22P016-O
Observation of laser ablation phenomena in super critical water
超臨界水中におけるレーザーアブレーション現象の観測
Hiroshi Goto, Noriharu Takada, Akihiro Kono and Koichi Sasaki
後藤 博1, 高田昇治1，河野明廣1，佐々木浩一2
Department of Electrical Engineering and Computer Science, Nagoya University
Furou-cho, Chikusa-ku, 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, 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13条西8丁目
1. Introduction
Conventional laser ablation, which is used forthin film deposition and nanoparticles syntheses,
has been carried out in gases. Recently, laser
ablation in liquids attracts significant attention ofmany researchers as a new technique to produce
nanoparticles. In this work, we employedsupercritical water as an alternative medium forlaser ablation. Supercritical water has high chemical
reactivity and properties of gas and liquid. Thus thesupercritical water is attractive state as a new laser
ablation medium. In this work, we observed theoptical emission intensity from laser ablation
plasmas produced in supercritical water and a
bubble-like phenomenon which was observed after
the disappearance of the laser ablation plasma. We
compared the experimental results in supercriticalwater with those in liquid water and gas.
2. Experimental
The experimental apparatus is schematicallyshown in Fig. 1. A cell which was compatible witha high pressure of 35 MPa and a high temperatureof 500 oC was filled with distilled water. YAG laser
pulses at a wavelength of 1064 nm irradiated a Ti
target installed in water from the normal direction.
The duration of the YAG laser pulse was 10 ns. The
YAG laser beam was focused using a lens. Whenwe observed optical emission from ablation plasma
in liquid water at a pressure of 30MPa and atemperature of 230oC, the laser energy was adjusted
at 36 mJ/pulse, while in the case of supercriticalwater at a pressure of 30 MPa and a temperature of430oC, an laser energy of 360 mJ/pulse wasemployed. The difference in the laser energy was tocompensate the different sizes in the ablationcraters formed in water and supercritical water. The
different sizes of the ablation craters were due tothe fact that the tight focusing of the laser beam in
the vertical direction was impossible in supercriticalwater. When we observed bubble-like phenomenain the supercritical water by shadowgraph imaging,a laser energy of 22.4 mJ/pulse was employed. The
optical emission image of the ablation plasma wascaptured using a charge-coupled device camera
with a gated image intensifier (ICCD camera) fromthe side of the high-pressure, high-temperature cell.
The gate width was 2ns. In shadowgraph imaging,
the ablation space was illuminated by flash lamp
light, and the transmitted flash lamp light wascaptured using the same ICCD camera with a gate
width of 5 ns. The temporal variations wereobtained by changing the delay time between theirradiation of the YAG laser pulse and the trigger tothe gate of the ICCD camera.
3.Results and discussion
Figure 2(a) shows normalized temporalvariations of the optical emission intensities fromlaser ablation plasmas produced in liquid water (30MPa, 230oC) and supercritical water (30 MPa,430oC). The optical emission intensities plotted inFig. 2 were obtained by integrating the opticalemission images spatially. As shown in the figure,the rise parts of the optical emission intensities
were rather similar in liquid water and supercriticalwater, while the temporal decay of the opticalemission intensity in supercritical liquid water wasmuch slower than that in liquid water. Figure 2(b)shows spatial distributions of the optical emission

1230℃300atm
430℃300atm

0.1

0.01

0.001

(a)
0.0001050

100

150200

Delay time [ns]1.2(b)
1230℃300atm
430℃300atm

0.80.6Fig. 3 Shadow graph images of the ablation
space in supercritical water (30 MPa and430 ℃ ) at various delay times after laserablation..

0.4
0.2
000.2

0.4
0.6
0.8
1Distance from target [mm]1.2Fig. 2 (a) Temporal variations and (b) spatial
distributions of optical emission intensities fromlaser ablation plasmas produced in liquid water(30 MPa, 230℃) and supercritical water (30MPa, 430℃).
intensities. The delay times for capturing the opticalemission images were 14 and 16 ns in liquid waterand supercritical water, respectively. These delay
times corresponded to the maximum opticalemission intensities. As shown in the figure, theoptical emission region in supercritical water wasmuch broader than that in liquid water. The
experimental results shown in Fig. 2 indicate lesstighter confinement and less significant quenchingin supercritical water than liquid water. These
characteristics are reasonable since supercriticalstate has smaller density and is classified into theintermediate between liquid and gas.
In liquid-phase laser ablation, we observe theformation of a cavitation bubble after thedisappearance of the plasma with optical emission.
In the case of laser ablation in supercritical water,
we also observed a bubble-like phenomenon asshown in Fig. 3. We detected the formation of abubble-like density hollow at 4 s after laserablation. The density hollow expanded with time,
and the maximum size was observed at 18 s.
Remarkable differences from the cavitation bubble
observed in liquid-phase laser ablation aresummarized as follows; 1) the maximum size of thebubble-like density hollow was larger than that of acavitation bubble in liquid-phase laser ablation at
the same pressure, 2) the bubble-like density hollow
expanded for a longer time than a cavitation bubble,and 3) the bubble-like density hollow never shrank
after the maximum size while a cavitation bubble
shrinks and comes to the collapse. The last shape ofthe density hollow observed in supercritical water
was not the collapse but a kind of extinction into
supercritical water. The density hollow insupercritical water is remarkably different from thecavitation bubble, but is rather similar tophenomena observed in the behavior of ambient gas
in gas-phase laser ablation [1]. This would be also
due to the fact that supercritical state is theintermediate between liquid and gas.
References[1] K. Sasaki and H. Watarai, Jpn. J. Appl. Phys. 45,
L447 (2006).