Optical Storage Applications Research Articles

Preparation and trans–cis isomerization of azobenzene-containing TiO2/ormosils hybrid films derived at a low temperature sol–gel process for photonic applications

Azobenzene-containing TiO2/γ-glycidoxypropyltrimethoxysilane and methyltrimethoxysilane hybrid films are prepared by combining a low temperature sol–gel process and a spin-coating technique. The trans–cis isomerization of azobenzene small molecules inside organic–inorganic hybrid films is induced by a photoirradiation under UV light. It is noted that below a baking temperature of 150 °C is necessary for the hybrid films in optical storage or optical switching applications. The change of the refractive index and thickness of the hybrid films with real-time heating temperature are observed by using a prism coupling technique. The structural properties of the hybrid films are also characterized and investigated by thermal gravimetric analysis, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The results indicate that the as-prepared hybrid films might allow directly integrating the optical storage or optical switching devices with the waveguide devices on the same chip.

Cellular automata modelling of phase-change memories

A novel approach to modelling phase-transition processes in phase change materials used for optical and electrical data storage applications is presented. The model is based on a cellular automaton (CA) approach to predict crystallization behaviour that is linked to thermal and electrical simulations to enable the study of the data writing and erasing processes. The CA approach is shown to be able to predict the evolution of the microstructure during the rapid heating and cooling cycles pertinent to data storage technology, and maps crystallization behaviour on the nanoscale. A simple example based on possible future nonvolatile phase-change random access solid-state memory is presented.

Holographic Patterning of Composite Polymeric Materials for Photonic Applications

We report the results of an extended investigation on holographic patterning of novel liquid crystal-polymer materials, developed with the aim of producing one- and two-dimensional periodic structures to be used in optical devices for data storage and processing. Characterization of reflection gratings recorded at 405 nm resulted in values of diffraction efficiency (> 60%), sensitivity (up to 104 cm/J) and refractive index modulation (Δn ∼ 0.01) that make these materials interesting for optical storage applications. Large-area two-dimensional periodic structures have been also patterned using a multibeam photolitigraphic technique. These planar photonic crystals have many possible uses, both in fundamental and applied physics.

Blue emission from Eu2+-doped high silica glass by near-infrared femtosecond laser irradiation

Eu 2 + -doped high silica glass (HSG) is fabricated by sintering porous glass which is impregnated with europium ions. Eu2+-doped HSG is revealed to yield intense blue emission excited by ultraviolet (UV) light and near-infrared femtosecond laser. The emission profile obtained by UV excitation can be well traced by near-infrared femtosecond laser. The upconversion emission excited by 800 nm femtosecond laser is considered to be related to a two-photon absorption process from the relationship between the integrated intensity and the pump power. A tentative scheme of upconverted blue emission from Eu2+-doped HSG was also proposed. The HSG materials presented herein are expected to find applications in high density optical storage and three-dimensional color displays.

Tellurium based glasses: A ruthless glass to crystal competition

Although selenium and tellurium are very close in the periodic table, their ability to form glasses is totally different. Indeed, selenium based glasses are very common and stable whereas pure tellurium glasses need an extremely fast quenching such as splash cooling. Consequently, tellurium glasses are very suitable for the fast and reversible glass to crystal transformation for optical storage applications. In particular, glasses of the ternary system Ge/Sb/Te are the most appropriate to observe phase transformation under laser beam irradiation for the Digital Versatile Disk (DVD) technology. Also, Te based glasses could be very interesting to produce optics for the far infrared, thanks to their low phonon character due to the high atomic weight of Te. Especially, some new tellurium glasses of the system Ge/Ga/Te/I are currently in development for infrared detection of exoplanet within the framework of the Darwin mission. In this contribution an overview covering the past two decades is proposed on the chemistry and the potential applications of these original glasses.

