24F17
Time-resolved measurement of femtosecond laser ablation process
by using a soft X-ray laser pulse
軟X線レーザーを用いたフェムト秒レーザーアブレーションダイナミクスの時間分解計測
Masaharu Nishikino1, Minoru Yamamoto1,2, Noboru Hasegawa1, Takuro Tomita2, Kota Terakawa3,
Yasuo Minami3, Ryota Takei3, Ryo Ohnishi2, Masahiko Ishino1,Takeshi Kaihori1,
Tetsuya Kawachi1, Mitsuru Yamagiwa1, and Tohru Suemoto3
錦野将元1, 山本稔1,2, 長谷川登1, 富田卓朗2, 寺川康太3, 南康夫3, 武井亮太3, 大西諒2,石野雅彦1, 海堀岳史1, 河内哲哉1, 山極満1, 末元徹3
1

Quantum Beam Science Directorate, X-ray laser Application group, Japan Atomic Energy Agency
8-1-7 Umemi-dai, Kizugawa, Kyoto 619-0215, Japan
日本原子力研究開発機構	  量子ビーム応用研究部門 〒619-0215	  京都府木津川市梅美台8-1-7
2
Department of Ecosystem Engineering, The University of Tokushima
2-1 Minamijyousanjima, Tokushima 770-8506, Japan
徳島大学	  工学研究科	  〒770-8506	  徳島市南常三島 2-1
3Institute for Solid State Physics, The University of Tokyo
5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8581, Japan
東京大学物性研究所	  〒 277-8581	  千葉県柏市柏の葉 5-1-5

The x-ray reflective imaging using a soft x-ray laser was used to observe the early stage of a femtosecond laser
ablation process on the platinum. The fluence dependence of the soft x-ray reflectivity is classified into three
regions: (1) strongly excited, (2) moderately excited, and (3) weakly excited regions. In strongly excited region,
the rapid reduction of the reflectivity due to the explosive evaporation in the ablation process of the material
surface was observed. In the moderately excited region, the reflectivity reduction is far slower than that in thestrongly excited region. The reflectivity reduction is seemed to be caused by the growth of surface roughness on
the ablation front because of the formation of nano-bubbles in the irradiated material.

1. Introduction
The femtosecond laser attracts much attention
as the promising fabrication tool for various materials
such as metals, semiconductors, and insulators. One of
the excellent properties of the femtosecond laser
processing is the precise processing in the micro/nano
scaled region. To improve the femtosecond laser
processing technique more accurate and controllable,
the knowledge about the laser ablation process isimportant. In order to understand the dynamics of thefemtosecond laser ablation process, several studies on
the time-resolved imaging of femtosecond laser
ablation process have been performed on various
materials.[1-3] The dynamics of phase transition
related to the femtosecond laser ablation, such as
melting and re-solidification, were investigated byobserving the reflectivity change of visible laser probe
in the fluence regime below and around the ablationthreshold. In contrast to the visible light probe, x-ray
probe is essential to penetrate and diagnose the insideof the highly density electron plasma, and thepenetration depth decreases from visible to soft x-ray
region, while it increases from soft x-ray to hard x-ray.
Therefore, soft x-ray is the most suitable to observe
the solid surface morphology. [4] The reflectivity ofsoft x-ray depends simply on the atomic density andthe roughness of the interface. In this proceeding, wereport a pump and probe reflectivity imaging of theplatinum (Pt) surface during the femtosecond laser
ablation by using the laser-driven soft x-ray laser as aprobe beam. We found the temporal evolution ofreflectivity strongly depends on the fluence of thefemtosecond laser as a pump beam.

