Materials For Optical Data Storage Research Articles

Immiscible blend of cholesteric copolymers as a new type of material with photoregulated optical properties

A blend containing two immiscible cholesteric comb-shaped polyacrylates was prepared. Upon light irradiation, one of the components is able to change the pitch of the cholesteric helix due to the isomerization of chiral photochromic side groups. The phase behavior and optical properties of the above blend were studied. Planarly oriented films based on the above blend show the two well-pronounced peaks of selective light reflection: one peak is seen in the visible spectral region whereas the other peak is observed in the near-IR region. Under the action of light, the long-wavelength peak is shifted to higher wavelengths whereas the second peak remains almost unchanged. The specific features of the kinetics of the photooptical processes taking place in this system were studied. The above immiscible blend is shown to be a new unique material for optical data recording and storage.

Reversible dye-doped holographic photopolymer materials for reconfigurable optical data storage

New self-developing reversible dye-doped holographic photopolymer materials were recently developed in our laboratory, which are sensitive to light from the green to near infrared wavelengths (500–850 nm) [1]. Previously, we have used liquid crystals in similar polymer matrices to increase the refractive index modulation [2]. In this work, we show that a new functionality of optical reconfiguration may be introduced if a reversible photosensitive dye is doped in the material. The compound we have used in this experiment is a multifunctional acrylic-based photopolymerizable material. Reversible dye and organic compound are added to serve as solvent and refractive index increasing agents. In addition, this dye may partially replace the liquid crystal’s role. Holographic recording characteristics of these materials were investigated. We have demonstrated that, the diffraction efficiency of recorded holograms and material sensitivity are high. We believe that these materials are very promising for applications in permanent, but reconfigurable diffractive optical elements.

Mechanical stresses upon crystallization in phase change materials

Crystallization processes in different Te alloys, employed in phase change materials for optical data storage, have been investigated by in situ mechanical stress measurements. Upon crystallization a considerable stress buildup is observed, which scales with the volume change upon crystallization. Nevertheless the observed stress change only corresponds to approximately 9% of the stress estimated for a purely elastic transformation. Further evidence of stress relief phenomena comes from the temperature dependence of the stress in the crystalline and amorphous states. Ultrathin dielectric layers have a profound influence on the crystallization process as evidenced by simultaneous optical reflectance and mechanical stress measurements. This observation can be explained by heterogeneous nucleation of crystallites at the interface between the dielectric layer and the phase change film.

New Chiral-Photochromic Dopant with Variable Helical Twisting Power and its use in Photosensitive Cholesteric Materials

A new photosensitive chiral dopant based on cinnamic acid and isosorbide was synthesised. UV light action leads to E-Z photoisomerization of the dopant about C=C double bond of cinnamoyl fragment. Mixtures of the dopant with nematic and cholestric polymers were studied. It was shown, that planarly-oriented films of mixtures possess a selective light reflection in ultraviolet, visible and infrared regions of spectrum depending on the dopant concentration. The helical twisting power of the chiral dopant was calculated. UV light action on mixture films leads to a shift of selective light reflection peak to the long wavelength region of spectrum, which is explained by photoisomerization and decrease of the helical twisting power of the dopant. Kinetics of the process of photoinduced untwisting of the cholesteric helix was studied. Advantages of the use of the synthesised dopant and of mixtures based on it were demonstrated in comparison to earlier studied chiral-photochromic materials for optical data recording and storage.

Synthesis of New Poly(Silicate Esters) with Disperse Red 1 and Their Application as Optical Data Storage Materials

New silicate ester monomer was synthesized by the reaction of 3-methacryloxypropylmeth-yldichlorosilane with disperse red 1 in methylene chloride. The monomer was polymerized with a radical initiator to produce the corresponding homopolymer and copolymers. These polymers were found to be excellent as reversible optical information recording media for data storage and retrieval through a trans-cis isomerization of the azobenzene groups by Ar laser irradiation.

Five-membered rings as diazo components in optical data storage devices: an ab initio investigation of the lowest singlet excitation energies

The two lowest singlet excitation energies of 18 azo dyes have been studied by ab initio quantum-chemical methods within the second-order polarization propagator approximation (SOPPA). Various combinations of five-membered rings (furan, thiophene, pyrrole, oxazole, thiazole, and imidazole) have been investigated as diazo components for a potential use in optical data storage materials. It is found that the diazo compounds with two heterocyclic five-membered rings have π→π ∗ excitation energies corresponding to laser wavelengths in the region 450–500 nm whereas one five-membered ring and a phenyl group as diazo components results in wavelengths in the region 400–435 nm.

Characterization of azobenzene chromophores for reversible optical data storage: molecular quantum calculations

Azobenzene chromophores are promising as molecularly engineered materials for reversible optical data storage based on molecular reorientation. In this paper, the optical properties of several different azobenzene chromophores are studied using molecular quantum calculations. Special emphasis is put on molecular anisotropy since a high degree of anisotropy is essential for the storage performance. The trans isomers are all found to be practically one-dimensional whereas the anisotropy of the cis isomers is highly dependent on substituents. Molecular reorientation of chromophores in liquid-crystalline polymers is simulated in order to study the influence of lacking cis anisotropy. It is demonstrated that photoinduced birefringence is significantly reduced in materials characterized by a low degree of cis anisotropy.

