Scattering measurements on optical disks and their relation to media noise

We have conducted measurements of scattered light from bare polycarbonate and glass substrates and from complete optical disks using a He-Ne laser beam in different polarization states and at different angles of incidence. The results are compared with the measured media noise obtained from the same disks on a dynamic tester. Both the scattered light and the media noise originate from the jaggedness and other imperfections of the groove structure, the roughness of the substrate's surface, and the inhomogeneities of the bulk of the substrate. Although some sources of media noise manifest themselves in the scattered light distribution, others cannot be easily detected by this type of measurement.     

Measurement of birefringence for optical recording of disk substrates

We experimentally investigate the birefringence of bare and coated substrates for magneto-optical recording, using ellipsometry at wavelengths of 632.8 and 780 nm. The polarization rotation and ellipticity of reflected or transmitted light are measured for different incident angles and for different orientations of the incident linear polarization. The measured data are then fitted by the MULTILAYER computer program, which solves the Maxwell equations for a plane wave propagating in a multilayer structure. This approach makes it possible to determine, with high accuracy, the orientations of the principal axes of the substrate and the corresponding refractive indices. The results show that one of the principal axes is always along the substrate's normal direction, but the orientations of the in-plane principal axes can be much different from the radial and track directions. A special feature of the ellipsometers that were used is that a glass hemisphere is placed in contact with the substrate to eliminate refraction of the incident beam. This enables a maximum propagation angle of 70° (with respect to the normal) in the substrate and hence increases the measurement sensitivity. Certain anomalies have been observed, which we believe are associated with the variation of birefringence properties along the thickness direction.     

Effects of substrate birefringence and tilt on the irradiance and phase patterns of the return beam in magneto-optical disk data storage

Substrate birefringence in a magneto-optical disk system is shown to have a predictable effect on the return beam. The irradiance and phase patterns of the return beam at the exit pupil of the objective lens are calculated and experimentally verified for the cases of no substrate birefringence, birefringence aligned with the incident polarization, and birefringence aligned at 45° to the incident polarization. The irradiance at the exit pupil is also calculated (and experimentally verified) for a grooved substrate for various amounts of substrate tilt.     

High track density thin-film tape heads

A short review of thin-film write heads, all formed on magnetically flat substrates, is given, and properties of the several types are discussed. A novel structure is proposed to avoid many disadvantages of the former heads. It is formed on a magnetically grooved but physically planar substrate. Measured and calculated characteristics are given for this new configuration for which the name "groove structure" is proposed. The groove structure turns out to be more effective than a magnetically flat substrate. A thin-film write head on the groove structure has 30 (60 as stagger) tracks per quarter inch and a common biasing line effectively to reduce the signal write current.     

Modulation of osteogenic, adipogenic and myogenic differentiation of mesenchymal stem cells by submicron grooved topography

Topographic cues have been recognized crucial on the modulation of cell behavior, and subsequent important for the design of implants, cell-based biomedical devices and tissue-engineered products. Grooved topography direct cells to align anisotropically on the substrates, resulting in an obvious morphological difference compared with the flat and the other topographies. This study aimed at investigating the effects of grooved topography on the differentiation of mesenchymal stem cells (MSCs) into osteoblasts, adipocytes and myoblasts. A series of submicron-grooved polystyrene substrates with equal groove-to-ridge ratio but different width and depth (width/depth (nm): 450/100, 450/350, 900/100, and 900/550) were fabricated based on electron beam lithography and soft lithography techniques. Primary rat MSCs (rMSCs) were cultured on these substrates without induction for differentiation for 6?days, and then subjected to induction for osteogenesis, adipogenesis and myogenesis. While the alignment of rMSCs strongly complied with the direction of the grooves and increased with groove depths, cell attachment on day 1 (~1.5?×?10⁴/cm²) and cell proliferation after 6?days of culture (~5?×?10⁴/cm²) were not significantly affected by substrate types. Osteogenesis, indicated by alkaline phosphatase activities and calcium deposit, was not significantly modulated by the grooved substrates, compared with the flat control, suggesting that cell alignment may not determine osteoinduction of rMSCs. On the other hand, adipogenesis, indicated by lipid production, was significantly enhanced by the grooved substrates compared with the flat surface (P?<?0.001). On the other hand, myogenesis, indicated by desmin and MHC staining, was enhanced by the grooves in a time- and groove size-dependent manner compared with the flat control. The results suggested that grooved topography has an in-depth potential for modulating the commitment of the stem cell lineages, which could benefit the development of advanced biomaterials for biomedical applications.     

