applications and possible ideas for future work on this material are presented. Our scope is to present the properties of SmS, which were mainly measured in bulk crystals, while at the same time we describe the possible deposition methods that will push the study of SmS to nanoscale dimensions, opening an intriguing range of applications for low-dimensional, pressure-induced semiconductor–metal transition compounds.

One-pot synthesis of new acid photogenerators for Rhodamine laser dyes fluorescence activation

New aryl(hetaryl)pyrazolines fitted for fluorescence activation of Rhodamine laser dyes have been obtained via one-pot tandem Michael addition of two arylhydrazine molecules with 2-(4′-R-cinnamyliden)-2Н-furo[3,2-≿]coumarin-3-ones. These aryl(hetaryl)pyrazolines do not undergo tautomeric transformations both in organic solvents and in polymer films and behave themselves as effective acid photogenerators of Rhodamine dyes fluorescence activation. New aryl(hetaryl)pyrazolines can be used in the fabrication of two-photon volumetric optical memory materials.

Phase Transition of Sb-rich Alloy Films under Isothermal and Constant Heating Rate Annealings

The phase changed alloy films have great potential applications in the thermal sensor materials beside in the optical and electrical memory materials due to their stable phase transitions. The academic investigation methods on the phase transformation of the phase changed films are annealings either at constant heating rate or isothermal annealing. Both methods are applied and compared for the Sb-rich alloy films. The transitions from amorphous to crystalline of Sb-rich chalcogenide alloys with composition ratio of element group 14 to Sb ranged from 0.46 to 0.17 were investigated. The crystallization temperature at both annealing methods increases with the decrease of the ratio of element group 14 to Sb. The activation energies decided from both annealing methods are comparable, which implies the same crystallization mechanism. The Avrami exponents, ranged between 1.7 and 2.4, are extra values obtained from the isothermal measurements. The 2-D growth mechanism is implicitly dominated by the Avrami exponent values. Sb rhombohedral and Sb 7 Te are the major crystallized phases observed and SnTe and InTe phases are mixed among the major phase matrix when the samples are annealed at 250 o C for 30 min.

Molecular keypad locks based on gated photochromism and enhanced fluorescence by protonation effects.

Being a "lock-unlock" system, gated photochromism is generally applied in nondestructive readout for optical memory materials. In the presented paper, we successfully constructed molecular keypad locks by introduction of the gated photochromism. A series of diarylethenes compounds (DAEs) based on fluorescent 5-methoxy-2-pyridyl thiazoles, were prepared and then characterized as photochromic fluorescent switches. Protoantion of the reported DAEs resulted in both protonation-locked photo-reactivities, i.e., gated photochromism, and enhancement of fluorescence. Molecular keypad locks were then successfully constructed, which are also featured by "one-key" lock operation.

Two-Photon Absorbing Molecules as Potential Materials for 3D Optical Memory

In this review, recent advances in two-photon absorbing photochromic molecules, as potential materials for 3D optical memory, are presented. The investigations introduced in this review indicate that 3D data storage processing at the molecular level is possible. As 3D memory using two-photon absorption allows advantages over existing systems, the use of two-photon absorbing photochromic molecules is preferable. Although there are some photochromic molecules with good properties for memory, in most cases, the two-photon absorption efficiency is not high. Photochromic molecules with high two-photon absorption efficiency are desired. Recently, molecules having much larger two-photon absorption cross sections over 10,000 GM (GM= 10−50 cm4 s molecule−1 photon−1) have been discovered and are expected to open the way to realize two-photon absorption 3D data storage.

Materials for optical memory: Resolved hyperfine structure in KY3F10:Ho3+

Basic principles of creating a quantum optical memory (QOM) and requirements for relevant materials, in particular, for crystals doped with rare-earth ions, are briefly reviewed. A combined approach to studying the hyperfine structure, which is essential for QOM applications, is presented on the example of KY3F10:Ho3+.

