Hiroaki Kishimura

Enhanced generation of fast protons from a polymer-coated metal foil by a femtosecond intense laser field

The results of generation of fast protons from 5-μm-thick copper foil targets by 60fs laser irradiation at 1.5×1017W∕cm2 are presented. Both polyvinylmethylether (PVME)-coated and uncoated copper foil targets are examined. Fast protons are measured using a Thomson mass spectrometer and maximum proton energies are 570 and 280keV for the PVME-coated and the uncoated target, respectively. The intensity of fast protons with energy of 160keV from the PVME-coated target is approximately 80-fold higher than that from the uncoated target.

Picosecond structural dynamics in photoexcited Si probed by time-resolved x-ray diffraction

Direct observation of structural dynamics of a 300 ps laser irradiated silicon crystal is performed by means of picosecond time-resolved x-ray diffraction. Change in x-ray diffraction profiles corresponds to propagation of a strain pulse inside the sample with sound velocity. The strain profiles are simulated by considering carrier dynamics and thermoelastic treatment and well explain the experiments.

Effect of phase transition in shock-recovered silicon

A series of shock-recovery experiments on a single crystal of silicon up to 38 GPa and characterizations of the recovered samples by x-ray diffraction analysis, Raman spectroscopy, and microscopic observations were performed for a better understanding of residual effects after shock loading by using a propellant gun. The x-ray diffraction trace of each sample revealed the absence of additional constituents including metastable phases and high-pressure phases of silicon except for 11 and 38 GPa. At 11 GPa, small amounts of metastable phases of silicon were obtained. The formation of copper silicide (Cu3Si) was confirmed in the sample shocked at 38 GPa. Considering the surface morphology revealed by microscopic observation, a thermochemical reaction through the melting of silicon resulted in the formation of Cu3Si. An additional band and the center frequency deviation of a peak were shown in the Raman spectroscopy results. The results of x-ray diffraction and Raman spectroscopy indicated that crystalline si...

DC electrical conductivity study of amorphous carbon nitride films prepared by reactive RF magnetron sputtering

The effects of chemical bonding states on the electrical properties of hydrogen-free amorphous carbon nitride (a-CNx) films were reported. a-CNx films were prepared by reactive RF magnetron sputtering at various deposition temperatures. The electrical conductivity of the a-CNx films increased with increasing deposition temperature because of the predominant sp2C–C bonding sites. Their conductivity increased by almost one order of magnitude with a 25% decrease in the fraction of the N-sp3C bonding state. It was found that the fraction of the N-sp2C bonding state strongly contributed to the increase in the electrical conductivity. Nitrogen incorporation led to an increase in the sp3C–C bonding fraction in the films; as a result, the conductivity of the a-CNx films was found to be lower than that of the a-C films deposited under the same conditions.

Relativistic laser plasma from micron-sized argon clusters as a debris-free x-ray source for pulse x-ray diffraction

We have demonstrated diffraction from Si(111) crystal using x rays from highly ionized Ar ions produced by laser irradiation with an intensity of 6×1018W∕cm2 and a pulse duration of 30 fs acting upon micron-sized Ar clusters. The measured total photon flux and linewidth in the Heα1 line (3.14 keV) were 4×107photons∕shot∕4πsr and 3.7 eV (full width at half maximum), respectively, which is sufficient to utilize as a debris-free light source for time-resolved x-ray diffraction studies.

Influence of Chemical Bonding States on Electrical Properties of Amorphous Carbon Nitride Films

The electrical properties of amorphous carbon nitride (a-CNx) films have been investigated in terms of the nitrogen concentration (N/C) and chemical bonding states in the films. The films were deposited by the reactive rf magnetron sputtering method. Nitrogen concentration and chemical bonding states in the films were controlled by regulating the deposition temperature. C–C networks in the films changed to those having a graphite like structure with decreasing N/C, as deduced by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). In addition, the N–sp2C bonding state becomes more predominant. These results indicate the contributions of the N–sp2C component to the decrease in electrical resistivity and increase in photoconductivity.

LASER‐DRIVEN MINIFLYER SYSTEM FOR SHOCK COMPRESSION STUDIES

A laser‐driven miniflyer system has been built for shock compression experiments on inert and reactive materials. It consists of an Nd:YAG 3 J driving laser, beam shaping optics, an impact assembly, and velocity measurement diagnostics. In order to characterize achievable velocities and efficiencies of the miniflyer system, copper foils (flyers) of 25 μm, 50 μm or 100 μm thickness and 3.2 mm or 2.4 mm diameter were mounted onto composite coated glass substrates using thin‐film epoxy. VISAR and Photonic Doppler Velocimetry (PDV) were used to investigate the flyer velocity and acceleration as a function of the driving laser energy for varying thicknesses and diameters of flyer. Data from this study is used for calibration of shock compression experiments and comparisons between the diagnostic techniques. A novel approach to obtaining Hugoniot measurements using dual‐velocimetry diagnostics was also investigated and described.

Dehydration of potassium alum induced by shock loading

Potassium alum (KAl(SO4)212H2O) powder filled into a copper container were shock loaded up to 6.3 GPa by flyer plate impact. Recovered samples were characterized by X-ray diffraction (XRD) analysis, Raman spectroscopy, and scanning electron microscopy (SEM). XRD and Raman results of samples shocked at 4.4 GPa and below indicated that there was no sign of phase transition. In contrast, the XRD pattern of the sample shocked at 6.3 GPa was clearly different from the initial sample. Unlike previous results obtained from hydrostatic pressure experiments, an irreversible phase transition to an amorphous phase occurred under shock compression at 6.3 GPa. The morphology of the sample surface indicated the ejection of water vapor caused by shock loading. The amorphization may be attributed to the vaporization of water molecules caused by shock pressure and shock-induced heat.

Effect of shock compression on single crystalline silicon

A series of shock-recovery experiments were performed on single crystals of silicon and germanium using a propellant gun and the laser-driven miniflyer method. Characterizations of the recovered samples by X-ray diffraction (XRD) analysis and Raman spectroscopy revealed the absence of additional constituents such as metastable phases and high-pressure phases. The XRD patterns for shocked samples are consistent with a powder XRD pattern corresponding to the cubic-diamond phase. The formation of copper silicide (Cu3Si) was confirmed in the sample shocked at 38 GPa. The formation of an additional band and the deviation of a center frequency peak from the cubic-diamond phase of silicon and germanium were evident in the Raman spectroscopy results. The results of XRD and Raman spectroscopy indicated that crystalline size reduction, rather than the formation of metastable phases, occurred.

Effect of shock compression on optical and structural properties of Eu2O3 and Y2O3:Eu3+ powders

Shock-recovery experiments on Eu2O3 and Y2O3:Eu3+ powders using a metal plate projectile accelerated by a single-stage powder-propellant gun were performed to investigate phase stability and response at high pressures and temperatures. The recovered samples were characterized using powder X-ray diffraction analysis and photoluminescence spectroscopy. The onset of the structural phase transition from the cubic (C-type) to monoclinic (B-type) phase was observed for both Eu2O3 and Y2O3:Eu3+ powders at shock pressures of 8 and 13 GPa, respectively. For Eu2O3, the amount of B-type phase increases with increasing shock pressure up to 23 GPa, whereas for Y2O3:Eu3+, a maximum was reached at 25 GPa followed by a decrease with increasing shock pressure; only the C-type phase was detected in the sample shocked at 51 GPa. The change in the amount of B-type phase indicates stability for the monoclinic phase against shock-induced heat and mechanical deformation. The large range in shock pressure for which the C-type an...