22C10
Observation of the laser-induced surface dynamics using the single-shot soft
x-ray laser probepyg[d FVu^Iv!lmYNoboru Hasegawa1, Yoshihiro Ochi1, Tetsuya Kawachi1, Masaharu Nishikino1, Masahiko Ishino1,Takashi Imazono1, Takeshi Kaihori1, Akira Sasaki1, Kota Terakawa2, Yasuo Minami2, Ryota Takei2,Takuro Tomita3, Minoru Yamamoto3, and Tohru Suemoto2.
tn9_1oJi?1U *)1sr51arx@1+1C1X,7(1L
H149>.2%>/2T.23\$K36Ne3MB2ac

Japan Atomic Energy Agency, 8-1-7, Umemidai, Kizugawa, Kyoto 619-0215, Japan;
Department of Ecosystem Engineering, The University of Tokushima, 2-1, Minamijosanjima, Tokushima 770-8506, Japan;
bInstitute of Solid State Physics, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8581, Japan.
1GN&0"bfu^SRwkc2bfE 619-0215 q=LW9;Qh'8-1-7
2A8-2
:2  770-8506 A8`A8;%<8]2-13O-2ZDbfE 277-8581 #j`P;Pj5-1-5We have developed a soft x-ray laser (SXRL) interferometer capable of the single-shot imaging of
nano-scaled structure dynamics. The depth and lateral resolutions of the interferometer were about 1 nmand 1.8 μm, respectively. By using this interferometer, the initial stage (~50 ps) of the ablation process of
the Pt surface pumped by a 70 fs Ti:Sapphire laser pulse was observed. In order to observe the detaileddynamics, the temporal synchronization between the pump and probe pulses was improved to be 3 ps byadopting a portion of the SXRL and pump beams as the time fiducials, to which the pump and probe timing
was adjusted by using the x-ray streak camera.

1. Introduction
The dynamical processes of the femto-second
laser induced surface modifications such as thesub-micron scaled structures (the ripple andbubble structures) [1], attract much attention forthe micro processing by the ultra-short laser pulse.
However the details of the femto-second laser
ablation process have not been understood. The
direct observation of these phenomena is difficult
because they are non-repetitive, irreversible andoccur very rapidly (pico-second) with a verysmall (submicron) feature size.
In this study, we propose the pump and probe
experiment system using the soft x-ray laser(SXRL) as the probe beam to observe thefemto-second laser induced surface dynamics.
The SXRL at the wavelength of 13.9 nm issuitable for the observation of the solid surface
morphology irradiated by ultra-short pulse laser,
because the transmissivity for the high density
plasma locating near the solid surface is high andthe penetration depth for the solid surface is small(< 10 nm).
2. Pump and probe experiment for observationof the fs laser-induced surface dynamics

In order to obtain single-shot image of thetemporal evolution of the nano-scaled structures, apump and probe soft x-ray interferometer has beendeveloped (see in fig. 1). The spatially coherent
SXRL at the wavelength of 13.9 nm [2] isgenerated from the linearly formed nickel-likesilver gain-medium plasmas pumped by the linearly
focused CPA Nd:glass laser. The output energy
and duration of the SXRL were 0.5 J and 7 ps,
respectively.
Ti:Sapphire laser at the centralwavelength of 795 nm with the duration of 70 fs
was used as the pump source for the sample. The
oscillator of Ti:Sapphire laser was synchronized tothat of Nd:glass laser. The interferometer consistsof the reflection optics (Mo/Si multi-layer mirrors
or Pt mirror). The probe SXRL pulse illuminates
the sample with the oblique incidence angle of  =24 deg.

Fig.1. Single shot pump & probe interferometer.

