Near-field Optical Disk Research Articles

Recording characteristics of high-density thin optical disk using near-field optical recording

We report on the recording characteristics of a near-field optical recording thin optical disk (NFR-TOD). We proposed a new cooling-less-type write strategy for recording, applied it to the NFR-TOD experimental system, and also applied a partial response maximum likelihood (PRML) signal processing, which was evaluated and optimized the PR coefficient for playback signals of the prototype NFR-TOD. The NFR-TOD has a track pitch of 160 nm and improved recording characteristics. The experimental results of the prototype NFR-TOD and the system indicate sufficiently low bit error rates (bERs) for recorded data at a fourfold higher recording density than the current Blu-ray disk.

Numerical Analysis of Ridged-Circular Nanoaperture for Near-Field Optical Disk

A ridged-circular nanoaperture is investigated through three-dimensional (finite-difference time-domain) FDTD method. The motion equations of free electrons are inserted to analyze a metallic material. The electromagnetic field distributions of optical near-field around the aperture are investigated. The phase change disk illuminated by a near-field optical light through a ridged-circular nanoaperture is also analyzed. The far-field scattering patterns from the phase change disk and the crosstalk characteristics between plural marks are studied.

Near-Field Optical Recording Using Solid Immersion Lens for High-Density Flexible Optical Disks

We propose a near-field optical recording flexible optical disk (NFR-FOD) for high-density recording at a high data transfer rate. We built a prototype high-density NFR-FOD that had a track pitch of 0.16 µm and we stacked some recording layers on a thin substrate that was 0.1 mm thick. We rotated the NFR-FOD closing the mechanical stabilizer on a drive system with less than 10 µmp–p axial run-out and achieved precise gap servo operation at a high rotational speed. We demonstrated near-field optical recording with a solid immersion lens with a numerical aperture (NA) of 1.84 on the NFR-FOD. The NFR-FOD has a fourfold higher recording density than the current Blu-ray disc, corresponding to a capacity of 100 Gbytes per layer, and a high data transfer rate of 250 Mbps, while rotating at half the speed.

FDTD Analysis of a Near-Field Optical Disk with a Ridged-Square Nano-Aperture

FDTD Analysis of a Near-Field Optical Disk with a Ridged-Square Nano-Aperture

Analysis of a Near-Field Optical Disk with an Acute-Edged Metallic Nano-Aperture

Readout characteristics of a near-field optical disk with an acute-edged metallic nano-aperture were investigated. The electromagnetic field distributions of the near field around the aperture were analyzed, and the far-field scattering of the waves transmitted through the phase change disk was calculated by finite-difference time-domain method into which motion equations of free electrons were incorporated. Adjusting the edge angle of the aperture increased the field intensity of near-field light. The influence of the edge angle on output through the recorded mark was also investigated. It was found that there is an optimum edge angle that differs depending on the aperture width.

Ultra-high-density phase-change storage using AlNiGd metallic glass thin film as recording layer

A nonchalcogenide inorganic blu-ray super-resolution near-field optical disk was successfully developed using an Al-based metallic glass as the recording layer. Data were recorded based on the large reflectivity change obtained during transformation from the amorphous state to the crystalline state upon laser irradiation. The optical and thermal properties of the Al 90Ni 3Gd 7 metallic glass thin film were studied. The crystallization and melting temperatures were higher and more thermally stable than that of conventionally used Ge 2Sb 2Te 5 chalcogenide phase-change recording material. Al 90Ni 3Gd 7 metallic glass also demonstrated fast phase-change response to blue laser. Dynamic recording performance of this new structure showed good carrier-to-noise ratio and readout stability.

Super-Resolution Near-Field Disk with Phase-Change Sn-Doped GeSbTe Mask Layer

A new mask layer of Sn-doped (7 at. %) Ge2Sb2Te5 was developed and used on super-resolution near-field phase-change optical disks (super-RENS). Temperature-dependent reflectivity result showed a reflectivity change at 169 °C with Sn doping into Ge2Sb2Te5. The mask material also showed high thermal stability. Optical study indicated the suitability of the film for use in blue-laser recording and as a mask layer. Fast crystallization within 90 ns was achieved using a pulsed high-power laser beam of 405 nm wavelength. Dynamic recording performance of the new structure showed carrier-to-noise ratios (CNR) of 37 and 18 dB obtained for 80 and 50 nm mark sizes, respectively. Readout thermal stability of 12,000 cycles was realized for 80 nm mark sizes. The incorporation of Sn-doped GeSbTe (GST) as mask layer in the super-RENS structure significantly improved the CNR and thermal stability of the disk.

