Optical Data Storage Devices Research Articles

Synthesis and characterization of H-type amphiphilic liquid crystalline block copolymers by ATRP

H-type amphiphilic liquid crystalline block copolymers containing azobenzene were synthesized by atom transfer radical polymerization (ATRP). Macroinitiators prepared by the esterification between poly(ethylene oxide) (PEG) and 2,2-dichloroacetyl chloride were utilized to initiate the polymerization of 6-[4-(4-ethoxyphenylazo)phenoxy]hexyl methacrylate (M6C). The resulting macroinitiators and block copolymers were characterized by 1H NMR, gel permeation chromatography (GPC). Polarizing optical microscopy (POM) and differential scanning calorimetry (DSC) preliminarily revealed the liquid crystalline property of these block copolymers. This series of liquid crystalline block copolymers are promising in some areas, such as optical data storage, optical switch, and molecular devices.

Photoinduced dichroism and surface-relief-gratings of hyperbranched azo polymers synthesized by azo-coupling reaction

This article reports the synthesis and photoresponsive properties of a hyperbranched azo polymer containing electron donor-acceptor type azo chromophores both in backbone and peripheral positions. For the synthesis, a hyperbranched azo precursor polymer was synthesized by step-growth polymerization of an AB2 monomer, bis[2-(N-ethylanilino)ethyl] 5-aminoisophthalate, through azocoupling reaction. The hyperbranched precursor polymer was then reacted with the diazonium salt from 4-cyanoaniline to introduce donor-acceptor azo chromophores in the peripheral groups. The resulting azo polymers were characterized by the spectroscopic methods and thermal analysis. The photo-induced dichroism of the hyperbranched polymer was studied and its orientation order parameter was estimated to be 0.063. Thin films of the hyperbranched azo polymers were used to fabricate surface-relief-gratings (SRGs) by exposing them to an interference pattern of Ar+ laser beam at modest intensities (150 mW/cm2). The azo chromophores introduced by post-azo-coupling reaction at the para-positions of the peripheral azobenzenes shows a significant effect on the SRG inscription rate. The hyperbranched azo polymers can potentially be used for applications such as reversible optical data storage, photoswitch, sensors, and other photo-driven devices.

Tailored liquid‐crystal switching on ferroelectric polymer films

Abstract— The development of voltage‐controlled visible‐wavelength progression in displays and optical data storage devices using ferroelectric polymers and liquid crystals is described. Ferroelectric polymers are materials that have a ready distribution of dipoles which can be oriented by manipulating material composition and external fields. Utilizing the charge polarization distribution, their performance as an alignment layer for inducing liquid‐crystal alignment is presented. The switching response of the devices was tailored by changing the material composition through copolymers and nanoclay doping.

Optical switch and luminescence properties of sol–gel hybrid organic–inorganic materials containing azobenzene groups and doped with neodymium ions

TiO2/γ-glycidoxypropyltrimethoxysilane and methyltrimethoxysilane hybrid organic–inorganic material, which contains azobenzene groups and is doped with neodymium ions, was prepared by a low temperature sol–gel technique. The trans-cis-trans photoisomerization cycles of the hybrid film were investigated by a photoirradiation with UV light followed by visible light. An intense room-temperature upconversion emission at 397 nm was also measured from a hybrid film heated at a low temperature of 80 °C upon excitation with a xenon lamp at a wavelength of 580 nm. The mechanism of the upconversion is proposed and indicates that the sequential two-photon absorption process originating from the long-lived 4F3/2 excited state should be responsible for the up-conversion process. These results indicate that the prepared hybrid materials with multifunctional photonic properties are promising candidates for integrated optics and photonic applications, which would allow directly integrating onto a single chip, the upconversion device with the pump source, and optical data storage and optical switching devices.

Electron beam induced coloration and luminescence in layered structure of WO3 thin films grown by pulsed dc magnetron sputtering

Tungsten oxide thin films have been deposited by pulsed dc magnetron sputtering of tungsten in argon and oxygen atmosphere. The as-deposited WO3 film is amorphous, highly transparent, and shows a layered structure along the edges. In addition, the optical properties of the as-deposited film show a steplike behavior of extinction coefficient. However, the electron beam irradiation (3.0 keV) of the as-deposited films results in crystallization, coloration (deep blue), and luminescence (intense red emission). The above changes in physical properties are attributed to the extraction of oxygen atoms from the sample and the structural modifications induced by electron bombardment. The present method of coloration and luminescence has a potential for fabricating high-density optical data storage device.

