Near-field Optical Recording Research Articles

Round-tower plasmonic optical microfiber tip for nanofocusing with a high field enhancement

A round-tower (RT) plasmonic tip based on an optical microfiber for nanofocusing is proposed. Its operation principle is systematically analyzed by theoretical methods. Four operation regions of the theoretical model of the plasmonic tip are identified for the first time in this study. In addition, the near-field performances of the plasmonic optical microfiber tip are investigated by a finite-element simulation. The results reveal that the field enhancement at the tip apex can be improved by adjusting the slope angle of its surface plasmon polariton region. Remarkably, the proposed RT plasmonic tip can achieve a significantly enhanced field at the tip apex and simultaneously focus the optical spot to a few nanometers. The characteristics of superfocused optical spot and ultrahigh field enhancement are promising for applications in scanning near-field optical microscopy, optical recording, nanolithography, and biosensing.

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.

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.

High-Speed and Precise Gap Servo System for Near-Field Optical Recording

We propose a high-speed and precise gap servo control system that is based on the feed-forward control method that can reduce the harmonic disturbance (RHD-FFC) on a near-field optical recording system with a solid immersion lens. An optical disk of 1.2 mm thickness rotates with about 30 µmp–p of axial run-out. The axial run-out prevents a gap servo from performing precisely at high rotational speed. The RHD-FFC method can suppress harmonic disturbances of gap error signal and can perform at high speed while keeping less than 25 nm of gap length between an optical head and a rotational disk surface. We confirmed that the gap servo applying the RHD-FFC method performed precisely and achieved at high disk rotational speed of 11000 rpm with a 1.2-mm-thick optical disk with a track pitch of 0.16 µm, corresponding to a data capacity of 100 Gbyte.

High-Speed and Precise Gap Servo System for Near-Field Optical Recording

We propose a high-speed and precise gap servo control system that is based on the feed-forward control method that can reduce the harmonic disturbance (RHD-FFC) on a near-field optical recording system with a solid immersion lens. An optical disk of 1.2 mm thickness rotates with about 30 µmp–p of axial run-out. The axial run-out prevents a gap servo from performing precisely at high rotational speed. The RHD-FFC method can suppress harmonic disturbances of gap error signal and can perform at high speed while keeping less than 25 nm of gap length between an optical head and a rotational disk surface. We confirmed that the gap servo applying the RHD-FFC method performed precisely and achieved at high disk rotational speed of 11000 rpm with a 1.2-mm-thick optical disk with a track pitch of 0.16 µm, corresponding to a data capacity of 100 Gbyte.

Near-field optical recording with nanocomposite cover-layer for numerical aperture of 1.85

Near-field optical recording media with a nanocomposite cover-layer were prepared and tested using a gap servo, near-field recording system with an effective numerical aperture of 1.85. The refractive index of the nanocomposite cover-layer, which has a smooth surface, is 1.86, yielding a successful gap servo operation. However, the recording density cannot be maximized due to the refractive index of the cover-layer being lower than the required value for the effective numerical aperture. A recording density of 95 GB per disc can be achieved with a nanocomposite cover-layer having a refractive index of 1.86. The readout signal from 110 GB is not clear due to the reduced eventual numerical aperture in conjunction with the limited refractive index of the cover-layer.

High refractive index nanocomposite cover-layer for optical storage media

A nanocomposite cover-layer consisting of a UV curable resin and TiO 2 nanoparticles, a dielectric cover-layer, and a hybrid cover-layer for near-field optical recording media are prepared and characterized. The dielectric and the nanocomposite cover-layers with a refractive index of about 1.95 showed a rough surface, which yields collisions between a solid immersion lens and a cover-layer during the near-field gap servo test. A smooth surface with a nanocomposite cover-layer can be obtained by reducing the refractive index to less than 1.86, which results in a successful gap servo without the collision.

High density rewritable media for near-field optical recording

High density rewritable media for near-field recording are fabricated and their writing and readout characteristics are examined. High density substrates for rewritable media are prepared using a phase transition mastering technique to overcome the limitation of the deep UV laser recorder. A cover-layer with a UV curable material is coated on the rewritable media and its surface properties are discussed to apply for cover-layer incident near-field recording configuration. The effective numerical aperture of a pick up with a solid immersion lens is 1.46, which could cope with the recording density of about 70 GB per disc. The performance of readout signals were investigated, and compared the properties between 320 nm and 200 nm of the track pitch in the substrate.