Polarization dependence of thermal behavior in rewritable phase-change optical recording media

Thermal behavior in land-groove phase-change optical recording has been examined for different polarizations of the incident beam. Three-dimensional temperature distribution in the grooved medium is evaluated by the numerical solution of Maxwell's equations and the heat transfer equation with the finite-difference method. Both experiments and calculations have shown that the thermal behavior in the medium is dependent on the state of polarization and the nature of the track. The calculated mark shapes in a quadrilayer stack are in good agreement with experimental observations.     

Effect of beam profile on the cross talk of land and groove recording

Numerical analyses were conducted to simulate the effects of different Gaussian-weighted beams and disk geometries on the read-out signal cross talk of land and groove recording of phase-change optical disk systems. The optimized groove depth, which yields a minimum cross-talk noise, differs for different Gaussian-weighted beams and different track pitches. This beam profile dependence of the optimum groove depth is undesirable for practical application of this technique, because different optical systems may have different beam profiles at the lens aperture plane. It is found that this effect can be reduced to a certain extent by introduction of an appropriate shading band filter.     

Simulation of the cytoskeletal response of cells on grooved or patterned substrates

We analyse the response of osteoblasts on grooved substrates via a model that accounts for the cooperative feedback between intracellular signalling, focal adhesion development and stress fibre contractility. The grooved substrate is modelled as a pattern of alternating strips on which the cell can adhere and strips on which adhesion is inhibited. The coupled modelling scheme is shown to capture some key experimental observations including (i) the observation that osteoblasts orient themselves randomly on substrates with groove pitches less than about 150 nm but they align themselves with the direction of the grooves on substrates with larger pitches and (ii) actin fibres bridge over the grooves on substrates with groove pitches less than about 150 nm but form a network of fibres aligned with the ridges, with nearly no fibres across the grooves, for substrates with groove pitches greater than about 300 nm. Using the model, we demonstrate that the degree of bridging of the stress fibres across the grooves, and consequently the cell orientation, is governed by the diffusion of signalling proteins activated at the focal adhesion sites on the ridges. For large groove pitches, the signalling proteins are dephosphorylated before they can reach the regions of the cell above the grooves and hence stress fibres cannot form in those parts of the cell. On the other hand, the stress fibre activation signal diffuses to a reasonably spatially homogeneous level on substrates with small groove pitches and hence stable stress fibres develop across the grooves in these cases. The model thus rationalizes the responsiveness of osteoblasts to the topography of substrates based on the complex feedback involving focal adhesion formation on the ridges, the triggering of signalling pathways by these adhesions and the activation of stress fibre networks by these signals.     

Measuring distribution of the ellipsoid of birefringence through the thickness of optical disk substrates

We have measured the birefringence of polycarbonate optical disk substrates, using ellipsometry. For a more comprehensive characterization, the probe beam was incident upon substrates in a wide range of polar angles and from different azimuths relative to track direction (?). Our measurements show that the ellipsoid of birefringence is tilted in the plane of radial (r) and normal (z) directions. The tilt angle varies through thickness, with a maximum value of approximately 10°. For beams passing through the substrate in the ?-z plane and at large incident angles, this tilt causes significant conversion (up to 100%) between p- and s-polarized components. Distributions of other parameters, such as the values of in-plane and vertical birefringence, are obtained simultaneously in the measurements.     

The influence of surface chemistry and topography on the contact guidance of MG63 osteoblast cells

The purpose of the present study was to determine in vitro the effects of different surface topographies and chemistries of commercially pure titanium (cpTi) and diamond-like carbon (DLC) surfaces on osteoblast growth and attachment. Microgrooves (widths of 2, 4, 8 and 10 μm and a depth of 1.5–2 μm) were patterned onto silicon (Si) substrates using microlithography and reactive ion etching. The Si substrates were subsequently vapor coated with either cpTi or DLC coatings. All surfaces were characterized using atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and contact angle measurements. Using the MG63 Osteoblast-Like cell line, we determined cell viability, adhesion, and morphology on different substrates over a 3 day culture period. The results showed cpTi surfaces to be significantly more hydrophilic than DLC for groove sizes larger than 2 μm. Cell contact guidance was observed for all grooved samples in comparison to the unpatterned controls. The cell viability tests indicated a significantly greater cell number for 8 and 10 μm grooves on cpTi surfaces compared to other groove sizes. The cell adhesion study showed that the smaller groove sizes, as well as the unpatterned control groups, displayed better cell adhesion to the substrate.     

