Yuji Kuroda

Large Capacity and High-Data-Rate Phase-Change Disks

Large-capacity phase-change disks permitting higher data-transfer rates have been developed for 0.85-numerical-aperture (NA) recording systems. Three methods to improve the recording data rate were examined. First, the recording layer was sandwiched by ceramic layers for the purpose of promoting nucleation. Second, signal properties were evaluated with a disk that had a so-called absorptivity-controlled structure. Third, these two methods were combined so that they functioned together in one disk. The disk structure was optimized separately for 640 nm and 407 nm wavelengths. With the disk designed for 640 nm wavelength, a user capacity of 9.2 GB and a user data-transfer rate of 50 Mbps were obtained. These quantities were 22 GB and 35 Mbps with the disk designed for 407 nm wavelength. (The disk diameter was 120 mm and the format efficiency was 80%.)

Near-Field Phase-Change Optical Recording of 1.36 Numerical Aperture.

A bit density of 125 nm was demonstrated through near-field phase-change (PC) optical recording at the wavelength of 657 nm by using a supersphere solid immersion lens (SIL). The lens unit consists of a standard objective and a 2.5 mm SIL. Since this lens size still prevents the unit from being mounted on an air-bearing slider, we developed a one-axis positioning actuator and an active capacitance servo for precise gap control to thoroughly investigate near-field recording. An electrode was fabricated on the bottom of the SIL, and a capacitor was formed facing a disk material. This setup realized a stable air gap below 50 nm, and a new method of simulating modulation transfer function (MTF) optimized the PC disk structure at this gap height. Obtained jitter of 8.8% and a clear eye-pattern prove that our system successfully attained the designed numerical-aperture (NA) of 1.36.

Near-Field Optical Recording on a Pre-Grooved Phase-Change Disk in the Blue-Violet

Near-field optical recording was implemented on a pre-grooved phase-change disk by using a 1 mm superhemispherical solid immersion lens (SIL) of 1.5 numerical aperture. The SIL unit was mounted on a two-axis actuator for enabling both gap-height and tracking controls. The gap height was precisely adjusted with a capacitance formed between the SIL and disk materials. Groove structures were fabricated on a glass substrate by a reactive-ion-etching (RIE) process and flattened with spin-on-glass (SOG) coating for obtaining reliable push-pull tracking error signals. The phase-change layer stack was optimized by a vector diffraction analysis in order to maximize evanescent coupling at the gap height as small as 50 nm. A bit density of 80 nm was demonstrated on the land of a pre-grooved disk at the wavelength of 407 nm.

Near-field phase-change recording using a GaN laser diode

We demonstrated near-field recording with the combination of a GaN laser diode and a 1.5 NA objective lens. The realized linear bit density was less than 90 nm (corresponding to the areal recording density over 40 Gbit/in/sup 2/) regardless of using a small 1.0 mm-diameter super-hemispherical solid immersion lens (SIL). Further improvement of a near-field phase-change optical disk will enable a linear bit density of 80 nm or less. In this talk we shall also report the fabrication process of the plateau, and of the electrode on the SIL.

Fourier modal method applied to the electric field calculation for groove-only recording media

We describe a FMM (Fourier Modal Method) based rigorous vector diffraction method that can be applied to trapezoidal groove structures. Applying this method to the calculation of the electric field on the surface of the recording layer for DVR groove-only recording media, we found a significant difference of the electric field distributions between two recording schemes, one when the light spot is focused on-groove, the other when it is focused in-groove. These calculation results clearly explained the measured differences of the recording power sensitivities and the cross-writing characteristics between the two schemes.

Challenge of near-field recording beyond 50.4 Gbit/in2

The possibility of an areal density over 50 Gbit/in2 was examined in near-field phase-change recording. The disk structure was optimized to maximize readout signals under the land-and-groove recording condition at a tracking pitch of 160 nm. We also evaluated the signal crosstalk from adjacent tracks. Eye diagrams of 50.4 Gbit/in2 areal density were demonstrated using 1.5 NA optics and a GaN laser diode. The track pitch and linear bit density are 160 nm and 80 nm/bit, respectively. The transmission electron microscope (TEM) micrograph of recorded amorphous marks at an areal density of 50.4 Gbit/in2 is also presented.

Near-field phase-change optical recording over 1.2-numerical-aperture

Recently, solid immersion lenses (SILs) have yielded a great interest in near-field optical data storage. However, convincing data on areal recording density have yet to be reported. The difficulty is attributed to the fact that an air-bearing slider needs to be kept optically contacted and also that a tiny SIL is vulnerable to aberrations caused by an air gap between a lens and a disk. The air-bearing system cannot easily achieve the condition above because its linear velocity changes the air gap. We have developed a new SIL device mounted on an actuator, which allows the air gap to be varied arbitrarily and enables optical contact independent of disk rotation. The obtained eyepattern of (l,7)-coded data on a phase-change disk demonstrates near-field recording over 1.2- numerical-aperture (NA).