N. Ozaki

Silicon nanowhiskers grown on a hydrogen-terminated silicon {111} surface

Using a hydrogen-terminated Si {111} surface as a substrate, we have grown Si nanowhiskers along the 〈112〉 direction by the vapor–liquid–solid mechanism. The minimum silicon core diameter was 3 nm and the maximum length was about 2 μm. The minimum silicon core diameter is close to the critical value for visible light emission due to the quantum confinement effect. In contrast to an oxidized Si surface, the hydrogen-terminated surface facilitates the formation of small molten Au–Si catalysts at a lower temperature (500 °C) which is slightly above the eutectic temperature. The formation of catalysts and the subsequent growth at the low temperature yield thin Si nanowhiskers on a Si substrate.

Pure-tin microdroplets irradiated with double laser pulses for efficient and minimum-mass extreme-ultraviolet light source production

Laser-driven expansion of pure-tin microdroplets was demonstrated to produce an efficient and low-debris extreme-ultraviolet (EUV) light source. The pre-expansion is indispensable for resolving the considerable mismatch between the optimal laser spot diameter (∼300μm) and the diameter (∼20μm) of microdroplets containing the minimum-mass Sn fuel for generating the required EUV radiant energy (∼10mJ/pulse). Explosive expansion of microdroplets was attained by using a laser prepulse, whose intensity was at least 3×1011W∕cm2. The expanded microdroplet was irradiated with a CO2 laser pulse to generate EUV light. A combination of low density and long-scale length of the expanded microdroplet leads to a higher EUV energy conversion efficiency (4%) than that (2.5%) obtained from planar Sn targets irradiated by a single CO2 laser pulse. This scheme can be used to produce a practical EUV light source system.

Inhibition of fast electron energy deposition due to preplasma filling of cone-attached targets

We present experimental and numerical results on the propagation and energy deposition of laser-generated fast electrons into conical targets. The first part reports on experimental measurements performed in various configurations in order to assess the predicted benefit of conical targets over standard planar ones. For the conditions investigated here, the fast electron-induced heating is found to be much weaker in cone-guided targets irradiated at a laser wavelength of 1.057μm, whereas frequency doubling of the laser pulse permits us to bridge the disparity between conical and planar targets. This result underscores the prejudicial role of the prepulse-generated plasma, whose confinement is enhanced in conical geometry. The second part is mostly devoted to the particle-in-cell modeling of the laser-cone interaction. In qualitative agreement with the experimental data, the calculations show that the presence of a large preplasma leads to a significant decrease in the fast electron density and energy flux...

High-Mach number collisionless shock and photo-ionized non-LTE plasma for laboratory astrophysics with intense lasers

We propose that most of the collisionless shocks in the Universe, for example, supernova remnant shocks, are produced because of the magnetic field generated by Weibel instability and its nonlinear process. In order to verify and validate the computational result confirming this theory, we are carrying out model experiments with intense lasers. We are going to make a collisionless counter-streaming plasma with intense laser ablation based on the scaling law to laser plasma with the particle-in-cell simulation resulting in Weibel-mediated shock formation. Preliminary experimental data are shown. The photo-ionization and resultant non-LTE plasma physics are also very important subjects in astrophysics related to mainly compact objects, for example, black hole, neutron star and white dwarf. Planckian radiation with its temperature 80–100 eV has been produced in gold cavity with irradiation of intense lasers inside the cavity. The sample materials are irradiated by the radiation inside the cavity and absorption and self-emission spectra are observed and analyzed theoretically. It is demonstrated how the effect of non-LTE is essential to reproduce the experimental spectra with the use of a precision computational code.

Shock Hugoniot and temperature data for polystyrene obtained with quartz standard

Equation-of-state data, not only pressure and density but also temperature, for polystyrene (CH) are obtained up to 510 GPa. The region investigated in this work corresponds to an intermediate region, bridging a large gap between available gas-gun data below 60 GPa and laser shock data above 500 GPa. The Hugoniot parameters and shock temperature were simultaneously determined by using optical velocimeters and pyrometers as the diagnostic tools and the α-quartz as a new standard material. The CH Hugoniot obtained tends to become stiffer than a semiempirical chemical theoretical model predictions at ultrahigh pressures but is consistent with other models and available experimental data.