Photochromic and irreversible photofluorescent organic materials for 3D bitwise optical memory

This paper presents latest own results in the development of reversible photochromic and irreversible photofluorescent polymer systems providing fabrication of multilayer recording media for 3D optical memory with super high information capacity. It was shown that synthesized thermal irreversible photochromic diarylethenes into plastic binders allow to prepare photochromic polymer layers providing nondestructive refractive read-out of optical information. These photochromic polymer layers were used for preparation of six-layer recording media tested with the positive results using the framed optical device. This device imitated layer-by-layer writing, erasure and read-out of optical signals. Experimental evidences for making photochromic polymer layers based on a mixture of photochromic diarylethene and dye — phosphor with nondestructive fluorescent read-out are presented too. Polymer systems based on light-sensitive chromones manifest an irreversible photochemical transformations of these non-fluorescing compounds into the fluorescent products under UV irradiation. Received results open perspectives for making multilayer optical discs for bitwise working (based on photochromic systems) and archival (based on irreversible photofluorescent systems) optical memory with information capacity more 1 TB.

The impact of film thickness and melt-quenched phase on the phase transition characteristics of Ge2Sb2Te5

This study investigates the detailed phase transition behavior of Ge2Sb2Te5 (GST) thin films in the thickness range between 4–30 nm using a static laser tester. It was found that the crystallization time has a minimum for both as-deposited (around 12 nm) and melt-quenched (around 10 nm) amorphous materials. Capping the GST with a thin SiO2 layer changes this behavior and leads to a monotonic increase in crystallization time with film thickness for as-deposited amorphous samples but a decrease in time for melt-quenched, amorphous samples. The shortest crystallization time of about 7 ns was observed for the melt-quenched, amorphous 6 nm thick film. It was also found that the recrystallization time of melt-quenched, amorphous GST was shorter when the melt-quenched area was surrounded by GST in the hexagonal phase compared to GST in the rocksalt phase. The shorter recrystallization times for thinner, capped GST films indicate a promising scaling behavior of these materials for rewritable optical storage and phase change memory.

On the atomic structure of thin amorphous Ge-Sb-Te films

The structure of thin films of Ge2Sb2Te5 was studied by high-resolution transmission electron microscopy (HRTEM) and by X-ray diffraction (XRD). The as-evaporated films prepared by flash evaporation were amorphous. Nearly, all the atoms were randomly distributed but even in as-evaporated films small areas with some ordering were noticeable. The number and size of the ordered areas increased with thermal treatment of films to temperatures ∼80 °C, i.e., ∼70 °C below the crystallization temperature of amorphous Ge2Sb2Te5. After thermal treatment, small embryos of nucleation centers (size <8 nm) were formed. According to XRD measurements the films as a whole were still fully amorphous. The crystallite model of the structure of amorphous solids is then partly valid, at least for Ge2Sb2Te5. We expect that it can be applied also for other Ge–Sb–Te-based films, as well as for the other amorphous chalcogenides. The possible impact of the formation of nucleation “embryos” on storage time of data in optical and electrical phase-change memory materials and devices is discussed.

Electrical Resistance and Structural Changes on Crystallizaiton Process of Amorphous Ge-Te Thin Films