Ab Initio Calculation of the Electronic Spectrum of Azobenzene Dyes and Its Impact on the Design of Optical Data Storage Materials

Electronic excitation energies of 16 azobenzene dyes have been calculated by ab initio methods within the second-order polarization propagator approximation (SOPPA). Good agreement with expriment is found for the lowest singlet and triplet states for both the trans- and cis-azobenzene molecules. The differences are in the range of ±0.3 eV, with the exception of the lowest n → π* transition in trans-azobenzene, where a deviation of −0.64 eV is found. The lowest π → π* transition in trans-azobenzene, on the other hand, is particularly well reprensented with a deviation of only −0.15 eV. Furthermore, the experimental singlet π → π* transitions are reproduced for a set of azobenzene dyes with different electron donor and acceptor groups and the correct shifts in excitation energy are obtained for the different substituents. It has also been demonstrated that ab initio methods can be used to determine suitable candidates for azo components used in materials for data storage.

Some considerations of organic materials for high density optical disk data storage

The application possibilities of organic materials for high density optical disk data storage are discussed. Several points, such as physical and chemical stabilities, wavelength match and reversible property changes, which should be taken into consideration, are presented.

Photoaddressable Polymers: A New Class of Materials for Optical Data Storage and Holographic Memories

Photoaddressable Polymers: A New Class of Materials for Optical Data Storage and Holographic Memories

Chiroptical Switching between Liquid Crystalline Phases

The modulation of the mesophases and physical properties of liquid crystalline (LC) materials by reversible photochemical reactions, i.e., photoswitching of LC phases, is a major challenge for information technology. The photochromic behavior of organic molecules has also attracted renewed interest with a view to the development of materials for optical data storage and molecular optical devices. It is well-known that small amounts of optically active guest molecules added to a nematic host induce a cholesteric phase and that the helical pitch in the twisted nematic phase is very sensitive to chemical modifications in the chiral guest molecule. Reversible switching of a liquid crystal between a twisted and a nontwisted phase by photochemical modification of the chirality of the doping molecule, although predicted, has thus far not been realized. In their pioneering efforts to develop a chiral liquid crystalline phototrigger, Schuster and co-workers 6 showed that photoracemization of optically active binaphthyl derivatives induces a cholesteric to nematic transition in a doped phase. We describe herein the first successful demonstration of the reversible photochemical modulation of the helical pitch of a twisted nematic LC phase and the reversible transition between cholesteric and nematic phases using as dopant an optically active photoresponsive molecule. The chiroptical molecular switch employed is based on the donor-acceptor substituted inherently dissymmetric alkenes cisand trans-2-nitro-7-(dimethylamino)-9-(2 ,3 -dihydro-1 H-naphtho[2,1-b]thiopyran1 -ylidene)-9H-thioxanthene (1 and 2) (Scheme 1). Addition of 1 wt % of enantiomerically pure P-trans-2 to the nematic phase, obtained from 4 -(pentyloxy)-4-biphenylcarbonitrile 3,

Theoretical investigation of the relation of hole-burning properties and the electronic structure of chemisorbed dyes

The ground and first excited states of the organic dyes quinizarin (Q) and tetrahydroxyphenylporphyrin (TOHP) chemisorbed on γ-Al2O3 were investigated with semiempirical PM3 and multireference CI methods. Q/γ-Al2O3 and TOHP/γ-Al2O3 represent promising materials for frequency domain optical data storage (FDOS) based on persistent spectral hole burning (PSHB), since it is possible to detect spectral holes at high temperatures (77 K). To find out whether differences in the electronic structure between the free and the adsorbed molecules account for the hole-burning properties, we performed calculations of Q–Al complexes with various coordination spheres. The calculated vertical excitation energies for the first electronically excited state of the investigated structures are in good agreement with experimental data. Analysis of the molecular orbitals shows that complexation of the investigated molecules with Al affects the electronic structure of the dyes only little. Furthermore, the character of the first excited state is conserved. The change of the dipole moment between the ground and the first excited states is about the same for the free and for the complexated systems. Therefore, it is concluded that hole burning of dye molecules chemisorbed on γ-Al2O3 at high temperatures is not due to a change of the electronic structure of the adsorbed dye molecules compared to the free dye molecules, but has to be attributed to the special interactions of Q with the γ-Al2O3 surface.

Optimization of physicomechanical properties of photopolymeric materials for optical data storage by treatment with high-frequency currents

We study the effect of the duration of ultraviolet irradiation of D-1 and D-2 photopolymerizable compositions on some physicomechanical properties of a photopolymeric material obtained in the process of photopolymerization. It is shown that these properties become most pronounced for a certain fixed duration of ultraviolet treatment and, hence, continuation of this process becomes unreasonable. The degree of conversion α of photopolymeric materials does not exceed 70%. Strength and some other characteristics of photopolymers can be improved by additional treatment of the material with high-frequency currents. This kind of treatment is characterized by lower power consumption and can be performed much faster than routine thermal treatment (by about two orders of magnitude).