Wavefront error measurement of high-numerical-aperture optics with a Shack-Hartmann sensor and a point source

We developed a new, to the best of our knowledge, test method to measure the wavefront error of the high-NA optics that is used to read the information on the high-capacity optical data storage devices. The main components are a pinhole point source and a Shack-Hartmann sensor. A pinhole generates the high-NA reference spherical wave, and a Shack-Hartmann sensor constructs the wavefront error of the target optics. Due to simplicity of the setup, it is easy to use several different wavelengths without significant changes of the optical elements in the test setup. To reduce the systematic errors in the system, a simple calibration method was developed. In this manner, we could measure the wavefront error of the NA 0.9 objective with the repeatability of 0.003 lambda rms (lambda = 632.8 nm) and the accuracy of 0.01 lambda rms.

Photo-responsive properties and heating-induced surface relief patterns from azobenzene-doped GeO_2/γ-glycidoxypropyltrimethoxysilane organic-inorganic hybrid films

GeO(2/ gamma-)glycidoxypropyltrimethoxysilane organic-inorganic hybrid optical waveguide films, which contain azobenzene small molecular groups, have been prepared by combining a low temperature sol-gel process with a spin-coating technique. The azobenzene in hybrid films can undergo trans-cis-trans photoisomerization efficiently by a photoirradiation with UV light. It is also found interestingly that surface relief patterns can be heat-induced on such azobenzene-containing small molecular hybrid films, but can be erased by further heating the hybrid film and no permanent damage of the hybrid film is observed. The refractive index, thickness, and morphologic properties of the hybrid films have also been investigated by a prism coupling technique and atomic force microscopy. These results indicate that the azodye-doped hybrid films are promising candidates for integrated optics and photonic applications, which allow directly integrating on the same chip waveguide device with the optical data storage and optical switching devices.

Halbach-Magnet-Array-Based Focusing Actuator for Small-Form-Factor Optical Storage Device

Small-form-factor optical data storage devices are being developed rapidly nowadays. In the case of a CF-II-type optical data storage device (43×36×5 mm3), its components such as the disk, head, focusing actuator, and spindle motor should be assembled within a 5 mm thickness. Since the thickness specification is tight, each component should be designed to have a small thickness. In this paper, a Halbach magnet array is proposed for the focusing actuator. The proposed Halbach magnet array has the advantage of a thin actuation structure without sacrificing flux densities due to its special magnet array feature that increases the magnetic flux on one side without using a yoke. By finite element method (FEM) analysis, flux density, actuation force and actuator thickness are compared with those of conventional methods. Each dimension of the array is obtained to achieve higher performances. Finally, the working range and the resolution of the focusing actuator are experimentally obtained to verify the feasibility of the proposed idea.

초소형 광정보저장기기용 웨이퍼 스케일 대물렌즈 제작을 위한 회절광학소자 성형기술 개발

The demand fer small and high-capacity optical data storage devices has rapidly increased. The areal density of optical disk is increased by using higher numerical aperture objective lens and shorter wavelength source. A wafer-scale stacked micro objective lens with a numerical aperture of 0.85 and a focal length of 0.467mm for the 405nm blue- violet laser was designed and fabricated. A diffractive optical element (DOE) was used to compensate the spherical aberration of the objective lens. Among the various fabrication methods for micro DOE, the UV-replication process is more suitable fur mass-production. In this study, an 8-stepped DOE pattern as a master was fabricated by photolithography and reactive ion etching process. A flexible mold was fabricated for improving the releasing properties and shape accuracy in UV-replication process. In the replication process, the effects of exposing time and applied pressure on the replication quality were analyzed. Finally, the surface profiles of master, mold and molded pattern were measured by optical scanning profiler. The geometrical deviation between the master and the molded DOE was less than <TEX>$0.1{\mu}m$</TEX>. The diffraction efficiency of the molded DOE was measured by DOE efficiency measurement system which consists of laser source, sample holder, aperture and optical power meter, and the measured value was <TEX>$84.5\%$</TEX>.

Predictive aberration correction for multilayer optical data storage

The recording of data in multiple layers, rather than a single layer, permits a significant increase in the capacity of optical data storage devices. However, focusing to the different layers introduces different amounts of depth-dependent aberrations. Variable aberration correction is therefore necessary to maintain diffraction-limited operation. We demonstrate the use of adaptive optics to predict and correct these aberrations for both the recording and read-out of such media.