Spectral encoding on Gold Nanorods Doped in a Silica Sol–Gel Matrix and Its Application to High‐Density Optical Data Storage

AbstractMetallic nanorods exhibit fascinating optical properties due to surface plasmons—collective oscillation of the electron cloud within a particle. They exhibit two principle absorption bands that correspond to surface plasmon resonance (SPR) along the longitudinal and transverse directions of the nanorod. Most importantly, the longitudinal band can be tuned with the aspect ratio of the rod, making it a spectrally tuneable optical material, which can be applied to a variety of devices from bioimaging to high‐density optical storage. Here, spectral encoding for high‐density optical storage applications is demonstrated on two sizes of gold nanorods (aspect ratios of three and five) doped in a silica sol–gel matrix by femtosecond pulsed laser irradiation. It is widely known that high‐power pulsed laser irradiation causes metal nanorods to undergo shape transformations via the process of melting or fragmentation. The process is enhanced if the laser wavelength is tuned at the longitudinal surface plasmon resonance peak of the nanorods, which results in a significant reduction or shift in the surface plasmon resonance peak. As such a shape change occurs only on the subpopulation of rods that have a longitudinal plasmon band matching the laser wavelength, a size‐ or spectrum‐selective shape transition is possible in a rod mixture with varying aspect ratios. The current spectral encoding technology can be incorporated into existing optical disc technology, such as three‐dimensional bit‐by‐bit and holographic, and can increase the capacity limit by utilizing the spectral domain.

Intense greenish emission from d0 transition metal ion Ti4+ in oxide glass

Ti 4 + -doped oxide glass is revealed to yield intense greenish emission by excitations with ultraviolet (UV) light and near-infrared femtosecond pulsed laser (NIFPL). The emission profile obtained by UV excitation can be well traced by NIFPL. Compared with Ta5+-doped oxide glasses reported previously, Ti4+-doped glasses exhibit distinctive characteristics in optical absorption, excitation, and emission spectra, indicating that intense tunable emissions can be readily achieved by selecting nd0 ions as dopants in glasses. The glass materials containing nd0 ions are expected to find applications in high density optical storage and three-dimensional color displays.

Photo-responsive properties of azobenzene small molecules in sol-gel hybrid TiO_2/ormosil organic-inorganic matrices

Azodye-doped TiO(2)/ormosil hybrid materials for photonic applications were prepared by a low temperature sol-gel process from an organic-inorganic hybrid system. Acid-catalyzed solutions of gamma-glycidoxypropyltrimethoxysilane and methyltrimethoxysilane mixed with tetrapropyl orthotitanate were used as hybrid matrix precursors. The trans-cis-trans photoisomerization of azobenzene small molecules in sol-gel hybrid organic-inorganic matrices was induced by a photoirradiation with UV light and subsequent visible light. It was found that the hybrid film doped with azodyes and heated at a lower temperature was much better for applications in optical storage or optical switch. The planar waveguide properties of the hybrid films were also investigated by using a prism coupling technique. These results indicates that it is possible for the as prepared hybrid films to allow directly integrating on the same chip the optical storage or optical switch devices with the pump source.

Quantum coherence effects in quasidegenerate two-level atomic systems

The wealth of quantum coherence effects depending on the orientation of external magnetic field, the polarization of coupling and probe lights, and the Rabi frequency of the coupling beam are studied in transition F{sub e}=2{r_reversible}F{sub g}=3 of Cs D{sub 2} line. The split of electromagnetically induced transparency (EIT) resonances on two or three resonances determined by the different combination of the polarization of interaction lights and the direction of applied magnetic fields is obtained. The shifting and widening of the EIT resonances with the strength of the magnetic field (i.e., Zeeman splitting in the upper and lower levels) and Rabi frequency of the coupling beam increasing are also discussed. It may develop into the potential application for tunable multichannel optical information storage. On the other hand, an explanation of observed asymmetry of spectra by laser frequency offset from the optical resonance is given with theoretical calculation, which is in good agreement with the experimental results.