High-Density Recording with a Near-Field Optical Disk System Using a Medium with a Top Layer of High Refractive Index

A near-field optical technology using a solid immersion lens (SIL) has been actively studied to expand the storage capacity higher than 100 Gbytes per layer in a 12-cm-sized optical disk. However, the working distance of an objective lens in a near-field optical disk system should only be 25 nm or less. Therefore, from the practical viewpoint, a topcoat layer is required to protect the recording layer when the SIL collides with a disk surface because of disturbances such as dust, shock and vibration. From a mechanical viewpoint, the topcoat should have the mechanical toughness to protect the disk surface. Moreover, from an optical viewpoint, it should have refractive index higher than the numerical aperture of an SIL to achieve a sufficient evanescent couple between the SIL and disk surface. In this study, we describe a topcoat for a near-field optical recording. First, we investigate the topcoat performance from the optical viewpoint. Second, we evaluate the topcoat performance from a mechanical viewpoint. Finally, we report the results of recording experiments for a disk with the topcoat and discuss its performance.

Super-Resolution Near-Field Blue Laser Type Optical Disk with Modified Coumarin Dye Recording Layer and Poly(methyl methacrylate) Contrast Enhanced Layer

A new coumarin derivative with blue light absorption was designed and synthesized. The absorption properties and thermal stability of the coumarin derivative were discussed. We also studied the characteristics of a super-resolution near-field blue laser type optical disk with a modified coumarin recording layer and a poly(methyl methacrylate) (PMMA) contrast enhanced layer. A sample disk, which consists of a spin-coated coumarin derivative recording layer and a PMMA contrast enhanced layer, showed excellent recording and readout characteristics with a carrier-to-noise ratio (CNR) of about 33 dB for a mark length of 100 nm (laser wavelength = 405 nm, numerical aperture = 0.65). These results indicate that the coumarin derivative dye is a promising candidate for optical recording by a super-resolution near-field structure (super-RENS) technique.

Blu-ray Type Super-Resolution Near-Field Phase Change Disk with In2Ge8Sb85Te5 Mask Layer

A new mask layer of In2Ge8Sb85Te5 was developed and used on super-resolution near-field phase change optical disks (super-RENS). The thermal and optical properties of the mask layer were investigated. Thermal analysis showed higher crystallization temperature and activation energy but lower melting temperature than Ge2Sb2Te5 phase change material. Ultra-fast crystallization within 30 ns was obtainable using a pulsed high-power laser beam of 405 nm wavelength. The recording performance of the new structure was evaluated on an optical recording system with a numerical aperture of 0.85 and a 405 nm blue-violet laser diode. Carrier-to-noise ratio (CNR) of 32, 25, 15, and 10 dB were obtained for 80, 50, 40, and 30 nm mark trains, respectively. Readout thermal stability of 20,000 cycles was realized for 80 nm mark trains. These performances were by far the highest obtainable for blu-ray aperture-type super-RENS disk.

Readout Signals Enhancements of Subwavelength Recording Marks via Random Nanostructures

Super-resolution properties of super-resolution near-field structures (super-RENS) are analyzed by a simplified Fourier optics approach. Statistical averages of near-field optical disks with random nanostructures are calculated to realize their general behaviors. Averaged readout waveforms of near-field optical disks with random apertures are calculated numerically. The resolution of a near-field optical disk is determined by the size and shape of the random nanostructure.

Servo Technologies in a Near-Field Optical Disk Drive System

A near-field optical disk drive (NFDD) system has been predicted as a storage system with a capacity higher than 100 Gbytes in a 12-cm-sized disk and with a data transfer rate higher than 100 Mbps. The major advantage of an NFDD system with a solid immersion lens is that it is possible to utilize the technologies accumulated through conventional optical drive systems. However, the working distance in an NFDD system is only 25 nm or less, which is much smaller than that in a conventional optical disk drive system. The nano-working distance must be successfully maintained without regard to any tilts over the entire disk radius. In this study, we discuss the servo technologies in the NFDD system by focusing on the tilt servo technology which contributes to the stable gap servo performance over the entire disk radius. First, we evaluate the tilt in the NFDD system by utilizing the optically detected tilt error. Second, we propose specifications for an NFDD system on the basis of the results. We also report a tilt servo system that satisfies the specifications and discuss its effect from the viewpoint of drive performance over the entire disk radius. In addition, we suggest a novel gap servo technology utilizing the tangential tilt information and discuss its performance.

Study of the optical response of phase‐change recording layer with zinc oxide nanostructured thin film

Recently, use of nanostructured materials as a near-field optical active layer has attracted a lot of interest. The non-linear optical properties and strong enhancements of metallic oxide nanostructured thin films are key functions in applications of promising nanophotonics. For the importance of ultra-high density optical data storage, we continue investigating the ultra-high density recording property of near-field optical disk consisting of zinc oxide (ZnO(x)) nanostructured thin film. A carrier-to-noise ratio above 38 dB at a recording mark size of 100 nm can be obtained in the ZnO(x) near-field optical disk by a DVD driver tester directly. In this article, we use an optical pump-probe system (static media tester) to measure the optical response of a phase-change recording layer (Ge(2)Sb(2)Te(5)) and demonstrate the high contrast of optical recording with a ZnO(x) nanostructured thin film in short pulse durations. Also, we investigate the dependence of writing power and the optical response in conventional re-writable recording layers and the phase-change material with ZnO(x) nanostructured thin film.

