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Kumada, Takayuki; Otobe, Tomohito; Nishikino, Masaharu; Hasegawa, Noboru; Hayashi, Terutake*
Applied Physics Letters, 108(1), p.011102_1 - 011102_4, 2016/01
Times Cited Count：11 Percentile：48.05(Physics, Applied)The dynamics of photomechanical spallation during femtosecond laser ablation of fused silica was studied by time-resolved reflectivity with double pump pulses. Oscillation of reflectivity was caused by interference between the probe pulses reflected at the sample surface and the spallation layer, and was enhanced when the surface is irradiated with the second pump pulse within a time interval of several picoseconds after the first pump pulse. However, as the time-interval was increased, the oscillation amplitude decreased with an exponential decay time of 10 ps. The oscillation disappeared when the interval exceeded 20 ps. This result suggests that the formation time of the spallation layer is approximately 10 ps.
Zherebtsov, S.*; Maekawa, Katsuhiro*; Hayashi, Terutake*; Futakawa, Masatoshi
JSME International Journal, Series A, 48(4), p.292 - 298, 2005/10
The effect of temperature on the structure and properties of the type 316 stainless steel alloyed with Al-Si has been reported in the present paper. It has been found that four different types of structure are formed in the alloyed zone depending on the temperature of the substrate. These structures differ from each other in phase composition, microhardness and relation to cracking. Hard, crack-free microstructures are formed at temperatures of about 350 and 750 
C. Maintaining the temperature of the sample at 350 C a uniform, crack-free layer with a high hardness is produced by laser alloying with an energy density of 0.76 W/mm
.
Kumada, Takayuki; Otobe, Tomohito; Nishikino, Masaharu; Hasegawa, Noboru; Hayashi, Terutake*
no journal, ,
Dynamics of the formation of a spallation layer during femtosecond laser ablation of fused silica has been studied using time-resolved reflectivity with double pump pulses. Oscillation of the reflectivity, which is due to interference between the reflected probe pulses at the bulk sample surface and the spallation layer, is enhanced when the second pump pulse is irradiated within the time interval, 
s, of a few picoseconds after the first pump pulse. However, as s increases, the oscillation amplitude decreases with an exponential decay time of 10 ps. This result suggests that it takes approximately 10 ps for formation of the spallation layer.
Kumada, Takayuki; Otobe, Tomohito; Nishikino, Masaharu; Hasegawa, Noboru; Hayashi, Terutake*
no journal, ,
We studied femtosecond laser ablation dynamics using time-resolved reflectivity with double pump pulses. We found that the time-scale of spallation was comparable to the theoretical estimation of stress relaxation time, which corresponds to the penetration depth of the pump laser divided by sound velocity.
Kumada, Takayuki; Otobe, Tomohito*; Nishikino, Masaharu*; Hasegawa, Noboru*; Hayashi, Terutake*
no journal, ,
The non-thermal effect on femtosecond laser ablation has been focused on for development of high-precision micromachining technique. In this study, we newly focused on a time-resolved grazing-incidence small-angle X-ray scattering (GI-SAXS) measurement. In principle, the GI-SAXS can measure time-evolution of the morphology of the voids, which would be stretched in normal to the sample surface by the non-thermal effect. We can quantitatively evaluate the non-thermal effect from the asymmetric morphology of the voids. We succeeded in observing the scattering from a gold-deposited optical grating (1740 lines / mm), but not from a post-ablated crater on a gold plate. We have to decrease the noise level of the camera to pick up dispersed signal from disordered system such as the crater.
