Tsutomu Ishi

Si nano-photodiode with a surface plasmon antenna

We developed a Si nano-photodiode with a surface plasmon (SP) antenna for a SiON waveguide-integrated structure. We showed that interfacial periodic nano-scale metal-semiconductor-metal Schottky electrodes function as an SP optical antenna and also as an optical coupler between a SiON waveguide and a very thin Si-absorption layer.

On-Chip Optical Interconnect

We describe a cost-effective and low-power-consumption approach for on-chip optical interconnection. This approach includes an investigation into architectures, devices, and materials. We have proposed and fabricated a bonded structure of an Si-based optical layer on a large-scale integration (LSI) chip. The fabricated optical layer contains Si nanophotodiodes for optical detectors, which are coupled with SiON waveguides using surface-plasmon antennas. Optical signals were introduced to the optical layer and distributed to the Si nanophotodiodes. The output signals from the photodiodes were sent electrically to the transimpedance-amplifier circuitries in the LSI. The signals from the photodiodes triggered of the circuitries at 5 GHz. Since electrooptical modulators consume the most power in on-chip optical interconnect systems and require a large footprint, they are critical to establish on-chip optical interconnection. Two approaches are investigated: 1) an architecture using a fewer number of modulators and 2) high electrooptical coefficient materials.

Proposal for real-time terahertz imaging system with palm-size terahertz camera and compact quantum cascade laser

This paper describes a real-time terahertz (THz) imaging system, using the combination of a palm-size THz camera with a compact quantum cascade laser (QCL). The THz camera contains a 320x240 microbolometer focal plane array which has nearly flat spectral response over a frequency range of ca. 1.5 to 100 THz, and operates at 30 Hz frame rate. The QCL is installed in compact cryogen-free cooler. A variety of QCLs are prepared which can cover frequency range from ca. 1.5 to 5 THz. THz images of biochemical samples will be presented, using the combined imaging system. Performance of the imaging system, such as signal-to-noise ratio of transmission-type THz microscope, is predicted.

Large optical transmission through a single subwavelength hole associated with a sharp-apex grating

The effect of grating shapes on optical transmission in a bulls eye structure (a single subwavelength hole surrounded by a concentric grating in a metal) is discussed. Finite-difference time-domain calculations predict that a sharp-apex shape gives as high a reflective structure to the propagating surface plasmon polaritons (SPPs) as does a rectangular shape. Fabricated samples with a sharp-apex grating actually show large optical transmission (a factor of 400 greater than that of samples with a single hole) even when the number of corrugations is three. This result indicates that a sharp-apex grating acts as an effective SPP reflector to confine the energy around the hole, resulting in high optical throughput.

Palm-size and real-time terahertz imager, and its application to development of terahertz sources

This paper describes features of uncooled palm-size and real-time Terahertz (THz) imager. The THz imager and powerful THz quantum cascade laser were assembled into THz microscope with which THz images of narrow string were obtained at 4.3 and 2.0 THz. The analyses on these images show that spatial resolutions evaluated at two frequencies are consistent with Fraunhofer diffraction limit. THz imager has been applied to investigate beam patterns for a variety of THz sources. The experimental results on beam patterns show that THz imager plays an important role in developing THz sources. A method for reducing non-uniformity due to strong coherency of THz sources is finally presented.

Low resistance magnetic tunnel junctions and their interface structures

Effects of interface structure and oxidation state were studied in stacked magnetic tunnel junction (MTJ) structures with top and bottom antiferromagnetic layers to obtain optimum resistance and high tunneling magnetoresistance (TMR) ratios for read heads. The roughness of the NiFe surface and the Al coverage were significantly improved by introduction of O2 surfactant gas on the Ta-seed-layer surface, which increased TMR ratios of the MTJ with low resistance area (RA) products of less than 10 Ω μm2. Furthermore, it was found that avoidance of Ni oxidation and Co oxidation at the tunnel barrier interface is essential to obtaining high TMR ratios, and that a good Al coverage and Fe–oxide formation may enhance TMR ratios when Fe-rich magnetic materials are used. For the top-type and bottom-type structures, a TMR ratio of 12%–17% with RA products of 6–7 Ω μm2 was obtained, which provides sufficient performance for read heads.

Co-Ni-Fe write heads with a 10-/spl mu/m yoke length for high-speed recording

We have developed a Co-Ni-Fe write head with a short yoke length for high-speed recording. By reducing the yoke length to 9.5 /spl mu/m, the eddy currents induced in a yoke with a relatively low resistivity (0.2 /spl mu//spl Omega/m) were reduced. The head of this short yoke had good write performance for a medium with a coercivity of 400 kA/m (5000 Oe) at frequencies up to 250 MHz (the overwrite less than -30 dB, and nonlinear transition shift less than 7%).

Optical interconnect technologies for high-speed VLSI chips using silicon nano-photonics

Optoelectronic and electrooptic elements are integrated on VLSI chips. The junction capacitance of a nano-photodiode is extremely low (<10aF), which permits a high load resistance to be used, resulting in higher output voltage at high frequencies. A ceramic Pb(,ZrTi)O3 film with average crystallite diameter below 20nm has a high electro-optical coefficient (>150pm/V) suitable for on-chip modulators. This paper introduces a new approach for realizing high-speed optical interconnects on silicon chips. This concept uses nano-photodiodes on silicon with extremely low parasitic capacitance (less than 10aF) enabling robust communication at very high frequencies. The results demonstrate 5GHz clocking with the promise of up to 20GHz. The authors will also discuss how the silicon nano-photodiode can be used for wavelength-division multiplexing and low-voltage electro-optic modulators for on-chip and off-chip optical communications

Highly defined narrow track write heads fabricated by focused ion beam trimming with the Al/sub 2/O/sub 3/ refilling process

We have developed a focused ion beam trimmed write head using Al/sub 2/O/sub 3/ refilling, in which an Al/sub 2/O/sub 3/ film was deposited and a two-step lapping process was performed after FIB trimming at the air bearing surface. This process provided highly defined pole edges without any rounded corners, well-filled hollows, and a damage-free read element. Good environment-proof characteristics were also obtained, because a diamond-like carbon film was deposited as a protective layer on the element after FIB trimming. The fabricated head showed good overwrite characteristics in half-micron width writing.

Anisotropy control in fabrication process for NiMn spin-valve dual element heads

NiMn antiferromagnets with a high blocking temperature are attractive for application to the pinning layer in spin-valve heads, because a highly stable exchange coupling field is expected through their use. We have fabricated NiMn spin-valve heads having a CoZrTa bottom shield and a CoZrTa/NiFe write element, using an anisotropy control process which provides a desirable magnetization configuration in each magnetic layer. These heads showed highly stable, symmetrical, and Barkhausen noise free readback waveforms with output voltage of over 800 /spl mu/V/sub pp///spl mu/m.