S4-3
Production and diagnostics of multiphase plasmas
using laser ablation and ultrasonic wave
レーザーおよび超音波誘起気-液マルチフェーズプラズマの創製とプラズマ診断
Koichi Sasaki
佐々木浩一
Division of Quantum Science and Engineering, Hokkaido University,
Kita 13, Nishi 8, Kita-ku, Sapporo 060-8628, Japan
北海道大学大学院工学研究院量子理工学部門 〒060-8628 札幌市北区北40条西8丁目
A topic in this talk is liquid-phase laser ablation, which is a method for producing a multiphase plasma
consisting of a solid (the ablation target), a liquid (the ambient medium), and a gas (the cavitation bubble).
In addition, the liquid-phase laser ablation plasma has a temperature and a pressure corresponding to asupercritical fluid. Another topic is “sonoplasma”, which is a plasma produced at the collapse of acousticcavitation. The acoustic cavitation is induced by the irradiation of ultrasonic wave onto a liquid. The
sonoplasma is rather new terminology, but we believe that it has been used in the field of sonochemistry
for a long time. The study of sonoplasma may open a new interdisciplinary research field.

1. Introduction
In this talk, we mainly focus on plasmasproduced in liquids without using electricity. An
alternative to electricity is to use optical power.
Irradiating an intense laser pulse onto a solid
target immersed in liquid produces a dense
plasma with a high pressure. This is calledliquid-phase laser ablation, which attracts muchattention recently as a new method forsynthesizing
functional
nanoparticles.
Liquid-phase laser ablation induces the formationof a cavitation bubble. The cavitation bubble is
not a static bubble but has dynamics of expansion,shrinkage, and collapse. The inside of thecavitation bubble at the collapse has a hightemperature and a high pressure, resulting in theproduction of secondary plasma. This plasma
does not need electricity, and is produced by themechanical power of the collapsing bubble. We
have found the importance of the cavitation
bubble in the synthesis of nanoparticles byliquid-phase laser ablation.
Another alternative to electricity is to use
acoustic power or ultrasonic wave. It has been
known from many years ago that a liquid
irradiated by intense ultrasonic wave emits
optical radiation (sonoluminescence). The
mechanism of the sonoluminescence was amystery for a long time, but rather recently, it has
been shown that the sonoluminescence isoriginated from a plasma produced at the collapseof cavitation bubble induced by the ultrasonicwave [1]. This plasma is sometimes called“sonoplasma”. We have found a simple way for

producing standing sonoplasmas efficiently.
2. Liquid-phase laser ablation
Laser ablation plasmas have high densities since
they are produced from solid targets directly. Thisis also the case in the very early stage of laser
ablation in vacuum; however, the lifetime of thehigh-density plasma is short in vacuum because of
the rapid expansion of the laser ablation plume. The
ambient medium confines the plume tightly inliquid-phase laser ablation, resulting in in a small,dense plasma. We observed no line emissions fromliquid-phase laser ablation plasmas [2]. The
blackbody radiation indicates a high density andrapid quenching in the liquid-phase laser ablation
plasma.
After the disappearance of optical emission, weobserve the formation of a gas phase; that is acavitation bubble. Since the cavitation bubble isformed by abrupt vaporization, it has a highpressure at the beginning. The high pressure drives
the expansion of the bubble. It is important herethat the expansion cannot stop due to the inertia
even after the inside of the bubble has a negative
pressure. The cavitation bubble begins to shrink
after the maximum size because of the negative
pressure, and the shrinkage cannot also stop evenafter the bubble pressure reaches the ambientpressure. This results in the high bubble pressure atthe collapse [3]. In addition, since the bubble
dynamics is adiabatic, the bubble temperature in theexpansion phase is ultralow, while the bubble
temperature at the collapse is ultrahigh [3]. The
cavitation bubble provides us an interesting reaction

Ti target

Cavitation
bubbleScatteredlaser light

Water surface
Punching plate
Holes

Scattered light

Fig. 1 Superimposed image of shadowgraphand laser light scattering observed inliquid-phase laser ablation of a titanium target.