Vector diffraction and polarization effects in an optical disk system

The track pitch of current optical disks is comparable with the wavelength of the laser source. In this domain of the pitch-to-wavelength ratio, the complex-diffraction amplitudes are different for different incident polarization states, and the validity of the scalar diffraction theory is questionable. Furthermore, the use of multilayer coatings and high-numerical-aperture beams in modern optical disk technology inevitably entails the excitation of surface waves, which can disturb the baseball pattern significantly. To describe the interaction of a focused beam with a grooved multilayer system fully, it is necessary to have a rigorous vector theory. We use a rigorous vector theory to model the diffraction of light at the optical disk. We present the simulation and the experimental results and demonstrate the ability of this approach to predict or model accurately all essential features of beam-disk interaction, including the polarization effects and the excitation of surface waves.     

Dependence of the tracking performance of an optical disk on the direction of the incident-light polarization

In optical-disk data-storage systems, the signal that provides tracking information is dependent on the groove shape, the optical constants of the materials involved, and the polarization state of the incident light. In this paper, we show that the tracking signal can be described by two measurable quantities, both of which are largely independent of aberrations in the optical system. Using these two quantities, we match the tracking performance of a given disk to an equivalent disk having rectangular grooves-the adjustable parameters being the rectangular groove depth and the duty cycle. By assumption, these rectangular grooves modulate only the phase of the incident beam and disregard its state of polarization. The effective groove depth and the duty cycle thus become dependent on the polarization state of the incident beam. We examine these dependences for various disks having different groove geometries and different combinations of materials.     

Effect of surface plasmon excitations on the irradiance pattern of the return beam in optical disk data storage

The excitation of surface plasmons at a dielectric-metal interface is responsible for dips in the zeroth order diffraction efficiency of a metal grating at certain angles of incidence. The dips appear as dark bands in the returning irradiance pattern in an optical disk system and are seen only when there is a component of incident polarization that lies perpendicular to the tracks. The location of these bands is derived from theoretical considerations and is shown to depend on the track pitch and the materials involved, but not on the groove depth or width. The band locations are confirmed by zeroth order diffraction efficiency measurements as a function of incident angle. A possible negative effect of these bands on an optical disk system is the introduction of additional fluctuations and noise into the focusing and push-pull tracking signals.     

Recording and readout using clock marks premastered by groovewobbling

A new recording and readout technique for land and groove recording on a magnetic super-resolution (MSR) disk is described. The technique uses specifically premastered clock marks. To generate a stable clock signal, the clock marks are fabricated with short bursts in grooves by wobbling at a different frequency from address information. The clock marks and the address information can be separated from a track error signal. When the extracted clock signal is applied to precise recording and readout of the magneto-optical data marks, the bit error rate becomes lower than that of the conventional clock recording/readout system. Additionally, the crosstalk of the wobbled address information to the magneto-optical signal can be canceled out electronically. This new format is not only suitable for high-density recording but also convenient in the disk manufacturing process     

Recording performance of discrete track patterned media fabricated by focused ion beam

We report on the recording performance of discrete track patterned media fabricated by focused ion beam (FIB). We investigated performance over a small area by spinstand read/write testing. Discrete track patterned regions show smaller magnetic track width and better signal separation between adjacent tracks and therefore higher track density than that of nonpatterned continuous media as a result of reduced side fringe effect and edge noise. We found that, at a designed groove depth of 4-8 nm, the shallow FIB etched grooves already provide good isolation between adjacent tracks, indicating the superiority of ion beam induced modification of magnetic properties in film media over physical modification of disk surface topography. This has implications for discrete track recording and media fabrication.     

Instrument-invariant method of film thickness determination by means of substrate-to-film X-ray peak intensity ratioing

A simple method for the determination of thin film thickness using X-ray spectrometry is described. The ratio of the substrate X-ray peak intensity to that of the film is taken as a measure of the thickness of the film. A calibration curve, constructed using specimens of known thickness, can then be used for thockness determination. The results are independent of the incident electron beam current. The calibration curve is made instrument invariant by means of a normalization procedure and by taking into account the respective X-ray take-off angles. Normalized calibration curves are reported for 13 different elements on a silicon substrate, covering thicknesses between 2.5 and 650 nm. The method, ideally suited to thin films on solid substrates, can also be applied to films not on substrates, and in the presence of a thin organic interfacial layer between film and substrate.     