AbstractGeTe-Sb2Te3 pseudobinary compounds are attracting considerable attention as phase change materials for optical disk and phase change random access memory (PRAM). In these compounds, Ge2Sb2Te5 (GST) has been used for an optical disk memory such as DVD-RAM because the crystallization by laser beam heating is very fast (∼20ns). Recently, the GST has been much considered as material for PRAM and, therefore, the electrical resistance change due to crystallization and the phase change by applying an electrical current have been widely investigated. On the other hand, although GeTe compound has been suggested as the phase change material for the optical disk by Chen et al in 1986, the study focusing on the phase change material for PRAM is limited. Since GeTe is known to show the phenomenon of electrical switching, this compound has a potential of PRAM. In this study, the electrical resistance and crystalline structural changes on crystallization process in Ge-Te thin films were investigated.Films of amorphous Ge100–xTex (x : 46-94) with 200 nm thickness were deposited by sputtering of GeTe alloy target or co-sputtering of GeTe and Te targets on SiO2/Si substrates. In-situ electrical resistance measurements during heating process of these films were performed by two point probe method in a heating rate of 2∼50°C/min. X-ray diffraction (XRD) analysis was employed for the structural identification of thin films for 10-60° in 2′ using X-ray diffractometer with Cu-K. Transmission electron microscope (TEM) analysis was carried out to investigate the microstructure and to identify crystalline structure. The compositions of these films were confirmed by energy dispersive X-ray spectroscopy (EDS) attached TEM.All as-deposited Ge-Te films were confirmed amorphous by XRD and TEM. From the in-situ electrical resistance measurements, it is found that resistance change with crystallization process depends on the composition and the stoichiometric GeTe compound shows abrupt electrical resistance change at around 190 °C. The crystallization temperature of GeTe was higher than that of GST and resistance difference between the amorphous and the crystal was also larger. While the electrical resistance of GST film gradually decreased with increasing temperature after the crystallization at around 160 °C, that of GeTe film showed small temperature dependence after crystallization. It was found by X-ray measurement observation that the amorphous GeTe compound film crystallized first into a cubic state, and then into a stable rhombohedral state by further heating. The crystallization kinetics of Ge-Te thin films will be also presented.

Design of two-photon absorbing materials for molecular optical memory and photodynamic therapy

Molecular design of two-photon absorption materials toward three dimensional high-density optical memory and highly selective photodynamic therapy at deep tissue sites is introduced. Functionality such as photochromism was attached to highly efficient 2PA materials. In the case of PDT, solubilization of the 2PA agents in water without any aggregation is important.

Transient spectra, kinetics and mechanism of Rhodamine 700 dye precursor photoreaction

A new dye precursor (DP) molecule was synthesized and its photochromic, spectroscopic and kinetic properties were determined. This dye precursor yields the strongly fluorescent Rhodamine 700 dye when it is exposed to UV light. The ability to form a highly fluorescent dye was utilized in the fabrication of two-photon volumetric optical memory materials. The spectral characteristics, kinetics and photochemistry were measured and the photoreactions mechanism is proposed.

Stable Photochromism and Controllable Reduction Properties of Surfactant-Encapsulated Polyoxometalate/Silica Hybrid Films

In this paper, we present a novel strategy for fabricating polyoxometalate (POM)-based photochromic silica hybrid films. To combine metal nanoparticles (NPs) into the POMs embedded silica matrix, furthermore, we realized the controllable in situ synthesis of metal NPs in the film by utilizing the reduction property of POMs existing in the reduced state. Through electrostatic encapsulation with hydroxyl-terminated surfactants, the POMs with good redox property can be covalently grafted onto a silica matrix by means of a sol-gel approach, and stable silica sol-gel thin films containing surfactant-encapsulated POMs can be obtained. The functional hybrid film exhibits both the transparent and easily processible properties of silica matrix and the stable and reversible photochromism of POMs. In addition, well-dispersed POMs in a hydrophobic microenvironment within the hybrid film can be used as reductants for the in situ synthesis of metal NPs. More significantly, the size and location of NPs can be tuned by controlling the adsorption time of metal ions and mask blocking the surface. The hybrid film containing both POMs and metal NPs with patterned morphology can be obtained, which has potential applications in optical display, memory, catalysis, microelectronic devices and antibacterial materials.

Bulk photochromism in a tungstate-phosphate glass: A new optical memory material?

In this work, we present a new photochromic tungstate based glass which have both absorption coefficient and refractive index modified under laser exposure. The photosensitive effect is superficial under ultraviolet (UV) irradiation but occurs in the entire volume of the glass under visible irradiation. The effect can be obtained in any specific point inside the volume using an infrared femtosecond laser. In addition, the photosensitive phenomenon can be erased by specific heat treatment. This glass can be useful to substitute actual data storage supports and is a promising material for 3-dimensional (3D) and holographic optical storage.