Fig. 2 Photograph of a sonoplasma production
system with the help of a punching plate.
Cavitation bubbles are seen by light scattering.

field with unique physical parameters.
We have found that the cavitation bubble plays
an essential role in the synthesis of nanoparticles byliquid-phase laser ablation. Figure 1 shows asuperimposed image of shadowgraph and laser lightscattering. The location and the size of thecavitation bubble is observed by shadowgraphimaging, while the location of nanoparticles isdetected by laser light scattering. As shown in thefigure, nanoparticles are stored in the cavitation
bubble until the collapse [4]. Since nanoparticles
areimmersedinthehigh-pressure,high-temperature reaction field at the collapse of
the cavitation bubble, the size [5] and the structure[6,7] of nanoparticles are controlled by the physicalcontrol [8,9] of the dynamics of the cavitation
bubble. The importance of the cavitation bubble inthe synthesis of nanoparticles is also reported byanother group [10].

ultrasonic clear used in a glasses shop. The water isilluminated by white lamp light, and cavitation
bubbles are seen like white cloud by light scattering.
We did not observe remarkable formation ofcavitation bubbles in the absence of the punchingplate. However, as shown in Fig. 2, we obtained theefficient formation of cavitation bubbles below thepunching plate. The group of crowded cavitation
bubbles did not move and were standing. We
confirmed that the production rate of gold
nanoparticles from HAuCl4 solution was enhanced
by a factor of 8 by inserting the punching plate.
Now we are trying to have electrical discharges
inside collapsing cavitation bubbles [12].

3. Sonoplasma
We believe that sonoplasmas are used bychemists for a long time. The enhancement ofliquid-phase chemical reactions with the help ofultrasonic wave is called sonochemistry. It utilizes
radicals produced at the collapse of cavitation
bubbles. Chemists are not so interested in theplasma at the collapse of cavitation bubbles, but the
study of sonoplasma may open a newinterdisciplinary research field. Of course
sonochemistry is also in a close position to thestudy of liquid-phase plasmas.
We have noticed that the insertion of a punchingplate is a simple, efficient way for producing
standing sonoplasmas [11]. Figure 2 shows apicture of a vessel which is filled with water. Thevessel has an ultrasonic transducer at the bottom,and ultrasonic wave at a frequency of 32 kHz is
propagating from the bottom toward the watersurface. This situation is rather similar to anReferences[1] D. J. Flannigan and K. S. Suslick, Nature 434, 52(2005).
[2] H. Ushida, N. Takada, and K. Sasaki, J. Phys. Conf.
Series 59, 563 (2007).
[3] W. Soliman, T. Nakano, N. Takada, and K. Sasaki,Jpn. J. Appl. Phys. 49, 116202 (2010).
[4] W. Soliman, N. Takada, and K. Sasaki, Appl. Phys.
Express 3, 035201 (2010).
[5] W. Soliman, N. Takada, and K. Sasaki, Jpn. J. Appl.
Phys. 50, 108003 (2011).
[6] W. Soliman, N. Takada, N. Koshizaki, and K. Sasaki,
Appl. Phys. A, 110, 779 (2013).
[7] N. Takada, A. Fujikawa, N. Koshizaki, and K. Sasaki,
Appl. Phys. A, 110, 835 (2013).
[8] K. Sasaki, T. Nakano, W. Soliman, and N. Takada,Appl. Phys. Express 2, 046501 (2009).
[9] N. Takada, A. Fujikawa, and K. Sasaki, Jpn. J. Appl.
Phys. 50, 126201 (2011).
[10] S. Ibrahimkutty, P. Wagener, A. Menzel, A. Plech,and S. Barcikowski, Appl. Phys. Lett. 101, 103104(2012).
[11] Y. Iwata, N. Takada, and K. Sasaki, Appl. Phys.
Express 6, 127301 (2013).
[12] N. Takada, Y. Hayashi, M. Goto, and K. Sasaki, in
this conference, 18pD1-6.