Theoretical investigation of InGaN self-pulsating laser diodes for optical storage applications

Self-pulsating laser diodes operating at 420 nm are required in high-density optical storage devices. By growing saturable absorbing quantum wells in the p-doped cladding layer, of a semiconductor laser, it is possible to obtain the interplay between gain and absorption, which is required for pulsation. The dynamics of self-pulsating InGaN laser diodes are investigated to gain an insight into how the quantum-well configuration in the absorber and the cavity length affect the laser output. In particular, the work shows how a carefully designed structure gives rise to stable self-pulsation up to a temperature of 100degC while offering high powered emission at low operating currents. As a result, such blue-emitting laser diodes are highly desirable for use within next-generation optical storage devices

Optimisation of high power AlGaInP laser diodes for optical storage applications

The use of expanded mode layers in lasers designed for DVD read/write applications is investigated and their performance is compared with that of a more conventional structure. The expanded mode design achieves a higher confinement factor and hence much lower threshold currents than the conventional structure, for the same measured vertical farfield divergence of 18°. T0 is increased from 51 to 65 K (20–70°C) for the expanded mode structure for 320-µm long devices with uncoated facets. It is also found that zinc or magnesium can be used interchangeably for the p-dopant in a given structure without any impact on device performance.

A new three-zone amplitude-only filter for increasing the focal depth of near-field solid immersion lens systems

The near-field solid immersion lens (SIL) system with high numerical aperture exhibits high-quality resolution but its focal depth is at the nanoscale. This drawback limits its applications in high-density optical storage and photolithography. A new three-zone amplitude-only filter is proposed, to increase the focal depth and further improve the resolution of SIL systems. The procedure for designing a rotationally symmetric pupil-plane filter is presented, to control the three-dimensional intensity distribution of a SIL system near focus. The spot size, focal depth, Strehl ratio, and sidelobe intensity are analysed in detail. Numerical simulations show that, using the proposed filter, focal depth and resolution can be increased and sidelobe intensity can be suppressed.

Photoinduced birefringence in blends of a polyurethane bearing azobenzene moieties and a poly(amide–imide)

AbstractA polyurethane (MDI‐DR19Cl) bearing azobenzene moieties was prepared by condensation of 4,4′‐diphenylmethylenediisocyanate and an azobenzene compound, 2‐chloro‐4‐nitro‐4′‐[bis‐(2‐hydroxyethyl)]amine‐azobenzene (DR19Cl). Blends of MDI‐DR19Cl and a poly(amide‐imide) were prepared and characterized by UV‐visible and Fourier transform infrared spectroscopy and by DSC. The purpose of using the blends was to overcome the lack of stability of cast films of pure MDI‐DR19Cl, which tend to crack on storage under ambient conditions. This was successfully achieved and, depending on blend proportions, flexible self‐standing films could be prepared after thermal treatment. Optical storage experiments performed with the blends and pure MDI‐DR19Cl cast films showed a decrease in the remnant values of photoinduced birefringence in the blends (60 % for the blend with 5 wt% MDI‐DR19Cl) compared to pure MDI‐DR19Cl (75%), but, taking into account the low content of MDI‐DR19Cl used, the results obtained are probably sufficient to show the viability of these blends in optical storage applications. Copyright © 2006 Society of Chemical Industry

Effects of writing conditions on the photoinduced birefringence in azodye-doped polymers by a femtosecond laser

Birefringence in azodye-doped polymethylmethacrylate (PMMA) and azodye-doped inorganic–organic hybrid silica was induced by multiphoton excitation of a femtosecond laser. Comparing the growth kinetics of the photoinduced birefringence (PB) in the two matrixes, we concluded the mechanisms of photoinduced anisotropy were different and azodye-PMMA was much better for applications in optical storage. We also found that the growth rate and saturation value of the PB in azodye-PMMA were dominated by either peak or average power of the laser, strongly depended on writing conditions. An optimal control of the PB by varying writing conditions was indispensable to fully realize optical storage.