Advancing nanograined/ultrafine-grained structures for metal implant technology: Interplay between grooving of nano/ultrafine grains and cellular response

Nanograined/ultrafine-grained (NG/UFG) metals provide surfaces that are different from conventional coarse-grained polycrystalline metals because of the high fraction of grain boundaries. In the context of osseointegration of metal implants, grooving of nanograins/ultrafine grains by electrochemical grooving is a potential approach to increase the biomechanical interlocking and anchorage with consequent enhancement of cellular response. The primary objective of the research described here is to advance science and technology of metal implants by making a relative comparison of osteoblast response of grain boundary grooved and planar NG/UFG surfaces. The NG/UFG substrates were obtained using an ingenious concept of controlled phase reversion and the grain boundaries were electrochemically treated to induce grooving of large fraction of grain boundaries of NG/UFG substrate. Experiments on the effect of grooving of grain boundaries of NG/UFG metal indicated that cell attachment, proliferation, viability, morphology, and spread are favorably modulated and significantly different from planar (non-grooved) NG/UFG substrates. Furthermore, immunofluorescence studies demonstrated stronger vinculin signals associated with actin stress fibers in the outer regions of the cells and cellular extensions on electrochemically grooved NG/UFG substrate. These observations are indicative of accelerated response of cell–substrate interaction and activity. The differences in the cellular response of planar and grain boundary grooved NG/UFG surface are attributed to favorable surface topography that accelerates the cellular activity.     

The influence of surface morphology and rigidity of the substrata on cell motility

The purpose of this study is to measure the cell motility under different stiffness and pattern of the substrata. Human melanoma cells were used for this study. Three surface patterns including flat, 6 µm-cone, and 6 µm-groove on polydimethylsiloxane (PDMS) were created by the micro-electro-mechanical system (MEMS). Glass dish was used as control substrate. After cells were seeded for 30 min, the cell images were taken every minute for 2 h. Each group had 4-5 dishes and 27-38 cells were calculated. The cell motility was 1.13 ± 1.30, 2.12 ± 1.21, 2.39 ± 1.11 and 3.08 ± 1.49 µm/min on glass, flat, cone, and groove PDMS, respectively. Cells in PDMS groups moved significantly faster than the control group (glass) due to smaller stiffness of the former substrates. More than 80% of cells on grooved PDMS moved along the grooves, indicating the grooved surface morphology could control the direction of cell movement. Our results display that substrate modulus and pattern can influence cell motility.     

Computation of effective groove depth in an optical disk with vector diffraction theory

Results of vector diffraction simulations pertaining to the effective groove depth for various disks with different groove parameters, different coatings, and different incident polarizations are presented. The effective depth deviates from the physical depth if the track pitch approaches the wavelength of the light source. Moreover, the difference of the effective depth for the two polarization states is demonstrated. The effective depth is usually shallower than the physical depth, especially for deeper grooves. The ray-bending mechanism associated with the objective lens and the different response to s- and p-polarized light on reflection from the disk surface impact the effective depth for objective lenses with different numerical apertures.     

Optically addressed ferroelectric memory with nondestructive readout

We present a review of the emerging optically addressed ferroelectric memory with nondestructive readout as a nonvolatile memory technology, identify its high-impact applications, and project on some novel device designs and architectures that will enable its realization. Based on the high-speed bidirectional polarization-dependent photoresponse, simulation of a readout circuit for a 16-kbit VLSI ferromemory chip yields read-access times of ~20 ns and read-cycle times of ~30 ns (~34 ns and ~44 ns, respectively, within a framework of a radiation-hard environment), easily surpassing those of the conventional electrical destructive readout. Extension of the simulation for a 64-kbit memory shows that the read-access and -cycle times are only marginally increased to ~21 ns and ~31 ns, respectively (~38 ns and ~48 ns, with a radiation-hard readout circuitry). Commercial realization of the optical nondestructive readout, however, would require a reduction in the incident (optical) power by roughly an order of magnitude for the readout or an enhancement in the delivered power-to-size ratio of semiconductor lasers for compact implementation. We present a new two-capacitor memory-cell configuration that provides an enhanced bipolar optoelectronic response from the edges of the capacitor at incident power as low as ~ 2 mW/μm(2). A novel device design based on lead zirconate titanate with the c axis parallel to the substrate is suggested to reduce the requirement of incident optical power further by orders of magnitude.