Tracking Characteristics of a Triangular-Aperture Mounted Optical Head Slider to Which a Polarized Violet Laser Source Is Applied

In near-field optical recording, the combination of a triangular aperture and a polarized illuminating light is thought to be one of the most promising breakthroughs for improving both spatial resolution and signal-to-noise ratio. In light of this, we have already fabricated a triangular-aperture mounted optical head slider and demonstrated its superior performance while clarifying the influence of the polarization direction on the spatial resolution in the circumferential direction. When the polarization direction was perpendicular to the bottom side (which is parallel to the slider trailing edge) of the aperture, the highest spatial resolution and signal contrast were obtained, in spite of the usage of a fairly large aperture, indicating the presence of clear readout signal waveforms corresponding down to 100 nm line-and-space (L/S) patterns. In this study, we tried to experimentally clarify the influence of the polarization direction of the illuminating light on an aperture's field spread in the radial direction. In order to concretely evaluate the field spread, we prepared 1-mm-long linearly arranged (in the circumferential direction) L/S patterns on a metal-layered medium, and a piezo-electric actuator combined positioner. Intersecting the aperture at two portions of the tracks, directly acquired signal waveforms could be successfully transformed into the waveforms that would be obtained if the aperture had crossed the track at right angles. The field spreads in the radial direction were estimated to be approximately 250 nm when the polarization direction was perpendicular to the bottom side. In contrast, when the polarization direction was 45 degrees, the stationary field spread in the radial direction was estimated to be approximately 350 - 370 nm. It could be confirmed experimentally that both the highest spatial resolution in the circumferential direction and the smallest field spread in the radial direction were realized with the combination of the triangular aperture and the illuminating polarized light whose direction was perpendicular to the bottom side. Based on these results, the signal-to-noise ratio will be evaluated and discussed in the future with respect to the above-mentioned optimum aperture structure and conditions.

Cover-layer-protected solid immersion lens-based near-field recording with an annular aperture

Currently, data recording density in cover-layer-protected near-field-recording (NFR) and multiple-recording layered NFR optical data storage technology is limited by the difficulty in obtaining high-refractive-index cover layer materials. In addition, with the exception of improved resolution, the higher the numerical aperture (NA), the poorer the optical characteristics. However, in this study, we present novel cover-layer-protected solid immersion lens (SIL)-based NFR optics that provide superior optical performance with higher recording density, greatly enhanced focal depth, and less sensitivity to near-field air-gap-distance variation by modulating the amplitude and phase in the entrance pupil using annular pupil zones. Using an annular aperture consisting of three concentric annular zones to effect amplitude and phase modulation, the 1.45 NA cover-layer-protected SIL-based NFR optics achieved a data recording density as high as that of conventional 1.80 NA SIL-based NFR optics. These 1.45 NA optics yielded a full-width at half-maximum (FWHM) spot size of 0.315 lambda, a focal depth of 0.82 lambda, a focused beam spot sensitivity to air-gap-distance within the near-field region of 0.04 lambda, and a sidelobe intensity lower than 7%. In comparison with conventional 1.80 NA SIL-based NFR optics, the annular aperture optics achieved 3.5 times longer focal depth and much lower focused beam spot sensitivity to air-gap distance while maintaining the same high resolution. The introduction of this novel specially designed NFR optics could greatly improve data capacity in multiple-recording layered NFR.

Cover-layer-protected solid immersion lens-based near-field recording with an annular aperture

Currently, data recording density in cover-layer-protected near-field-recording (NFR) and multiple-recording layered NFR optical data storage technology is limited by the difficulty in obtaining high-refractive-index cover layer materials. In addition, with the exception of improved resolution, the higher the numerical aperture (NA), the poorer the optical characteristics. However, in this study, we present novel cover-layer-protected solid immersion lens (SIL)-based NFR optics that provide superior optical performance with higher recording density, greatly enhanced focal depth, and less sensitivity to near-field air-gap-distance variation by modulating the amplitude and phase in the entrance pupil using annular pupil zones. Using an annular aperture consisting of three concentric annular zones to effect amplitude and phase modulation, the 1.45 NA cover-layer-protected SIL-based NFR optics achieved a data recording density as high as that of conventional 1.80 NA SIL-based NFR optics. These 1.45 NA optics yielded a full-width at half-maximum (FWHM) spot size of 0.315λ, a focal depth of 0.82λ, a focused beam spot sensitivity to air-gap-distance within the near-field region of 0.04λ, and a sidelobe intensity lower than 7%. In comparison with conventional 1.80 NA SIL-based NFR optics, the annular aperture optics achieved 3.5 times longer focal depth and much lower focused beam spot sensitivity to air-gap distance while maintaining the same high resolution. The introduction of this novel specially designed NFR optics could greatly improve data capacity in multiple-recording layered NFR.

Surface-plasmon resonance of a planar lollipop near-field transducer

Surface-plasmon resonance of a lollipop near-field transducer integrated in a planar solid immersion mirror for heat-assisted magnetic recording has been studied by pump-probe photothermal measurement and recording on a phase-change medium. Lollipops of 190 nm in disk diameter and 20-nm-thick were found to exhibit two resonant modes in 710–950 nm. Recording on a phase-change medium showed that the wavelength at the short resonant mode is not sensitive to the presence of a medium, while the resonance at the long wavelength exhibits significant redshift. On resonance, the plasmon efficiency enables the pronounced near-field optical recording.

