st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Observation of Optical Emission Intensity of Plasma Induced by Nanosecond
Laser Pulses in Supercritical CO2 Medium
S. Machmudah1,2, Wahyudiono1, N. Takada3, K. Sasaki4, M. Goto1
1
Department of Chemical Engineering, Nagoya University, Nagoya, Japan
Department of Chemical Engineering, Sepuluh Nopember Institute of Technology, Surabaya, Indonesia
3
Department of Electrical Engineering and Computer Science, Nagoya University, Japan
4
Division of Quantum Science and Engineering, Hokkaido University, Japan
2
Abstract: In this work, the optical emission intensity from plasmas induced by nanosecond
laser ablation of a solid-state target in supercritical CO2 was investigated. Comparative of the
properties of plasmas generated from laser ablation of gold and silver target was also presented. Laser ablation was carried out in a high-pressure cell with three sapphire windows for observing optical emission from laser ablation plasmas. Gold or silver plate was used as a target
located in the center of the cell and ablated by a fundamental Nd:YAG laser ( =1064 nm).
Energy of the laser pulse was ~160 mJ/pulse. The duration and the repetition rate of the laser
pulse were 10 ns and 10 Hz, respectively. The distribution of the optical emission intensity
was captured using a charge-coupled device camera with a gated image intensifier (ICCD
camera). In addition, high resolution spectrophotometer was used to examine the propagation
loss of laser ablation in supercritical CO2 medium. Laser ablation was conducted at various
CO2 densities. The result showed that saturated emission intensity for ablation from silver
target was observed due to its high conductivity. At constant pressure, transmittance did not
significantly change with the temperature changed. It indicated that there was no propagation
loss during the ablation process. In conclusion, the experimental results suggest a possibility
that chemical reactions and physical states (pressure and temperature) of supercritical-phase
laser-ablation plasmas can be controlled by changing density of CO2 medium.
Keywords: Nanosecond laser pulses; Optical emission intensity; Plasma; Supercritical CO2.
1. Introduction
High-power lasers are being used in a variety of applications, including pulsed laser deposition of thin films
and laser-assisted machining. During these processes, the
laser beam evaporates and ionizes the target material,
creating a plasma plume above the target surface. Understanding the transport process of the laser beam in the
laser-induced plasma plume is essential for controlling the
interaction of the laser and the materials being used, and
for optimizing the high-power laser processes. In the case
of laser ablation in liquid-phase, the plasma can be considerably dense since the expansion of the laser ablation
plume is restricted significantly due to the tight confinement effect of ambient liquid.
Since supercritical fluid has liquid-like density, laser
ablation in supercritical fluid medium may produce similar plasmas generated by laser ablation in liquid-phase.
We have reported the formation of nanoparticles using
laser ablation in supercritical CO2 medium [1-2]. In order
to optimize parameters for controlling nanoparticles formation, it is important to investigate laser ablation plasma
formation in supercritical CO2 by observation of its emission intensity. Investigation of the optical emission intensity of plasma produced by laser ablation in distilled water has been reported by Takada et al. [3].
In this work, the optical emission intensity from plasmas produced by laser ablation of a solid-state target in
supercritical CO2 was investigated. The fluctuation generated by supercritical CO2 medium on laser ablation plasma was observed.
2. Experimental
Gold and silver plates (purity: 99.99%, thickness: 0.5
mm) used for PLA target were purchased from Nilaco,
Japan. CO2 (purity: 99.95%) was obtained from Soho Co.,
Japan. Laser ablation was performed in a high-pressure
cell made of SUS equipped with sapphire windows for
observing optical emission from laser ablation plasmas.
Schematic diagram of the experimental apparatus is
shown in Fig.1. PLA was carried out in a high-pressure
cell (SUS 316, 110 ml volume, AKICO, Japan) with three
sapphire windows. The fundamental of a Q-switched
pulsed Nd:YAG laser (Spectra-Physics Quanta-Ray
INDI-40-10, wavelength: 1064 nm, pulse energy: maximum 320 mJ/(cm2.pulse), pulse duration: 10 ns, repetition
frequency: 10 Hz) was used. Target was fixed in the center of a high-pressure cell. Incident angle of the laser
beam was 30o and the laser was located 1 m from the target. Liquid CO2 was pressurized and pumped into the cell
using a high-performance liquid chromatography (HPLC)
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
ence in the maximum optical emission intensities was
observed. Moreover, the difference in the optimal delays
and width of delay time distribution was also observed. It
illustrates the diverse composition and dynamics of the
ablation plasma in supercritical CO2 [4].
Integrated optical emission intensity (arb. unit)
pump (Jasco PU-1586). The cell temperature was controlled by a temperature controller at various temperatures
of 40-80oC. The pressure of the system was varied from 5
to 15 MPa. After the setting temperature and pressure
were reached, PLA was performed. The laser beam was
collimated by a 1-mm-diameter of aperture without any
focusing lens. The distribution of the optical emission
intensity was captured using a charge-coupled device
camera with a gated image intensifier (ICCD camera).
The delay time tD between the irradiation of the YAG laser pulse and the trigger to the gate of the ICCD camera
was controlled using a digital delay pulser. The origin of
delay time (tD) was defined as the time when a weak optical emission intensity corresponding to the detection
limit of the ICCD camera was observed. The delay time
was scanned with a step of 0.1 ns to find the appearance
time of the optical emission intensity.