The image of the illuminated area on the sample istransferred to the CCD camera surface by theimaging mirror with the magnification factor ofabout 20. A double Lloyd's mirror is put between
the imaging mirror B and CCD. One of themirrors covers the reflection from the observation
area on the sample, and the other covers that fromanother area used as the reference, and they areoverlapped at the CCD position to makeinterference pattern.
The depth and lateral
resolutions were estimated to be 1 nm and 1.8μm,respectively [3].
The temporal resolution isrestricted by the duration of SXRL pulse (= 7 ps).
We observed the ablation dynamics of Pt (100nm thickness) surface by using this system. Thetypical pump beam energy, fluence and intensity on
the sample surface were 130 μJ, 2 J/cm2 and 3 x
1013 W/cm2, respectively. The focal spot shapewas ellipsoidal (see in fig. 2(d)). Figure 2 shows
the temporal evolution of the interferogram. The
apparent fringe shift to right-ward, implying
positive deviation of the sample, was obtained at t =50 ps. The height at the center of the ablating area
was about 34 nm. The left-ward fringe shift,
implying negative deviation of the sample, wasobtained at t = . The origin of surface dilation issupposed as follows; (i) thermal expansion of solid
Pt film, (ii) the volume expansion associated withthe phase transition from the solid to molten Pt. In
the case of (i), thermal expansion will be 2 nm if we
assume the thermal expansion of the 100 nm Pt
layer associate with the temperature rises fromroom temperature to the melting point. In the caseof (ii), the volume of the melted Pt increases about
8.5 % compared with Pt at room temperature. It
corresponds to the expansion by 8 nm in 100 nm Pt
film.
Thus the surface dilation at the center of
the ablating area cannot be explained neither by thecase of (i) and (ii). This result implies that thedensity under the Pt surface decreased by theformation of the nano-bubbles structures. The
nano-bubble formations are predicted by Monte
Carlo and molecular-dynamics simulations for Si[4]. The time-scale of nano-bubble formations intheir report is consistent with our results.

Fig.2. Temporal revolution of the interferogram andoptical microscope image of the laser induced Pt surface.

3. Real time monitoring system for the temporaladjustment between the pump and probe pulses

In this experiment, the standard deviation oftiming jitter between Ti:Sapphire laser and SXRL
was 10 ps, which was measured by the x-ray streakcamera (HAMAMATSU 4575-01). In order toimprove the precision of the temporal adjustment
between the pump and probe pulses, we constructed
the real time direct monitoring system by adopting
a portion of the Ti:Sapphire laser and SXRL as thetime fiducials. The experimental set up is shownin figure 3(a). The SXRL was spatially divided
into the probe beam and the fiducial beam (fiducial(X)) at the edge of the mirror 1. The Ti:Sapphirelaser was divided into the pump beam and thefiducial beam (fiducial (T)) by the plate beamsplitter. The fiducial beams are measured by thex-ray streak camera in every shot (see in fig.3(b)).
The influence of the timing jitter between the pumpand probe pulse can be removed by this system.
The precision of the temporal adjustment between
the pump and probe pulses was improved to bebetter than 3 ps [5].

Fig.3. Real time pulse monitoring system for thetemporal adjustment between pump and probe pulses.

4. Summary
We have developed a soft x-ray laserinterferometer capable of the single-shot imaging of
the nano-scaled structure dynamics. The surface
dilation speed at the center of the ablating area of
the Pt surface was about 700 m/s (34 nm/50 ps),and it implies that the nano-bubble structures wereformed in the initial stage of the ablation. In
addition, we have improved the temporalsynchronization technique between the pump andprobe pulses by using the fiducial beams. We areplanning to adapt this system to the measurement of
the laser ablation dynamics in the next experiment.
References[1] T. Tomita, et. al.: Jpn. J. Appl. Phys. 45, (2006) pp.
L444.
[2] M. Nishikino, et. al.: Appl. Opt. Vol. 47 Issue 8,(2008) pp.1129.
[3] T. Suemoto, et. al.: Opt. Exp. 18, (2010) 14114.
[4] P. Lorazo, L. J. Lewis and M. Meunier: Phys.Rev. B.
73, (2006) 134108-1.
[5] N. Hasegawa, et. al.,: Proc. of SPIE2011 optics andphotonics, in press.