Multigigahertz range-Doppler correlative signal processing in optical memory crystals

Analog optical signal processing of complex radio-frequency signals for range-Doppler radar information is theoretically described and experimentally demonstrated using crystalline optical memory materials and off-the-shelf photonic components. A model of the range-Doppler processing capability of the memory material for the case of single-target detection is presented. Radarlike signals were emulated and processed by the memory material; they consisted of broadband (> 1 GHz), spread-spectrum, pseudorandom noise sequences of 512 bits in length, which were binary phase-shift keyed on a 1.9 GHz carrier and repeated at 100 kHz over 7.5 ms. Delay (range) resolution of 8 ns and Doppler resolution of 130 Hz over 100 kHz were demonstrated.

Nanoscale cube-on-cube growth and characterization of SrS:Eu, Sm optical memory material

We report the growth and characterization of SrS codoped with europium and samarium ions. The material was grown on (0 0 1) MgO substrate, using solid-source molecular-beam epitaxy technique. We show that the film epitaxial planes conserved the substrate surface symmetry—cubic. X-ray diffraction results establish the epitaxial relationship as cube-on-cube (0 0 1) SrS ||(0 0 1) MgO with [1 0 0] SrS ||[1 0 0] MgO for growth temperature between 400°C and 600°C. Infrared-stimulated luminescence with a peak at 612 nm is observed at room temperature in SrS: Eu, Sm which is stimulated with infrared light after irradiation with visible light.

Stability of Photochromism in New Bifunctional Copolymers Containing Spiropyran and Chalcone Moiety in the Side Chain

We synthesized three copolymers bearing photochromic spiropyran dye and chalcone moiety in the side chain for studying the dynamic properties of their photochromism. They contain methacrylate-spiropyran (MAspiropyran) and methacrylate-chalcone (MA-chalcone) with the different concentration. The photosensitivity of the newly synthesized copolymers was investigated by using UV-Vis absorption spectroscopy. We observed photodimerization and photochromic behavior under UV irradiation at the same time. The effect of photocrosslink on the rate and stability of photochromism in three copolymers was considered in this study. This study might be helpful to design photochromic materials for irreversible optical memory by virtue of photocrosslinking reaction.

Crossed-beam two-photon readout system for three-dimensional radiation dosimeters

Three-dimensional optical random access memory (3D ORAM) materials with enormous capacity and fast access speed have shown a great potential in overcoming limitations of access and storage capacity in current memory devices. As another useful development of this 3D ORAM, we have shown the application of 3D ORAM materials as a practical dosimeter. The local heating of the polymer matrix by the deposited energy of ionizing radiation is thought to contribute to the conversion of the fluorescent photochromic dye to a nonfluorescent form. The two-photon readout system is very useful in tracking the interactions of energy of ionizing radiation deposited in a polymer matrix. However, the polymer fracturing that has occurred during two-photon readout has been an obstacle in utilization of 3D ORAM materials as a dosimeter. In this work, we further evaluated the readout system using a high-energy variable attenuator in order to prevent polymer fracturing due to the strong absorption of the 1064 nm beam by the polymer matrix. Through adjustment of the 1064 nm beam intensity using this attenuator, two-photon excited fluorescence of anthracene-doped 3D ORAM materials could be obtained without polymer fracturing. As a result of this improved procedure, a highly spatially resolved fluorescence image of anthracene-doped 3D ORAM material could be observed with the two-photon readout system.

Structural and thermal analysis of Ag-Sb-Te alloy and its films for phase change optical memories

An Ag-Sb-Te alloy and its films are prepared as a new optical recording amorphous crystalline (a?c) phase transformation material. The crystallization process of Ag-Sb-Te films is systematically studied through measurement of recording characteristics to solve the trade-off problem between data (amorphous) stability and erasing sensitivity. phase change optical recording disks demonstrate long thermal stability of the amorphous recording marks. The crystallization process of Ag- Sb-Te material was studied using differential thermal analysis (DTA), and the nature of the material was studied by x-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), respectively. The films were studied for both cases of before and after annealing. It was concluded that the alloy (Ag-Sb-Te) could be used as a phase change optical memory material.