Effects of Surface Roughness and Mechanical Properties of Cover-Layer on Near-Field Optical Recording

Narrow gap distance in cover-layer incident near-field recording (NFR) configuration causes a collision problem in the interface between a solid immersion lens and a disk surface. A polymer cover-layer with smooth surface results in a stable gap servo while a nanocomposite cover-layer with high refractive index shows a collision problem during the gap servo test. Even though a dielectric cover-layer, in which the surface is rougher than the polymer, supplements the mechanical properties, an unclear eye pattern due to an unstable gap servo can be obtained after a chemical mechanical polishing. Not only smooth surface but also good mechanical properties of cover-layer are required for the stable gap servo in the NFR.

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.

Improving the recording ability of a near-field optical storage system by higher-order radially polarized beams

Distributions of the optical field in a solid immersion lens recording system are calculated for higher-order radially polarized modes of the incidence. Results show that two higher-order radially polarized modes of R-TEM(11) (* )and R-TEM(21) * are useful to near-field optical recording, but further higher-order modes such as R-TEM(31) (* ), R-TEM(41) (* ), and R-TEM(51) (* ) are not useful due to the strong side-lobe intensity. Compared with R-TEM(01) (* ) beam focusing, the full width at half-maximum of the recording spot is decreased markedly and the focal depth is increased substantially by using R-TEM(11) (* )beam focusing. The effect of the beam width of the R-TEM(11) (* ) mode is also discussed.

Progress of super-resolution near-field structure in near-field optical storage technology

Super-resolution near-field structure (Super-RENS) is one of the most promising near-field optical recording schemes with significant application prospects. The development of Super-RENS from the basic type to the third-generation is introduced. The development of mask material and the application of Super-RENS in different recording systems are summarized.

Improvement of Cover-Layer Surface Properties for Near-Field Optical Recording

Scratches in a soft polymer cover-layer on near-field recording (NFR) media are observed after a servo test with a rotating disc owing to collision between a solid immersion lens (SIL) and a NFR disk. A dielectric protective film is coated on the cover-layer to enhance surface properties, which reduces the scratch problem of the cover-layer. The refractive index of the dielectric film can be tailored by controlling process parameters during the sputtering and matches that of the polymer cover-layer to reduce light loss at an interface between the dielectric film and the polymer cover-layer.

Head–disk interface problems in first-surface near-field optical recording with flying optical head

Abstract Near-field recording (NFR) technology using a flying optical head is applied for high density optical data storage. Diamond-like carbon (DLC) film and PFPE lubricant film are coated on utmost NFR disk to reduce the tribological problems. A contamination at the head–disk interface (HDI), especially on a solid immersion lens (SIL) is observed. The contaminants are composed of dust materials from the surroundings and lubricant materials from a disk surface. The contaminants are supposed to be condensed by heat from a writing laser beam at the HDI. To avoid the contamination problem, a cover-layer is coated and a focal plane moved under the cover-layer from the HDI. First-surface near-field recording configuration can be modified by laminating a cover-layer, which enhances the possibility of a practical application with a media removability function.

Effect of Elliptical Gaussian Beam in Solid-Immersion-Lens-Based Near-Field Recording Optics

In this study, we summarized the characteristics of beam spot profiles, exit pupil images, the gap error signal and the push–pull signal obtained when using a Gaussian beam in solid immersion lens (SIL) based near-field recording (NFR) optics. As the rim intensity of the Gaussian beam decreases and the tracking direction shift of the SIL assembly increases, the intensity of the beam spot is decreases. The exit pupil image is distorted asymmetrically by the tracking direction shift of the SIL assembly. We confirmed that the tracking direction shift of the SIL assembly affects the gap error signal and push–pull signal. When using linearly polarized incident light, it is observed that a tracking direction shift of 500 µm induces a gap error signal reduction of 3.2%. The push–pull signal is offset by 18.7% in the case of a tracking direction shift of when the tracking direction is shifted by 20 µm.

Performance Investigation of Soft-Decodable Runlength-Limited Codes With Different Minimum Runlength Constraints in High-Density Optical Recording

This paper investigates the performance of runlength-limited (RLL) codes with different d constraints in high-density optical recording, for both the low-density parity-check (LDPC)-coded Bliss scheme and the standard concatenation scheme. For the d = 5 constraint, we propose a low-complexity compression/decompression scenario that maps each 6-bit channel to a 3-bit compressed codeword by Gray-alike coding of the position of the transition in the 6-bit codeword, to compensate the density loss introduced by the redundancy of LDPC coding in the LDPC-coded Bliss scheme. Using a scalar diffraction channel model, we ran simulations to compare the performance of RLL codes with four different d constraints: d = 0,1,2, and 5, in near-field optical recording systems. The simulation results address the average bit error number in erroneous symbols after soft-input soft-output channel detection. Further, we evaluated the net density with different sets of simulation parameters, for both the LDPC-coded Bliss scheme and the standard concatenation scheme, considering jitter noise and additive white Gaussian noise, respectively.