Kazutaka G. Nakamura

Generation of picosecond hard x rays by tera watt laser focusing on a copper target

Hard x-ray generation from tera-watt laser irradiation (4 TW, 42 fs at 780 nm) on a solid copper target has been studied in a range of the laser-power density between 3×1016 and 2×1017 W/cm2. Short-pulsed x rays are generated within 6.4 ps and include Cu Kα and Kβ line emissions and a continuum between 3 and 6 keV. The maximum intensity of Cu Kα radiation has been estimated to be 6.5×1010 photons (4π sr pulse)−1. The mechanism of hard x-ray generation has been discussed.

Dependence on laser intensity and pulse duration in proton acceleration by irradiation of ultrashort laser pulses on a Cu foil target

The dependence on laser intensity and pulse duration in energetic proton acceleration by irradiation of ultrashort laser pulses on a 5μm thick copper tape target was measured. The laser intensity was varied from 8.5×1017W∕cm2 to 1.1×1019W∕cm2, and the pulse duration from 55 fs to 400 fs. The maximum proton energy increased as the pulse duration was increased while the laser intensity was kept constant. The dependence of the maximum proton energy on laser intensity and pulse duration was in good agreement with an analytical plasma-expanding model.

The rate constants for the H+H2 reaction and its isotopic analogs at low temperatures: Wigner threshold law behavior

Variational transition‐state theory rate constants for the H+H2 reaction and its isotopic analogs at very low temperatures have been calculated using the LSTH potential surface. The Wigner threshold law is found to hold quantitatively below 1 K and to within 34% on average at 5 K. The transition‐state theory rate constants are found to be consistent with the requirements of the Wigner threshold law provided that a tunneling probability having the correct threshold behavior is used in calculating the transmission coefficient. Using these calculated rate constants, decay rates of H and D atoms in solid H2, D2, and HD were estimated and compared with experiment. The good agreement found suggests that these decays occur through quantum mechanical tunneling exchange reactions.

Real‐time Raman measurements of graphite under Ar+ irradiation

The time dependence of the Raman spectra of graphite is measured under 3‐keV Ar+‐ion irradiation by developing a real‐time Raman measurement apparatus. An ion flux of 3×1011 ions/cm2 s is used and the time resolution of the Raman measurement is about 6 s. The lattice damage of graphite is evaluated by the relative intensity of the disorder‐induced peak (∼1360 cm−1) with respect to the Raman active E2g2 mode peak (∼1580 cm−1). An initial steep rise and a successive slow increase to saturation of the relative intensity ratio are observed.

Generation of giant carbon hollow spheres from C60 fullerene by shock-compression

Abstract We report on the generation of carbon hollow spheres up to several micrometers in diameter on an internal hollow surface of the recovered sample from C 60 fullerene powder after shock-compression up to 57 GPa. Scanning electron microscopy demonstrates see-through spheres, a collapsed one like a Ping-Pong ball and bowl type objects, suggesting that the spheres are empty. Raman spectroscopy reveals that the carbon hollow spheres are in highly graphitized state of which crystalline size is several hundred of nanometers. The possible formation mechanism is proposed.

Thin tape target driver for laser ion accelerator

A thin tape target driver for laser ion acceleration was developed. The driver can move a copper tape of 5 μm thickness with a positioning reproducibility of less than 30 μm (peak to valley), which is sufficient for a laser irradiation target. Using this tape target and laser pulses of energy 350 mJ and duration 60 fs, protons of energies of over 1 MeV were accelerated in the forward direction.

Evolving shock-wave profiles measured in a silicon crystal by picosecond time-resolved x-ray diffraction

Picosecond time-resolved x-ray diffraction is used to probe single-crystal silicon under pulsed-laser irradiation (300 ps pulse at 1.4 J/cm2) at an interval of 60 ps. The observed rocking curves show shock compression of the silicon lattice by the laser irradiation. Uniaxial strain profiles perpendicular to the Si(111) plane are estimated using dynamical x-ray diffraction theory. The temporal and spatial evolution of the profiles indicates a propagating shock wave with the velocity of 9.4 km/s inside the silicon crystal. The observed maximum compression is 1.05%, which corresponds to a pressure of 2.18 GPa.

Glass cutting by femtosecond pulsed irradiation

We report on quartz and glass cutting by a lateral scanning of femtosecond pulses (150 fs at 1 kHz repetition rate) of 800 nm wavelength at room and low pressure (5 Torr) air ambience. Pulses were focused by a low numerical aperture (NA0.1) objective lens. Optimization of fabrication conditions: pulse energy and scanning speed were carried out to achieve large-scale (millimeter-to-centimeter) cutting free of microcracks of submicron dimensions along the edges and walls of the cut. Cutting through out the samples of 0.1-0.5 mm thickness was successfully achieved without apparent heat affected zone. At low air pressure (5 Torr) ambience, redeposition of ablated material was considerably reduced. It is demonstrated that the damage on the rear surface was induced by the stress waves, which originated from the plasma ablation pressure pulse. The mechanism of femtosecond-laser cutting of transparent materials at high irradiance and the influence of stress waves generated by plasma plume are discussed.

Purification of single-wall carbon nanotubes by using ultrafine gold particles

Abstract Ultrafine gold particles with a diameter of 20 nm were dispersed in carbonaceous soot containing single-wall carbon nanotubes (SWNTs). These gold particles catalyzed the oxidation of carbonaceous impurities at about 350°C. This selective oxidation enabled us to purify SWNTs.

Hugoniot measurement of diamond under laser shock compression up to 2 Tpa

Hugoniot data of diamond was obtained using laser-driven shock waves in the terapascal range of 0.5–2TPa. Strong shock waves were generated by direct irradiation of a 2.5ns laser pulse on an Al driver plate. The shock wave velocities in diamond and Al were determined from optical measurements. Particle velocities and pressures were obtained using an impedance matching method and known Al Hugoniot. The obtained Hugoniot data of diamond does not show a marked difference from the extrapolations of the Pavlovskii Hugoniot data in the TPa range within experimental errors.