22P072-P
Influence of the frequency of superposed supersonic wave
on laser ablation plasma produced in water
水中レーザーアブレーションプラズマに対する重畳超音波の周波数の効果
Noriharu Takada and Koichi Sasaki*
高田昇冶, 佐々木浩一*
Department of Electrical Engineering and Computer Science, Nagoya University, Nagoya 464-8603, Japan
名古屋大学大学院工学研究科電子情報システム専攻，〒464-8603 名古屋市千種区不老町*Division of Quantum Science and Engineering, Faculty of Engineering, Hokkaido University,
Sapporo 060-0828, Japan*北海道大学大学院工学研究院量子工学部門, 〒060-0828 札幌市北区北13条西8丁目
Laser ablation of a solid target was carried out in water in the presence of supersonic wave. The opticalemission intensity from the ablation plasma was examined as functions of the phase of the supersonic wave
at the timing of the laser irradiation and the frequency of the supersonic wave between 32 and 135 kHz.
The enhancement of the optical emission intensity was observed when a laser pulse irradiated the target at
a negative phase of the supersonic wave at all the frequencies.

1. Introduction
Laser ablation in liquid-phase has beeninvestigated as a novel effective method forsynthesizing crystalline nanoparticles [1-3]. In
this process, a dense plasma with a hightemperature and a high pressure is induced in thevicinity of the target by the irradiation of an
intense laser pulse onto a solid target immersed inliquid [4-5]. It is pointed out that the inside of theablation plasma with the unique special state andthe interface between the plasma and the liquidplay important roles as reaction fields in thesynthesis of crystalline nanoparticles [6-8]. We
have proposed adding acoustic pressure by
exciting supersonic wave in ambient liquid as amethod for controlling the dynamics of thereaction field [9]. To date, chemical reactions andphysical phenomena caused by supersonic wave
in water have been investigated in the field of
sonochemistry [10]. It is reported that acousticcavitation in water induces the formation ofactive hydrogen and hydroxy radicals, andenhances chemical reactions [11-13]. Hence, we
expect that the characteristics of nanoparticles
synthesized by liquid-phase laser ablation arecontrolled by supersonic wave. In this paper, we
examined the effect of the acoustic pressure on
the change in the plasma state. In particular, we
focused our interest on the influence of the phaseof the supersonic wave at the timing of thelaser-pulse irradiation and the frequency of thesupersonic wave on the optical emission intensityfrom the ablation plasma.

Fig. 1. Variations of integrated optical emission
intensity from ablation plasma as a function of thephase of the supersonic wave.

2. Experimental
The details of the experimental apparatus
have been reported elsewhere [9]. Briefly, a
rectangular container was filled with distilledwater. A Ti target was immersed in water and wasirradiated by YAG laser-pulses with an energy of360 J/pulse at a wavelength of 1064 nm fromthe normal direction. The supersonic wave waspropagated in water from the bottom of thecontainer toward the top. The frequency of thesupersonic wave ranged between 32 and 135 kHz.
A supersonic power of 30 W was used. We
observed the optical emission image from theablation plasma using an ICCD camera. The
phase of the supersonic wave at the timing of theYAG laser irradiation was controlled using a
digital delay system. An image of the opticalemission intensity was obtained by accumulating
signals for 1000 laser shots.
3. Results and discussionFigures 1(a)-1(c) show the variation of the
optical emission intensity from plasmas as afunction of the phase of the supersonic wave. The
optical emission intensity was obtained by thespatial integration of the optical emission image.
We observed the enhancement of the opticalemission intensity at the negative phase of thesupersonic wave in all the supersonic frequencies
between 32 and 135 kHz. The magnitude of theenhancement decreased with the frequency. In thecase of 32 kHz, the widths (FWHM) of the
optical emission image were examined at
different phases as shown in Figs. 2(a) and 2(b).
We observed no change in the spatial distribution of
the optical emission intensity with the phase,indicating that the stronger optical emission was not
attributed to the more significant expansion of theplasma at the negative phase of the acousticpressure. Hence, we speculated that gases dissolvedin water contributed to the strong optical emission.
In a previous paper, we reported the enhancementof the optical emission intensity by pressurizing
water using nitrogen gas [14]. Dissolved gas was
the key for the enhancement of the optical emission
intensity. In the case of this experiment, the boilingtemperature of water becomes low at the negative
phase of the acoustic pressure, resulting in thesignificant degassing of dissolved gas from water.
The amplitude of the acoustic pressure driven at thelaser-irradiation position was larger at a higher
frequency of the supersonic wave. Indeed, weobserved more significant formation of bubbles on
observation windows of the container at a higher
frequency of the supersonic wave. Therefore, theweaker optical emission intensity at a higher
supersonic frequency may be attributed to a smaller
amount of dissolved gas in water.
Acknowledgment
This work was supported by a Grant-in-Aid for
Scientific Research C (No. 21510114) from theJapan Society for the Promotion of Science.

Fig. 2. Width of optical emission intensity as afunction of the phase of the supersonic wave.
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