Design and fabrication of a diffractive optical element for the objective lens of a small form factor optical data storage device

The demand for small and high-capacity optical data storage devices has rapidly increased. The areal density of the optical disk is increased using a higher numerical aperture objective lens and a shorter wavelength source. We have designed and fabricated a wafer-scale stacked micro objective lens with a numerical aperture of 0.85 and a focal length of 0.467 mm for the 405 nm blue–violet laser. To compensate the chromatic aberration of the objective lens, a diffractive optical element (DOE) was used. Among the various fabrication methods for micro DOE, the UV-replication process is more suitable for mass-production. In this study, an eight-stepped DOE pattern as a master was fabricated by photolithography and reactive ion etching processes. A flexible mold was fabricated for improving the releasing properties and shape accuracy in the UV-molding process. In the replication process, the effects of exposing time and applied pressure on the replication quality were analyzed. Finally, the shapes of the master mold and the molded pattern were measured by an optical scanning profiler. The deviation between the master and the molded DOE was less than 0.1 µm. The efficiency of the molded DOE was measured by a DOE efficiency measurement system which consists of a laser source, a sample holder, an aperture and an optical power meter, and the measured value was 84.5%.

Dielectric characterization of [Fe(NH2−trz)3]Br2•H2O thermal spin crossover compound by terahertz time domain spectroscopy

The complex optical index refraction of an iron (II) spin-crossover coordination polymer is measured in the Terahertz frequency range by Terahertz time-domain spectroscopy (THz-TDS). By scanning the temperature from 288 to 333 K, we have recorded the evolution of the THz spectrum within the low spin - high spin thermal hysteresis loop. We were able to simultaneously infer the refractive index and absorption variations. The low spin-high spin transition has a marked spectral signature in the millimeter wavelength. In the 0.1–0.6 THz frequency range, the variation of real and imaginary part of the index of refraction is 6% and 20%, respectively. A marked absorption is observed above 500 GHz. The THz-TDS provides a clear direct fingerprint of this class of materials, which are interestingly potential candidates for optical data storage and processing devices.

Spatial control of organic nanocrystal nucleation in sol–gel thin films for 3-D optical data storage devices or chemical multi-sensors

We have controlled the spatial distribution of organic nanocrystals grown in sol–gel thin films prepared by spin-coating. This spatial control of nucleation is carried out through the difference in confinement between the surface and the depth of the sol–gel thin films at the first stage of the coating process. Thus, we can direct the nucleation sites in grooves or wells of textured substrates. We have achieved, respectively 1- and 2-D ordering of nanocrystals while 3-D patterns have been obtained through a multi-layer process. This nucleation control gives an opportunity to design new 3-D optical data storage or 2-D arrays of luminescent crystals for chemical or biological multi-sensors.

Nondestructive Monitoring of the Photochromic State of Dithienylethene Monolayers by Surface Plasmon Resonance

The use of surface plasmon resonance (SPR) as a nondestructive, nonerasing readout of the isomerization state of a photochromic dithienylethene covalently linked to a chemically modified gold surface was investigated. Four different binding layers were examined: 11-mercaptoundecanol (MUO), an amine-modified 11-mercaptoundecanol (MUO-NH2), dextran, and an amine-modified dextran. The binding of dithienylethene to the modified gold surface and photoisomerization of the photochrome in the bound state were established by FTIR. Solvent effects were measured for every layer tested using ethanol and hexanes. In general, large, easily measurable SPR signal changes could be detected under conditions where photoisomerization of the dithienylethene photochrome was not quenched by the gold plasmon, establishing SPR as a viable form of readout for potential dithienylethene-based optical data storage or processing devices. Dextran-bound photochrome in ethanol exhibited the largest SPR response upon photoisomerization, but is more prone to time-dependent fluctuations resulting from swelling of the dextran layer (caused by slow diffusion of the solvent) than the other layers. Large responses are also provided by MUO-NH2 and MUO, and the signal is much more stable than that for dextran.

Amphiphilic Azo Polymer Spheres, Colloidal Monolayers, and Photoinduced Chromophore Orientation

In this work, azobenzene-containing colloidal spheres have been fabricated and used to construct photoresponsive monolayers. The colloidal spheres were prepared from an amphiphilic azobenzene-containing random copolymer through hydrophobic aggregation of the polymer chains, which was induced by adding the selective solvent (H2O) into a THF solution of the polymer. The size and size distribution of the spheres depended on the initial concentration of the azo polymer in THF and the H2O/THF ratio. Adjusting those factors and optimizing other preparation conditions, uniform colloidal spheres could be obtained. Monolayers composed of hexagonally close-packed colloidal spheres were prepared by the capillary-force-driven method. The colloidal monolayers showed obvious dichroism after laser irradiation due to the photoinduced azo-chromophore orientation occurred in the spheres. The orientation order parameter was related to the irradiation time and estimated to be 0.09 at the photostationary state. The colloidal spheres and their monolayers can potentially be used as building blocks or media for reversible optical data storage, photo-switching, sensors, and other photo-driven devices.