Rewrite Characteristics of a Scattered-Type Superresolution Near-Field Structure Optical Disk with a Ag2O Mask Layer

On the basis of the working mechanism of a silver-oxide-type superresolution near-field structure (super-RENS) disk, we realized excellent rewriting by performing the following steps: (1) initialize the disk at a continuous power of Pi=8.0 mW for 3 s, (2) write at a pulsed power of Pw=8.0–9.0 mW, (3) read (reinitialize only the mask layer) at a superresolution power of Psr=5.5 mW, which lies between the powers under reversible conditions of Pil=4.5 mW and Piu=6.0 mW for the mask layer, (4) erase at a continuous power of Pe=7.5 mW for 3 s, and (4) rewrite at a pulsed power of Pw=8.0–9.0 mW. The carrier-to-noise ratio (CNR) difference between writing and erasing slightly varies for successive rewritings, resulting in CNR=23 dB after 20 rewrite operations for a mark length of 400 nm, which is less than the resolution limit λ/4NA=413 nm. The Ag2O mask layer is decomposed into Ag nanoparticles at 160 °C that aggregate to form nanoclusters dispersed in the mask layer after the initialization operation at Pi=8.0 mW (3 s), and the recording layer made of Ge2Sb2Te5 (GST) is transformed by a second phase transition from a rocksalt crystal structure of fcc to a hexagonal crystal structure (the phase transition temperature range is estimated to be 250–450 °C) at a pulsed power of Pw=8.0–9.0 mW, and vice versa at a continuous power of Pe=7.5 mW for a disk velocity of 2 m/s.

Approach of Improving Disk Performance to High-Quality Gap Control in Near-Field Optical Disk Drive System

Near-field technology has been expected as one of the promising techniques for increasing the recording density in an optical storage disk system since it was introduced. In a near-field optical disk drive system (NFDD), the gap between a disk and a solid immersion lens (SIL) is required to be less than one-tenth laser wavelength with a high accuracy in order to generate an evanescent wave for reading or writing a signal. This is because the gap performance has detrimental effects on the RF amplitude vibration in a reading channel and on the signal laser peak power vibration in a writing channel. In this study, we first clarify whether specific disk resonances cause the deterioration of the gap performance in the NFDD. We then demonstrate an approach of improving the disk mechanical performance in order to achieve a highly precise gap performance. Finally, we investigate a high-performance gap servo system by utilizing the disk substrate suitable for the NFDD.

Slider Dynamics With Adhesive Force in Near-Field Recording

A near-field optical disk drive uses near-field optics to read/write disk data at a flying height of about 50 nm. Effects of interfacial adhesive forces, including van der Waals forces and electrostatic forces, become more evident during ultralow flying. However, the pickup head-flying-height variation is affected by the suspension, disk flutter, and air flow between the flying head and disk. An analysis model consisting of a simplified suspension and an air-bearing model with springs and dampers in three directions (focusing, pitching, and rolling) is used to simulate the dynamic behavior of the flying head. This study proposes an improved method for measuring the slider's flying-height variation. Simulation results with and without interfacial adhesive forces are compared

Readout contrast beyond diffraction limit by a slab of random nanostructures

A simplified Fourier optics approach is applied to study how near-field optical disks retrieve evanescent signals through random nanostructure. The statistical properties of the random nanostructures are used to realize the general behavior of near-field optical disks. The mechanism of its super-resolution capability and an analytical expression of the readout contrast of near-field optical disks with random apertures are derived. The resolution of near-field optical disk is determined by the size of the random nanostructure.

Approach of Improving Disk Performance to High Quality Gap Control in a Near-Field Optical Disk Drive System

Approach of Improving Disk Performance to High Quality Gap Control in a Near-Field Optical Disk Drive System

Adaptive inverse control for the pickup head flying height of near-field optical diskdrives

Near-field optical disk drives represent a promising technique for optical data recording toachieve an even larger storage density and capacity than DVD-ROM and Blu-ray diskdrives. To realize near-field optics, unlike conventional optical disk drives, a flying pickuphead is required. In this study, the pickup head consists of a suspension arm, a sliderand a bimorph piezoelectric bender between the suspension arm and the slider.The dynamic model of the pickup head is identified using measurement, but thewhole dynamics including both pickup head and interface dynamics between thedisk and slider is unmodeled. Adaptive inverse control of robustness is developedto track the vibratory deformation of rotating optical disks, so that the flyingheight of a pickup head can remain stable in the presence of modeling error.Experimental results demonstrate that using the proposed method the pickuphead can not only track disk deformation but also maintain the flying height.

Z-Scan Study of Nonlinear Optical Coupling of PtOx and Ge2Sb2Te5 of Near-Field Optical Recording Structure

The strong local nonlinear optical coupling properties of a 5 nm PtOx thin film sandwiched by two Ge2Sb2Te5 phase-change structures of a new type super-resolution near-field optical disk are investigated and observed by Z-scan experiments.