14
o
40 C
o
50 C
o
60 C
o
70 C
o
80 C
12
10
8
6
4
2
0
0
10
20
30
40
50
Delay time (ns)
Integrated optical emission intensity (arb. unit)
(a)
14
o
40 C
o
50 C
o
60 C
o
70 C
o
80 C
12
10
8
6
4
2
0
0
10
20
30
40
50
Delay time (ns)
Fig.1 Schematic diagram of experimental apparatus
(b)
3. Results and Discussion
Typical images of optical emission intensities from laser ablation plasmas above gold target at 8 MPa and 40 oC
for delay time of 6 and 14 ns are shown in Fig.2(a) and
(b), respectively. The maximum optical emission intensities for the figures are 14667 and 30369, respectively.
Au
Au
(a)
(b)
Fig.2 Optical emission images of laser ablation plasmas at 8 MPa and 40oC. (a) 6 ns; (b) 14 ns
Fig.3(a) and (b) show temperature dependence of temporal variations of the maximum optical emission intensities observed at 8 and 10 MPa, respectively. The differ-
Fig.3 Temperature dependence of integrated optical
emission intensity at 8 MPa (a) and 10 MPa (b)
At 8 MPa, the integrated optical emission intensity of
plasma increased as decreasing temperature. The decreasing temperature causes the increasing CO2 density.
At high CO2 density, the dense gas may absorbs the laser
beam and results in high emission intensity of laser ablation plasma. This result could confirm our previous work
[1, 2] for the depth of crater formed by pulsed laser ablation on gold and silver plates. We obtained that the deepest crater was formed at 40oC for both gold and silver
plates.
On the other hand, at 10 MPa, the integrated optical
emission intensity of plasma increased as increasing temperature due to the decreasing CO2 density. At low CO2
density, the confinement of plasma decreases, that results
in the increasing plasma diffusion.
In order to examine whether propagation lost occurred
during the ablation, laser transmittance was measured by
high resolution spectrometer. Fig.4(a) and (b) show tem-
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
100
80
80
60
60
40
40
20
15
20
Emission Intensity
Transmittance
0
0
40
50
60
70
80
o
Temperature ( C)
100
80
80
60
60
40
40
20
Emission intensity
Transmittance
0
Transmittance (%)
Normalized integrated optical
emission intensity (%)
(a)
100
20
0
40
50
60
70
80
o
Temperature ( C)
(b)
Integrated optical emission intensity (arb. unit)
Fig.4 Temperature dependence of integrated optical
emission intensity and transmittance at 8 MPa (a) and
10 MPa (b)
3.0
5 MPa
7 MPa
8 MPa
10 MPa
15 MPa
2.5
2.0
1.5
1.0
0.5
0.0
0
10
20
30
40
was obtained at 8 MPa. This result could be confirmed
with our previous work [2] for investigation of the depth
of crater formed by pulsed laser ablation on silver plate.
We observed that the deepest crater formed on the silver
plate was obtained at pressure between 7 and 9 MPa. At
this condition, the highest constant volume heat capacity
of CO2 is obtained.
50
Delay time (ns)
Fig.5 Pressure dependence of integrated optical emission intensity at 40oC
Pressure dependence of temporal variations of the
maximum optical emission intensities observed at 40oC is
shown in Fig.5. The maximum optical emission intensity
Integrated optical emission
intensity (arb. units)
100
Transmittance (%)
Normalized integrated optical
emission intensity (%)
perature dependence on the integrated optical emission
intensity and transmittance of ablation plasma. As shown
in the figures, although the optical emission intensity
changed with temperature, the transmittance almost no
changed for both 8 and 10 MPa. It indicated that the
change of optical emission intensity was not caused by
the propagation lost.
Density = 280 kg/m3
8M P a 40℃
10
10M P a 60℃
10.7M P a 70℃
11.6M P a 80℃
5
0
0
10
20
30
40
50
Delay Time (ns)
Fig.6 Integrated optical emission intensity at constant
CO2 density
Density of CO2 on the ablation plasma in supercritical
CO2 medium is an important factor to be examined. Fig.6
shows the integrated optical emission intensity at constant
CO2 density. At low pressure and temperature, no difference in the maximum optical emission intensities was
observed. Moreover, the temporal variations of the maximum optical emission intensities were not identical.
However, at high pressure and temperature, the optical
emission intensities decreased drastically. It might be due
to the change of other properties of CO2 that resulted in
the change of diffraction angle of measurement.
4. Conclusions
The optical emission intensity from plasmas produced
by laser ablation of gold and silver target in supercritical
CO2 has been investigated. The effects of pressure and
temperature on the optical emission intensity of plasma
were observed. Density of CO2 was the most parameter
that affected the ablation plasma formation.
5. Acknowledgement
This work was supported by the Grants-in-Aid for Scientific Research by the Ministry of Education, Culture,
Sports, Science and Technology, Japan
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
6. References
[1] S. Machmudah, Wahyudiono, Y. Kuwahara, M.
Sasaki, and M. Goto: J. Supercrit. Fluids 60, 63
(2011).
[2] S. Machmudah, T. Sato, Wahyudiono, M. Sasaki, and
M. Goto: High Pressure Res.: An
Internat. J.
32, 60 (2012).
[3] N. Takada, T. Nakano, and K. Sasaki: Appl. Surf. Sci.
255, 9572 (2009).
[4] M. Lopez-Arias, M. Oujja, M. Sanz, R.A. Ganeev,
G.S. Boltaev, N.Kh. Satlikov, R.I. Tugushev, T.
Usmanov, and M. Castillejo: J. Appl. Phys. 111,
043111-1 (2012).