Novel materials for non‐ablative optical recording

Novel materials for optical data storage based on a new recording method are described. Instead of pits or refractive index changes being produced in the recording medium, marks are the result of the generation of near ‐IR absorbing dyes, which leads to a change in the absorption of light from the read laser. The new materials, thin films of tellurium (IV) dihalide bound in a glass together with a sensitizing dye, undergo thermal reductive elimination of halogen on expousure to laser radiation. This technique could ind applications in electrophotographic mastering.

Sol-gel derived hybrid polymers as alternative materials for optical data storage in the frequency domain

Sol-gel derived hybrid organic-inorganic materials show attractive properties as alternative media for optical data storage in the frequency domain. Doping of the matrix is easily achieved by dissolving different types of organic dyes in appropriate amounts in the clear sols, obtained via cohydrolysis and -condensation of 3-glycidoxypropyl-trimethoxysilane and aluminium-tri (secondary butylate).

Liquid crystalline siloxanes containing spiropyran chromophores as reversible optical data storage materials

AbstractThe application of liquid crystalline siloxanes consisting of pentamethylhydrocyclosiloxane cores with attached cholesteryl‐, biphenyl‐ and photochromic spiropyran‐based mesogens as reversible optical data storage materials is demonstrated by grating formation with an argon ion laser beam operating in the UV and visible regions. Erasure of the gratings with heat or visible radiation was possible.

OPTICAL AND MAGNETO-OPTICAL PERFORMANCE OF OPTIMIZED DISK STRUCTURES

We explore the relative magneto-optic readback performance of three magnetic materials for erasable optical data storage. We modeled the system signal-to-noise ratio (SNR) of optimal film structures incorporating either TbFeCo, a Co/Pt multilayer, or BiDy-substituted FeGa garnet as a function of the wavelength of the reading light beam. The complex indices of refraction and Voigt parameters of the three magnetic storage materials were determined experimentally over the spectral range 300-800 nm by regression fitting of data from variable angle of incidence ellipsometry and normal incidence Kerr spectroscopy to an appropriate optical model. Making specific comparisons of the read back performance at 450, 550, and 800 nm, we find that for the MO materials we have characterized, the performance of TbFeCo is superior to that of either Co/Pt or the garnet at 550 and 800 nm. Only at 450 nm did the Co/Pt give slightly higher SNR than the TbFeCo. The garnet showed improving SNR in passing from 800 nm to the shorter wavelengths, but it had lower performance than the metal films.

ChemInform Abstract: Materials for Optical Data Storage

The outstanding feature of materials research in the eighties has been the convergence of basic research and practical application, leading to ever shorter cycles of innovation. This is especially true of materials which form the basis of key technologies. The mass storage units of the next generations of computers will be based on optical processes having a storage density which exceeds that of all hitherto known storage techniques that are practicable from the technical standpoint. In view of the fact that since 1982 read-only memories in the form of compact discs (CD-ROM) have become very successful in the field of audio electronics, research and development are now concentrated on materials for write-once (WORM) and erasable (EDRAW) storage systems. Suitable materials for optical data storage are substances in which data markings can be recorded and deleted respectively using semiconductor lasers. Materials development is centered on the synthesis of infrared-absorbing dyes for WORM memories and the production of rare earth/transition metal alloys for magneto-optical data recording. An introduction to CD-ROM technology will be followed by an overview of the state of development of the most important storage materials which are currently available commercially, and then the properties of these materials will be discussed with reference to selected examples.

Materials for Optical Data Storage

AbstractThe outstanding feature of materials research in the eighties has been the convergence of basic research and practical application, leading to ever shorter cycles of innovation. This is especially true of materials which form the basis of key technologies. The mass storage units of the next generations of computers will be based on optical processes having a storage density which exceeds that of all hitherto known storage techniques that are practicable from the technical standpoint. In view of the fact that since 1982 read‐only memories in the form of compact discs (CD‐ROM) have become very successful in the field of audio electronics, research and development are now concentrated on materials for write‐once (WORM) and erasable (EDRAW) storage systems. Suitable materials for optical data storage are substances in which data markings can be recorded and deleted respectively using semiconductor lasers. Materials development is centered on the synthesis of infrared‐absorbing dyes for WORM memories and the production of rare earth/transition metal alloys for magneto‐optical data recording. An introduction to CD‐ROM technology will be followed by an overview of the state of development of the most important storage materials which are currently available commercially, and then the properties of these materials will be discussed with reference to selected examples.

Kinetic study of selenium used as optical data storage and holographic recording materials

A kinetic study has been performed on selenium for isothermal and continuous heating processes. The activation energy (E a) and Avrami exponent (n) have been evaluated in the range 414 to 449 K for the isothermal process and at a heating rate of 5 to 40 K min−1 for continuous heating. A new iterative calculation to give exact values ofE a andn for a continuous heating process is also presented.