Masahiro Ojima

High Speed Overwritable Magneto-Optic Recording

To achieve high speed overwriting on magneto-optic disks, magnetic field modulation recording using a floating magnetic head has been developed. High reliability for the removable disks can be attributed to the large head-and-disk spacing(2~4µm), and thick coating(5~10µm) on the disk. Characteristics of high speed overwriting were investigated at recording frequencies over 10MHz. No residual pre-recorded signal was observed after overwriting.

Picosecond Time Analysis of Excitonic Molecule Luminescence Spectra in CuCl

Time-resolved luminescence spectra in the picosecond region of excitonic molecules in CuCl are studied at 4.2 K under band-to-band and exciton resonance excitations with time resolutions of ∼10 ps and ∼60 ps, respectively. The formation time and the lifetime of excitonic molecules are determined to be ({lesssim}10) ps and 300 ps, respectively, the latter of which agrees with the radiative lifetime calculated by using existing theories. The relaxation processes of the excitonic molecule system are discussed on the basis of the time variation of the luminescence line shape, and some analyses are made by considering both molecule-molecule collisions and molcule-acoustic phonon interactions. Transient behavior of the stimulated emission due to the excitonic molecule luminescence are also investigated.

Stimulated emission effects in excitonic molecule luminescence of CuCl under resonant two-photon excitation

Abstract Luminescence spectra of excitonic molecules in CuCl resonantly created by nano-second dye laser excitation are measured at 4.2 K changing the lenghts of the excited region. The spectra are dominated by stimulated emission. The gain coefficient for the MT line is saturated more easily than that for ML, though the former is larger under unsaturated conditions. These gain characteristics explain well various stimulated emission effects observed.

High-Density Magneto-Optic Disk Using Highly Controlled Pit-Edge Recording

High-density pit-edge recording on magneto-optic disks has been achieved with accurate edge control. Storage density of 0.7 µm/bit is obtained by suppressing timing jitters caused by (1) teardrop-shaped magnetic domains and (2) thermal interaction between magnetic domains.

Tunable Picosecond Pulse Generation by Optical Parametric Oscillator

An optical parametric oscillator has been constructed in which a temperature-controlled LiNbO3 crystal in a resonant cavity is pumped by the second harmonic of a mode-locked glass:Nd laser. It produces broadly tunable (λ: 0.62–3.5 µm), intense (peak power gtrsim 10 MW), ultrashort pulses (pulse width ~ 60 ps) with a narrow spectral width (Δλ~0.1 nm in the red region) and a small beam divergence (Δθ<1 m rad). The tunable range has been extended up to 0.31 µm by efficient frequecy doubling of the signal in an RDP crystal. The usefulness of this coherent light source in the picosecond optical spectroscopy has been demonstrated.

Pico-second time-resolved luminescence spectra of excitonic molecules in CuCl

Abstract Transient luminescence behavior of excitonic molecules in CuCl is studied at 4.2 K with a time resolution of ∼ 10 ps. The lifetime of the excitonic molecule state is determined to be several hundred pico-seconds. The spectral shape and the time characteristics of the luminescence intensity depend on excitation wavelength and also on the direction of observation. This is explained by the effect of the stimulated emission process.

Optical Studies of Resonantly Excited Excitonic Molecules in CuCl

Various optical spectra such as time-resolved luminescence spectra are studied for excitonic molecules in CuCl under two-photon resonance excitation by using picosecond and nanosecond frequency-tunable laser pulses. Two sharp emission lines observed under resonance excitation are ascribed to the radiative decay of “cold” excitonic molecules created directly at k ∼2 k 0 , while the broad emission band on the low energy side of the sharp line to the luminescence of “hot” molecules created through single excitons. The stimulated emission process gives a considerable influence on both the sharp luminescence lines and the hot molecule band, and further on two-photon resonant excitonic Raman lines as well. Discussions are made on the Bose condensation of the excitonic molecules and also on whether the emission under just-resonance excitation is due to Raman scattering or luminescence.

2 GB/130 mm Capacity Direct-Overwrite Magneto-Optical Disk

Precise mark-edge recording on a high signal-to-noise ratio (S/N) exchange-coupled direct overwrite magneto-optical (MO) disk provides 2 GB capacity on a 130 mm disk with a sufficient edge jitter margin. Write compensation technology was proven to be effective for both non- and direct-overwrite MO disks.

Compact Two-Beam Head with a Hybrid Two-Wavelength Laser Array for Magneto-Optic Recording

A compact two-beam optical head using a hybrid two-wavelength laser array is demonstrated for real time verification in a magneto-optic disk. The head height is 10mm. This head uses an aspherical plastic focusing lens and new compound polarized beam splitter ( PBS ) detection optics for detecting spot position signals and magneto-optic signals with the same optics making the head more compact.

High Linear Density Recording of 0.3 µm Domain Length on Magneto-Optical Disk

High linear density magneto-optical recording of 0.3 µm domain length is studied for both laser power modulation and magnetic field modulation methods, through optical observation and numerical simulation of recorded domains on a rotating 5.25 inch TbFeCo disk. In the laser power modulation method, domain length of around 0.3 µm changes largely with fluctuations in recording power. By decreasing the wavelength of the incident laser beam, the amount of change can be reduced. On the other hand, in the magnetic field modulation method, a 0.3 µm domain length can be recorded stably against power deviation even in the case of an 830 nm wavelength at linear velocities of 5~15 m/sec.