Noriharu Takada

Effect of Pressurization on the Dynamics of a Cavitation Bubble Induced by Liquid-Phase Laser Ablation

We applied external high pressure to ambient water in liquid-phase laser ablation. As a result, it was found that the maximum volume Vmax of a cavitation bubble induced by laser ablation satisfied a scaling law of Vmax∝Pext-1 with Pext being the pressure applied to water. The effect of the pressurization was also observed in the shape of the second bubble induced by the collapse of the first cavitation bubble. These experimental results indicate that the dynamics of a cavitation bubble induced by liquid-phase laser ablation is controlled by the external pressure.

Growth Processes of Nanoparticles in Liquid-Phase Laser Ablation Studied by Laser-Light Scattering

We applied laser-light scattering for investigating the growth processes of nanoparticles in liquid-phase laser ablation. We observed the growth of nanoparticles inside the cavitation bubble. This means that particles ejected from the target are transported into the cavitation bubble, and they condense into nanoparticles inside it. The production of nanoparticles was efficient until 3 µs after the irradiation of the laser pulse for ablation, indicating the fast growth of nanoparticles. A part of nanoparticles was transported from the cavitation bubble toward the water, but the great portion of nanoparticles was stored in the cavitation bubble until the collapse.

Liquid-phase laser ablation*

The irradiation of an intense laser pulse onto a solid target immersed in liquid produces dense plasma. The plasma produced by liquid-phase laser ablation has unique features at high pressure and temperature, which are never realized by liquid-phase discharges. Another unique characteristic of liquid-phase laser ablation is the formation of a cavitation bubble. This article reports the fundamental aspects of liquid-phase laser-ablation plasmas, cavitation bubbles, and the formation processes of nanoparticles, together with some applications of liquid-phase laser ablation.

Characteristics of hydrogenated amorphous carbon films deposited by large-area microwave-sustained surface wave plasma

Abstract In this study, we report the characteristics of hydrogenated amorphous carbon (a-C:H) films grown at room temperature by chemical vapor deposition (CVD) with a 40-cm diameter planar surface wave plasma in a He/CH 4 gas mixture. The film characteristics such as electron field emission and the atomic or molecular bonding structures, measured with XPS, FT-IR spectrophotometry and NMR analysis, have been studied. The electron emission current density of 1 μA/cm 2 was obtained at an electric field of 4 V/μm in the ‘as-grown’ a-C:H film. From the NMR measurements, it is found that the a-C:H film typically contains 25% of graphite-like sp 2 -carbon bondings and 75% of sp 3 -carbon bondings with one or more hydrogen atoms. The results of electrical conductivity measurements of a-C:H films suggest that spatially localized conduction channels formed by the graphite-like sp 2 -carbons might be responsible for the electron emission in the a-C:H films by locally-enhanced electric fields.

Modification of Rayleigh–Plesset Theory for Reproducing Dynamics of Cavitation Bubbles in Liquid-Phase Laser Ablation

The solution of the conventional Rayleigh–Plesset equation did not agree with the experimental results on the temporal variations of the sizes of cavitation bubbles produced by laser ablation in water. In this work, we modified the conventional Rayleigh–Plesset theory in the following two points to reproduce the experimental observation theoretically. One was to introduce the effect of the contact angle among the water, the cavitation bubble, and the ablation target. The other was to treat the surface tension and the kinematic viscosity coefficient of water as additional adjusting parameters to fit the theoretical result with the experimental observation. The latter modification was effective especially for laser ablation in the pressurized water. Better agreement between the theoretical and the experimental results was realized with the help of these modifications, but anomalous thermodynamic parameters were necessary to obtain the best fitting. We evaluated the pressures and the temperatures inside the cavitation bubbles.

Field-Emission Characteristics of Hydrogenated Amorphous Carbon Films Prepared by Surface Wave Plasma

Hydrogenated amorphous carbon films are prepared by a 40-cm-diameter planar surface wave plasma to apply them to field-emission display. The 2.45 GHz surface wave plasmas at 700 W give a film deposition rate of ~15 nm/min in He gas mixed with a small amount of methane gas at a relatively low pressure of 100 to 200 mTorr. Preliminary experimental results show that the hydrogenated amorphous carbon films deposited on silicon substrates have good field-emission characteristics: a threshold electric field defined at 1 µA/cm2 was roughly 4 V/µm and an emission current of 0.1 mA/cm2 was achieved at an electric field of 7.5 V/µm.

Enhancement of burning velocity in premixed burner flame by irradiating microwave power

It was observed experimentally that the length of a premixed burner flame was shortened by irradiating microwave power, indicating the enhancement of the burning velocity. To examine the mechanism of the enhancement of the burning velocity, we evaluated the rotational temperatures of CH and OH in the flame by optical emission spectroscopy. As a result, the rotational temperatures were unchanged by the microwave irradiation, suggesting that the enhancement of the burning velocity is not due to the increase in the gas temperature but is caused by direct effects of electron heating.

Diagnostics of liquid-phase laser ablation plasmas by spectroscopic methods

We adopted spectroscopic diagnostics for investigating plasmas produced by laser ablation of a graphite target in water. By taking pictures of optical emissions at various delay times after the irradiation of the ablation laser pulse, we examined the size, the lifetime, and the transient expansion of the plasma. The spectrum of the optical emission was also measured at various delay times. No line emissions were observed in the spectrum. The blackbody temperature of the plasma was evaluated by fitting the continuum spectrum with the Plank equation. In addition, we examined the propagations of compressional waves in water by shadowgraphy.

Role of Reaction Products in F ¡ Production in Low-Pressure, High-Density CF4 Plasmas

In this paper we report on the role of reaction products in F- production in low-pressure, high-density CF4 plasmas. The spatial distributions and temporal variations of F- density (n-), and plasma parameters in the discharge phase and afterglow of helicon-wave CF4 plasmas which had an electron density (ne) of 1011–1013 cm-3 were measured by the laser-photodetachment technique combined with a heated Langmuir probe. The relationship between the n-/ne ratio and the degree of ionization was investigated in the discharge phase. The n-/ne ratios in the plasma column of highly ionized plasmas were much higher than those expected from dissociative electron attachment to CF4, and n-/ne ratios were larger by several orders of magnitude in the outer region. The efficient increase in n- was observed in the afterglow and n-/ne was enhanced by increasing the discharge duration. It is concluded that the attachment to the reaction products contributes greatly to F- production in low-pressure, high-density plasmas.

Production of Ar metastable atoms in the late afterglow of pulse-modulated rf magnetron sputtering plasmas

We observed the production of Ar atoms at a metastable state (ArM) in the late afterglow of an rf magnetron sputtering plasma. The temporal variation of the ArM density distribution was measured by laser-induced fluorescence imaging spectroscopy. In the initial afterglow, the ArM density decreased rapidly with time and became negligible at several hundred microseconds after the termination of the rf power. However, at approximately 1 ms after the termination of the rf power, the ArM density increased and had the peak value at a region with a long distance from the target surface. The production of ArM in the late afterglow was remarkable at a high gas pressure and a high discharge power. In addition, we also measured the spatiotemporal variation of the optical emission intensity from Ar. As a result, we found that the population of radiative Ar increased and had a similar peak in the late afterglow. Since the electron temperature decreases drastically after the termination of the rf power, the production of ArM in the late afterglow is attributed to recombination processes between ions and electrons. Qualitative and quantitative discussions on the contribution of recombination processes to the production of ArM are described.