Kusato Hirota

Local structure of amorphous GeTe and PdGeSbTe alloy for phase change optical recording

The local structures of amorphous Ge0.52Te0.48 and Pd0.01Ge0.17Sb0.26Te0.56 were examined by extended x-ray absorption fine structure. In amorphous GeTe film, only Ge atoms were observed in the nearest-neighbor of Te atoms. The average coordination number around Te atoms in amorphous GeTe is 1.5 which is close to twofold coordination. These results support a chemically ordered structure model. In amorphous PdGeSbTe film, Ge and Sb (and/or Te) atoms were observed in the nearest-neighbor of Te atoms at distances of 2.61 and 2.84 A, respectively. Debye–Waller factors of the Te–Ge bonds in TeGe and PdGeSbTe are 0.076 and 0.081 A, respectively, and are larger than that of Te-(Sb and/or Te); 0.063 A. From these results, we argue that the softened Te–Ge bond plays an important role in the elementary process of crystallization of GeTe and PdGeSbTe alloys. The total average coordination number of Pd atoms in amorphous PdGeSbTe alloy is 4.0. This result suggests Pd atoms play the role of cross-linking element.

The Nature of the Coupling Field in Optical Data Storage Using Solid Immersion Lenses

The field incident onto the interface between the solid immersion lens (SIL) and recording medium is decomposed into two parts that exhibit different behavior as the beam propagates through the recording layers. The homogeneous part is shown to diffract, and the inhomogeneous part decays exponentially away from the bottom of the SIL. Reflection and signal contrast are calculated for a phase-change recording medium. The dependences of the signal contrast on the gap width, the refractive index of gap and the SIL are analyzed.

Near-Field Phase Change Optical Recording Using a GaP Hemispherical Lens

A GaP solid immersion lens and a modified liquid immersion lens (MLIL) are studied. The phase change marks exhibit a diameter of approximately 200 nm when recorded by the MLIL.

Design of a Near-Field Probe for Optical Recording Using a 3-Dimensional Finite Difference Time Domain Method.

Tapered dielectric near-field probes are designed for optical recording by means of a 3-Dimensional finite difference time domain (FDTD) method. Probe fabrication is attempted with a double-exposure holographic technique.

Pupil-plane filtering for improved signal detection in an optical data-storage system incorporating a solid immersion lens.

A pupil-plane filtering technique is applied to data-signal detection in an optical data-storage system that uses a solid immersion lens (SIL) and a four-layered phase-change recording medium. We have confirmed by numerical calculations and experiment that the technique improves signal contrast and makes the contrast less sensitive to the gap width between the bottom surface of the SIL and the top surface of the recording medium. Light that is incident upon the objective lens that is used with the SIL is linearly polarized, and the full vectorial feature of the light is considered in the calculations.

PdGeSbTe Alloy for Phase Change Optical Recording

Phase change optical recording disks using a Pd–Ge–Sb–Te quaternary alloy demonstrated high crystallization speed and long-term thermal stability of the amorphous recording marks. This alloy film can be crystallized by a short duration laser pulse of less than 100 ns. It is applicable to a single beam overwrite optical recording system. The crystallized portion of this recording layer on the disk is assigned to single phase and polycrystalline face-centered-cubic (fcc) crystals by transmission electron diffraction. A small amount of Pd atoms (typically 0.2 to 3 at.%) in this alloy improve the thermal stability of amorphous marks.

Pupil Plane Filtering for Optical Pickup Heads with Effective Numerical Aperture of 1.1 and 2.0

A pupil plane filtering technique is applied to data signal detection in an optical data storage system that uses a solid immersion lens (SIL) and a quadrilayer phase change recording medium. Two systems that have the effective numerical aperture (NAEFF) of 1.1 and 2.0 are investigated. A new filter is designed for the NAEFF=2.0 system on the basis of this study. We have confirmed by numerical calculations that the technique improves signal contrast, which is defined by the ratio of the difference signal to the sum signal between the homogeneous crystalline area and the homogeneous amorphous area. We have also confirmed that the technique makes the contrast less sensitive to the gap width between the SIL and the recording medium. The vectorial feature of the light is considered in the calculations. The calculated irradiance distributions and signal levels for the system with NAEFF =1.1 are confirmed by experiments.

Nearfield phase change optical recording using a GaP hemispherical lens

The data density ofthe optical recording medium depends on the focused beam spot size, which is limited by diffraction. The beam spot size can be reduced by using a shorter wavelength light source or a larger NA objective lens. Recently, near-field optical techniques using evanescent light, have been developed to overcome the diffraction limit of far-field optics.

Reliability of the Phase Change Optical Disk

Thermal stability of the Al-alloy reflective layer is essential for the archival life of a rewritable phase change optical disk. The new Al–Hf–Pd alloy reflective layer provides excellent thermal stability. A disk using this Al-alloy and a Pd–Ge–Sb–Te alloy recording layer is extremely stable. The bit-error-rate (BER) of the recorded signal did not change substantially after an acceleration test of 6,800 h under the conditions of 90° C, 80%RH. X-ray diffraction analysis showed that the new additive components (Hf, Pd) of the Al-alloy prevented the growth of the Al crystals.

High-density phase-change optical recording using a solid immersion lens

A near-field optical recording technique using a solid immersion lens (SIL) and a red semiconductor laser light source ((lambda) equals 650 nm) has been applied to the phase change optical recording. We have recorded and erased amorphous marks of 300 nm in diameter using a hemisphere SIL (n equals 1.84, designed effective NA equals 1.1).