2. Experiment
The laser-driven plasma soft x-ray laser (SXRL)in Japan Atomic Energy Agency was used for theprobe beam. [5] The pulse width of the SXRL beam at
the wavelength of 13.9 nm (89.2 eV) is 7 ps (FWHM)
and the output energy of the XRL beam is about 50
nJ/pulse. A schematic view of the experimental
setup of the femtosecond laser pump and the softx-ray probe microscopy is shown in Fig.1(a). Thesample was platinum (Pt) thin film with 300 nm
thickness evaporated on fused silica substrates. The
initial surface roughness of root mean square (RMS)
was measured by atomic force microscope (AFM) andfound to be below 1.0 nm. The refractivity isestimated about 30 % at 13.9 nm. [6] The sample
image was transferred onto the CCD camera by theimaging mirror with a magnification factor of 19.1.
The pumping laser used for ablating materials was aTi:Sapphire laser. The laser emitted 80 fs pulses oflinearly polarized light at a central wavelength of 795

nm. The emitted pulses were focused by a lens (f =600 mm) onto the sample surface at nearly normal
incidence. The focal spot size (1/e) on the sample
surface was measured to be about 73 µm. The typical
pump energy, peak fluence, and excitation intensity on
the sample surface were 170 µJ, 4.1 J/cm2, and 1 x1014 W/cm2, respectively.
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for each area, we have plotted the horizontal
cross-sections of the reflectivity in Fig. 1(e). In
strongly excited area (from A to B), drastic and fastreflectivity drop was observed. In moderately excitedarea (form B to C), the decay time of the reflectivity isslower compared with the strongly excited area, andthe reflectivity reaches its minimum at t=160 ps. In
weakly excited area (from C to D), laser-induced
reflectivity change was not observed.
According to X-Ray Database[5], when thesurface roughness increases from 1 nm to 5 nm, thereflectivity reduction exceeds 80% at the wavelengthof 13.9 nm. This high sensitivity of the reflectivity ofsoft x-ray to surface roughness may bring us thefruitful information about the temporal roughnesschange of the ablation front due to nanometer-scale
bubble formation in the early stage. We calculated thesurface roughness of ablation front assuming thedensity of solid phase and all the reflectivity change iscaused by the surface roughness. The surface
roughness of ablation front was estimated to be about
5 nm.

204060Space[µm]Fig.1. (a) Schematic view of experimental setup.
Snapshots of the sample surface at the timing of (b)+10 ps and (c) +160 ps. (d) Fluence distribution of thepump beam. (e) Cross-sections of reflectivity.

Figures 1(b) and (c) show the soft x-ray reflective
images of a Pt surface at the delay times after theirradiation of 10 and 160 ps, respectively. At the delay
time of 10 ps, a disk-shaped dark area was found on
the center of irradiated region. This means that theablation phenomena already started at t = 10 ps. The
diameter of disk-shaped dark area gradually increases
and reaches to maximum values at 160 ps. The
diameter of disk-shaped area at t =160 ps coincides
with that at t = +∞. The difference of decay time ofthe reflectivity indicates that the observed phenomena
are strongly fluence dependent. Therefore, we classify
the fluence area into three ranges as follows;(i) Strongly excited region (from 1.2 J/cm2 to 4.1
J/cm2) where the fast reflectivity drop was observed(ii) Moderately excited region (from 0.5 J/cm2 to 1.2
J/cm2) where the slow reflectivity drop was observed(iii) Weakly excited region (below 0.5 J/cm2) wheretransient reflectivity change was not observed.
The schematic of this classification is shown in Fig.
1(d). To evaluate the time evolution of the reflectivity

3. Summary
In conclusion, The Ti:Sapphire laser pump andthe soft x-ray laser probe microscopy was carried out
for the observation of femtosecond laser ablation
dynamics on the Pt film. The gradual decrease ofreflectivity is seemed to be caused by the increase of
surface roughness on the ablation front.
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Bialkowski, Appl. Surf. Sci. 109-110, 1-10
(1997).
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Yamamoto, N. Hasegawa, M. Deki, Y. Minami
and T. Kawachi, Opt.Express, 18, (2010) 14114.
[5] Y. Ochi, T. Kawachi, N. Hasegawa, M. Nishikino,T. Ohba, M. Tanaka, M. Kishimoto, T. Kaihori, K.
Nagahima, and A. Sugiyama, Jpn. J. Appl. Phys.
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[6] The center for X-ray optics:
http://henke.lbl.gov/optical_constants/