X-ray source studies for radiography of dense matter

Studies of short-pulse laser-generated hard x-ray (18–60 keV) sources, suitable for radiographs of large samples of dense matter, are presented. The spatial and dynamic resolutions for different target types and laser parameters have been investigated. A high quality radiograph with good spatial resolution in two dimensions was demonstrated by irradiating freestanding thin W wires. The influence of the geometry for the quality of the radiograph, which is crucial for the design of experiments probing laser-compressed matter, is reported.

Formation mechanism of nanocatalysts for the growth of silicon nanowires on a hydrogen-terminated Si {111} surface template

We have observed the formation process of nanocatalysts that act for the growth of Si nanowires by means of UHV scanning tunneling microscopy. Gold–silicon nanocatalysts that we have examined were thought to form on a hydrogen (H)-terminated [111] silicon surface and to expel Si nanowires of extremely high aspect ratio via the vapor-liquid-solid mechanism. We have observed that a nanocatalyst, that is, a droplet of melted gold–silicon alloy of about 5 nm in diameter, is actually formed in a pit on a H-terminated surface in the narrow temperature range around 500 °C. We have concluded that, in this specific temperature range, nanocatalysts can be melted, remain mutually isolated, absorb silicon effectively, and expel Si nanowires. Based on the result, we have proposed a method of making a thin template, which facilitates to decide the nucleation sites and the sizes of nanocatalysts, resulting in the precise control of those of Si nanowires.

Hard x-ray radiography for density measurement in shock compressed matter

In this letter we report on the direct density measurement in a shock compressed aluminum target using hard x-ray radiography. Experimental data employing a molybdenum Kα source at 17.5keV, generated with a short pulse laser are presented. High spatial resolution was obtained thanks to a new design for the backlighter geometry. Density values deduced from radiography are compared to predictions from hydrodynamic simulations, which have been calibrated in order to reproduce shock velocities measured from a rear-side self-emission diagnostic. Our results reveal the great potential of this technique as a diagnostic tool for direct density measurements in dense high-Z opaque materials.

Observation of silicon surface nanoholes by scanning tunneling microscopy

Abstract We have studied electron-irradiation-induced defects created on an electron exit surface of a Si thin film by means of scanning tunneling microscopy (STM). Several electron-irradiated areas with different electron doses are provided for STM observation. Transmission electron microscopy (TEM) observation reveals a number of silicon-surface-nanoholes of 2–3 nm in diameter and about 5 nm apart in an irradiated area whenever it receives the dose larger than 1.5×10 24 e/cm 2 , while no distinctive TEM contrast of defects is observed in an area with lower dose. STM observation has shown that electron-irradiated surfaces are rougher than a nonirradiated surface. Examining the depth distribution of the areas with different doses, we have found that each irradiated surface exhibits two depth levels which are attributed to a rough surface and a bottom of surface nanoholes, respectively. Even in an area with the lowest dose (1.5×10 22 e/cm 2 ) in this experiment we have observed distinctive STM contrasts, the arrangement and sizes of which are similar to those of the well-developed surface nanoholes observable by TEM. This STM observation shows that the arrangement of nanoholes on an electron exit surface is set up at the very early stage, followed by the excavating of nanoholes under prolonged electron irradiation. We suggest that nanoholes exist in the early stage when only a few atomic layers are removed from the initial surface.

Femtosecond Laser Synthesis of High-Pressure Phases of Si

The high pressure simple hexagonal structure of silicon, which has not been synthesized, is quenched using femtosecond laser-driven shock wave. Any high-pressure phases of silicon do not remain after the pressure release in the case of the hydrostatic and conventional shock compression methods. We found the existence of the simple hexagonal structure after the intense femtosecond laser irradiation to silicon by analyzing the crystalline structures using a synchrotron grazing-incidence XRD method. Femtosecond laser-driven shock wave is a useful tool for the synthesis of non-equilibrium high-pressure phases.