Synthesis and Photochromic Properties of Functional Diarylethenes with a [1,3]dithiol-2-one (Thione) Bridging Unit

ABSTRACT In designing new photochromic compounds for optical data storage and photoswitching devices, the synthesis of a series of 4,5-diaryl-1,3-dithiol-2-one and 4,5-diaryl-1,3-dithiol-2-thione, which are suitable building blocks for incorporation with fluorophore, is herein reported. Our strategy needs the preliminary preparation of diarylalkynes, obtained by successive Sonogashira-Hagihara coupling-reactions in good yields (63–92%). A combination of these synthetic precursors with diisopropylxanthogen disulfide leads to the corresponding 4,5-diaryl-1,3-dithiol-2-ones under radical conditions with excellent yields (54–70%). The thionation of these compounds is obtained nearly quantitatively with phosphorous pentasulfide.

Femtosecond laser assisted production of dichroitic 3D structures in composite glass containing Ag nanoparticles

Three-dimensional, permanent anisotropic modifications in glass containing spherical Ag nanoparticles are demonstrated using multicolor fs laser irradiation. The method can produce dichroism by deformation of nanoparticles to oblong shapes oriented parallel to the laser polarization. Using samples with a vertical gradient of the fill factor of Ag nanoparticles in the glass substrate and an accordingly inhomogeneous broadening of the surface plasmon band, modifications in various depths can be made using different excitation wavelengths. The induced modifications are reversible: heating to ≈600 °C restores the spherical shape of Ag nanoparticles. This technique could be used in manufacturing of different, 3D, polarization and wavelength selective micro-devices such as polarizers, filters, gratings, display and rewriting optical 3D data storage devices.

Development of Winged Microscanners: Architecture and Performance

Two-dimensional scanners have a variety of applications in displays, barcode readers, optical data storage devices, and free-space optical interconnects. In this paper, the modeling, simulation, fabrication, and testing of a MEMS-based winged microscanner are described. The microscanner is controlled using electrostatic force distributed across multiple electrodes. Compared to previous cantilever designs, the symmetric spring on the beam is adapted to reduce the spring constant of the suspension beam and increase flexibility. In addition, wings are added to further reduce the actuation voltage. A thorough finite element analysis (both static and dynamic) has been completed to study key design parameters such as driving voltage, operating frequencies, and tilt angle. The focus of this work was on the performance analysis of the microscanner, including squeeze film damping effects. Prototype microscanners and four different microscanner arrays were fabricated using the MUMPS process (MEMSCAP). The dynamic performance of the microscanner was then investigated. For a single electrode actuation, the pull-in voltage of the winged microscanner is as low as 4 V, and it is further reduced to 3 V with only two driving electrodes used to actuate the microscanner, which makes it compatible with off-the-shelf control logic circuits. The initial testing results demonstrated that the maximum tilt angle is approximately 2.5/spl deg/. To increase this tilt angle, a customized process is now being developed.

Integrated MEMS optical flying head with lens positioning actuator for small form factor optical data storage

An integrated and miniaturized optical flying head (OFH) is presented for a small form factor with high-density portable optical data storage. It is composed of a high numerical aperture (NA) objective lens, a microlens actuator for adjusting the position of the objective lens, an integrated air-bearing-slider (ABS), and an embedded magnetic-field-modulation (MFM) coil for magneto-optical recording. To adjust the optimal focal length of the lens, a parallel-plate vertical actuator is adopted. In addition, the area-optimized torsion type H-shape spring is used for vertical actuation. The glass ABS structure and the double metal layer MFM coil are fabricated on a transparent substrate, bonded to the silicon actuator. The overall size of the integrated OFH, assembled with the high numerical aperture (NA=0.85) objective lens, is 3 mm×1.6 mm×0.5 mm (length×width×thickness), which makes it small enough to be furnished in a portable optical data storage device. A prototype of small optical drive is also developed to satisfy the form factor of PCMCIA type II. The prototype includes the integrated and miniaturized OFH and the basic operation of the drive is tested.

Molecular reorientation by photoinduced modulation of rotational mobility.

To explain the large photoinduced molecular reorientation phenomena observed in dye-doped liquids and liquid crystals, the hypothesis was formed that the rotational mobility of dye molecules is strongly altered during their electronic excitation. Here, we report the direct measurement of a 30%-50% mobility decrease of photoexcited anthraquinone dye molecules dissolved in a cyanophenyl liquid host. This mobility reduction is ascribed to an excited-state reinforcement of intermolecular hydrogen bonding. These results provide fully independent evidence for the validity of current models of the photoinduced reorientation and a working demonstration of the design concepts of "fluctuating-friction" molecular motors. We propose that a light-induced modulation of molecular mobility associated with electronic photoexcitation is of general relevance to the behavior of photosensitive organic materials, currently investigated for applications in optical data storage, liquid-crystal displays, and organic optoelectronic devices.