Takehito Shimatsu

Lasing and thermal characteristics of Yb:YAG/YAG composite with atomic diffusion bonding

We demonstrated the laser performance of an Yb:YAG/YAG composite ceramic laser medium mounted on an aluminium heatsink via atomic diffusion bonding (ADB) technique using nanocrystalline metal films at room temperature in air. The surface temperature rise of the ADB bonded laser medium was linear with 57 °C lower than that of the commercially available soldered Yb:YAG thin disk at the pump power of 280 W. Moreover, the ADB disk was pumped 1.5 times higher (7.3 kW cm−2) than the typical damage threshold of the soldered disk without any sign of damage. The undoped capping may be effective for the suppression of ASE heating; however, according to the in situ OPD measurement it induces strong thermal lensing. The CW laser output power of 177 W was obtained at the pump power of 450 W with the optical-to-optical efficiency of 40% using V-shape cavity.

Influence of intergrain interactions and thermal agitation on microwave-assisted magnetization switching behavior of granular magnetic film

Microwave-assisted magnetization switching (MAS) in a granular magnetic film is examined by computer simulation. Contrary to the macrospin calculation and the experiments on single magnetic dots reported so far, in which the switching field linearly decreases with increasing rf frequency and then sharply increases at the critical frequency, the granular film exhibits considerably broad MAS behavior against the rf frequency. This broad MAS behavior is mainly caused by the dispersion of magnetic properties and thermal agitation. On the other hand, intergrain dipolar and exchange interactions enhance the MAS effect in the granular film and suppress the MAS broadening.

Interface structure of InGaAs wafers bonded using thin amorphous Ge films in vacuum

This study examines room-temperature bonding of InGaAs wafers in vacuum using thin amorphous Ge (a-Ge) films. InGaAs wafers bonded using 0.5–1-nm-thick a-Ge film on respective sides show great bonding strength comparable to the fracture strength of InGaAs after annealing at 340 °C. High-resolution images obtained using transmission electron microscopy show no vacancy at the bonded a-Ge/a-Ge interface and no clear damage of the deposited a-Ge films on the crystal lattices of the InGaAs surfaces. Analysis using electron energy-loss spectroscopy reveals a slight inter-diffusion of Ge and In at the a-Ge/InGaAs interface.

Layer-selective microwave-assisted magnetization switching in a dot of double antiferromagnetically coupled (AFC) layers

Layer-selective magnetization switching is a key technology for three-dimensional magnetic recording. In this study, layer-selective magnetization switching is demonstrated in a dot of double antiferromagnetically coupled (AFC) layers under an assistance of an rf field. Each AFC layer consists of two Co/Pt multilayers with a Ru interlayer. By optimally tuning the rf frequency, the layer-selective switching of each AFC layer is clearly confirmed. However, this layer-selective switching is probabilistic, with a maximum probability of about 65%. The stability of the layer-selective switching is improved if the dipolar field from the first switched AFC layer is suppressed.Layer-selective magnetization switching is a key technology for three-dimensional magnetic recording. In this study, layer-selective magnetization switching is demonstrated in a dot of double antiferromagnetically coupled (AFC) layers under an assistance of an rf field. Each AFC layer consists of two Co/Pt multilayers with a Ru interlayer. By optimally tuning the rf frequency, the layer-selective switching of each AFC layer is clearly confirmed. However, this layer-selective switching is probabilistic, with a maximum probability of about 65%. The stability of the layer-selective switching is improved if the dipolar field from the first switched AFC layer is suppressed.

Novel gratings for next-generation instruments of astronomical observations

We will introduce current status of development of a birefringence volume phase holographic (B-VPH) grating, volume binary (VB) grating and reflector facet transmission (RFT) grating developing as the novel dispersive optical element for astronomical instruments for the 8.2m Subaru Telescope, for next generation 30 m class huge ground-based telescopes and for next generation large space-bone telescopes. We will also introduce a hybrid grism developed for MOIRCS (Multi-Object InfraRed Camera and Spectrograph) of the Subaru Telescope and a quasi-Bragg (QB) immersion grating. Test fabrication of B-VPH gratings with a liquid crystal (LC) of UV curable and normal LCs or a resin of visible light curable are performed. We successfully fabricated VB gratings of silicon as a mold with ridges of a high aspect ratio by means of the cycle etching process, oxidation and removal of silicon oxide. The RFT grating which is a surface-relief (SR) transmission grating with sawtooth shaped ridges of an acute vertex angle. The hybrid grism, as a prototype of the RFT grating, combines a high-index prism and SR transmission grating with sawtooth shape ridges of an acute vertex angle. The mold of the SR grating for the hybrid grism on to a work of Ni-P alloy of non-electrolysic plating successfully fabricated by using our ultra-precision machine and a single-crystal diamond bite. The QB immersion grating was fabricated by a combination of an inclined QB grating, Littrow prism and surface reflection mirror.

Room temperature bonding of InGaAs wafers using thin Ge films

Room temperature bonding of InGaAs wafers using thin Ge films was studied. Wafers were bonded even with 0.5 nm thick Ge film on each side. Bonded wafers showed strong bonding force after annealing at 340 oC, with no vacancy at the bonded interface in TEM images.

Shear strength of room-temperature-bonded sapphire and metal substrates using Au films

For sapphire and various metals (Al alloys, Cu alloys, and stainless steel) bonded at room temperature using Au films, shear strength was assessed in this study. Results show extremely high shear strength of all samples: greater than 10 kN (50 MPa).

Enhancement of light transmittance for wafers bonded with thin Al films using atomic diffusion bonding and subsequent laser irradiation

Enhancement of light transmittance was examined for sapphire or synthetic quartz glass wafers bonded with thin Al films using atomic diffusion bonding (ADB). The results indicate that the thin Al film at the bonded interface diffuses into the wafer by subsequent laser irradiation, which enhances the light transmittance through the bonded interface.