Fast and stable recording of birefringence and holographic gratings in an azo-polymethacrylate using a single nanosecond light pulse

Pulsed light-induced recording in azobenzene polymers has recently been studied due to its potential use in optical storage applications. In this paper we study the photoinduced birefringence (Deltan) and holographic grating recording in an azobenzene side chain liquid-crystalline polymethacrylate irradiating with a single 4 ns light pulse at 532 nm. For some irradiation conditions, Deltan grows in less than 50 ns reaching an essentially stable value of about 10(-2). Holographic gratings have been registered using intensity and polarization patterns. Fast response and stability, similar to those of Deltan, was observed in the holographic recording process. Both light-induced anisotropy and relief contributions have been found in the case of gratings recorded using intensity patterns, relief being the dominant contribution at high recording energies. Polarization gratings have been recorded using two orthogonally circularly polarized beams. The resultant gratings showed stable efficiencies up to 0.8% (measured at 633 nm in 1-mum-thick films) and no measurable relief was observed.

One- and two-photon induced birefringence in Salen dye cast films

In this paper, we present the one- and two-photon optically induced birefringence in cast films of amorphous poly(methyl methacrylate) (PMMA), containing a new series of organometallic azoaromatic compounds, referred to as Salen dyes, which may be used in optical storage applications. For the one-photon optically induced birefringence, longer writing times were required for the organometallic compounds in comparison with the nonmetal one, probably due to the higher degree of aggregation in the azoaromatic moieties that precludes molecular orientation. Furthermore, as a novelty concerning optical storage, two-photon optically induced birefringence could be achieved in the Salen dyes guest-host films, which allows their application in three-dimensional optical memories.

Design theories and performance limits of diffractive superresolution elements with the highest sidelobe suppressed

For the application of optical data storage, theories that we present can be used to design a diffractive super-resolution element (DSE) with the highest sidelobe suppressed. A globally optimal solution among general hybrid-type filters can be solved through linear programming. The obtained globally optimal performances set the exact performance limits of a general hybrid-type DSE with the highest sidelobe suppressed. A comparison of our design theories and the previous design methods shows the advantages of the former.

Electron-beam-induced optical memory effect in metallized ZnO thin films for the application of optical storage

Optical memory effect has been observed in Al- and Mg-capped ZnO thin films. Irradiate a focused electron beam on metallized ZnO films can result in a strong enhancement in band edge emission. Time-dependent cathodoluminescence (CL) reveals that, under electron irradiation, the band edge emission increases dramatically within several minutes without any change in the deep-level emissions. It is also found that, after irradiation, the emission intensity in metallized ZnO increases by almost eight times stronger than that of the bare counterpart. To demonstrate the feasibility of using Al/ZnO and Mg/ZnO films as data storage material, micron-scale optical data array has been fabricated by using a write–read routine. The emission pattern is not self-erasable, and the memory effect is sustainable even after several months without much degradation. As Al forms a good Ohmic metallization on ZnO, they can be used for fabricating high-density optical storage device.

High-extinction-ratio micro polarizing beam splitter for short wavelength optical storage applications

A surface-micromachined high-extinction-ratio polarizing beam splitter (PBS) using low absorptive silicon nitride layers for blue wavelength applications are demonstrated. The micro polarizing beam splitter consists of novel stack of two silicon nitride layers separated by an air gap. A PBS optimization model is established to achieve both high extinction-ratio and adequate process margin. The polarization extinction ratios of 25 dB for the reflected light and 15 dB for the transmitted light were experimentally achieved at lambda=405 nm. The fabrication of the PBS is compatible with other micro diffractive elements to build a micro optical bench, thus, feasible for short wavelength optical storage applications.

Low complexity symbol detection method for multilevel 2-D optical storage based on a linear channel model

A symbol detection scheme based on the Viterbi algorithm that simultaneously processes subsets of 2-D data in the presence of Gaussian noise was recently proposed for binary 2-D optical storage (TwoDOS). In the case of multilevel TwoDOS, a straightforward full-fledged maximum likelihood symbol detector, or even the previous Viterbi-based algorithm, is not an ideal solution due to complexity restrictions. We propose a suboptimum low complexity symbol detector, which still performs within the accepted performance bound for optical storage. We describe the procedures involved in designing and developing a practical symbol detection scheme for multilevel TwoDOS by analyzing the signal values generated by a linear channel model in the presence of Gaussian noise. Our proposed detection scheme exploits the properties of the 2-D data format on the disk, and is flexible enough to accommodate performance and complexity restrictions for optical storage applications.