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Kitazato, Kohei*; Milliken, R. E.*; Iwata, Takahiro*; Abe, Masanao*; Otake, Makiko*; Matsuura, Shuji*; Takagi, Yasuhiko*; Nakamura, Tomoki*; Hiroi, Takahiro*; Matsuoka, Moe*; et al.
Nature Astronomy (Internet), 5(3), p.246 - 250, 2021/03
Times Cited Count：43 Percentile：96.93(Astronomy & Astrophysics)Here we report observations of Ryugu's subsurface material by the Near-Infrared Spectrometer (NIRS3) on the Hayabusa2 spacecraft. Reflectance spectra of excavated material exhibit a hydroxyl (OH) absorption feature that is slightly stronger and peak-shifted compared with that observed for the surface, indicating that space weathering and/or radiative heating have caused subtle spectral changes in the uppermost surface. However, the strength and shape of the OH feature still suggests that the subsurface material experienced heating above 300 
C, similar to the surface. In contrast, thermophysical modeling indicates that radiative heating does not increase the temperature above 200 C at the estimated excavation depth of 1 m, even if the semimajor axis is reduced to 0.344 au. This supports the hypothesis that primary thermal alteration occurred due to radiogenic and/or impact heating on Ryugu's parent body.
Kitazato, Kohei*; Milliken, R. E.*; Iwata, Takahiro*; Abe, Masanao*; Otake, Makiko*; Matsuura, Shuji*; Arai, Takehiko*; Nakauchi, Yusuke*; Nakamura, Tomoki*; Matsuoka, Moe*; et al.
Science, 364(6437), p.272 - 275, 2019/04
Times Cited Count：259 Percentile：99.73(Multidisciplinary Sciences)The near-Earth asteroid 162173 Ryugu, the target of Hayabusa2 sample return mission, is believed to be a primitive carbonaceous object. The Near Infrared Spectrometer (NIRS3) on Hayabusa2 acquired reflectance spectra of Ryugu's surface to provide direct measurements of the surface composition and geological context for the returned samples. A weak, narrow absorption feature centered at 2.72 micron was detected across the entire observed surface, indicating that hydroxyl (OH)-bearing minerals are ubiquitous there. The intensity of the OH feature and low albedo are similar to thermally- and/or shock-metamorphosed carbonaceous chondrite meteorites. There are few variations in the OH-band position, consistent with Ryugu being a compositionally homogeneous rubble-pile object generated from impact fragments of an undifferentiated aqueously altered parent body.
Baba, Motoyoshi*; Nishikino, Masaharu; Hasegawa, Noboru; Tomita, Takuro*; Minami, Yasuo*; Takei, Ryota*; Yamagiwa, Mitsuru; Kawachi, Tetsuya; Suemoto, Toru
Japanese Journal of Applied Physics, 53(8), p.080302_1 - 080302_4, 2014/08
Times Cited Count：7 Percentile：30.85(Physics, Applied)A grazing incidence reflection-type soft X-ay microscope, using a Fresnel zone plate and a soft X-ray laser with wavelength 13.9 nm and pulse width 7 ps, was developed. Submicron size groove structures made on a Pt film were clearly captured at a single shot exposure, with spatial resolution of about 360 nm. A wide field view of 100 
m square was secured under the Kohler illumination. This microscope also had a large depth of focus of more than 100 m and was proven to have a sufficient performance for observing surface morphological changes.
Tomita, Takuro*; Nishikino, Masaharu; Hasegawa, Noboru; Minami, Yasuo*; Takei, Ryota*; Baba, Motoyoshi*; Eyama, Tsuyoshi*; Takayoshi, Shodai*; Kaihori, Takeshi*; Morita, Toshimasa; et al.
Journal of Laser Micro/Nanoengineering, 9(2), p.137 - 142, 2014/06
Times Cited Count：5 Percentile：29.55(Nanoscience & Nanotechnology)Femtosecond laser ablation processes on platinum, gold, and tungsten were observed by the single shot pump and probe reflective imaging using a soft X-ray laser probe. To avoid the timing error due to the jitter, we adopted a posteriori correction technique by simultaneous measurement of timing between the pump and probe pulses for every single shot, using a soft X-ray streak camera. A clear difference was found in the temporal behavior of the dynamical response of the soft X-ray reflectivity depending on the irradiated laser fluence in these three materials. On the other hand, the narrow dark rings were found in Pt and W, while an additional bright ring was found outside the dark disk in Au. Our result gives the experimental data comparable with various numerical simulations.
Nishikino, Masaharu; Hasegawa, Noboru; Ishino, Masahiko; Yamagiwa, Mitsuru; Kawachi, Tetsuya; Minami, Yasuo*; Terakawa, Kota*; Takei, Ryota*; Baba, Motoyoshi*; Suemoto, Toru; et al.
JAEA-Conf 2013-001, p.16 - 19, 2013/09
The dynamics of the femtosecond laser ablation is still not clear at the early stage. Several studies on the time-resolved imaging of femtosecond laser ablation process have been performed on various materials. In this study, we have demonstrated a pump and probe imaging of the metal surface morphology during the femtosecond laser ablation by using the laser-driven plasma induced soft X-ray laser (SXRL) as a probe beam. The pumping laser used for ablation was a Ti:Sapphire laser system based on chirped pulse amplification. The pump beam had a Gaussian profile and the focal spot size on the metal surface was about 70 micron at FWHM. The dependence for the pump laser fluence of the ablation surface expansion and the surface condition (roughness and density gradient) were observed from the soft X-ray interferogram and reflective image, respectively.
Nishikino, Masaharu; Hasegawa, Noboru; Tomita, Takuro*; Minami, Yasuo*; Takei, Ryota*; Baba, Motoyoshi*; Eyama, Tsuyoshi*; Takayoshi, Shodai*; Kawachi, Tetsuya; Hatomi, Daiki*; et al.
Proceedings of SPIE, Vol.8849, p.88490E_1 - 88490E_6, 2013/09
Times Cited Count：2 Percentile：73.12(Optics)We have developed the pump and probe interferometer and reflective imaging technique of the metal surfaces during the femtosecond laser ablation by using the laser-driven soft X-ray laser at the wavelength of 13.9 nm. The pumping laser used for ablation was a Ti: Sapphire laser pulse with the duration of 80 fs pulse at a central wavelength of 795 nm, and had a gaussian spatial profile. By using the X-ray imaging technique, the time resolved image of nano-scaled ablation dynamics of the platinum and the gold pumped by a femtosecond laser pulse was obtained.
Takei, Masanobu; Katanishi, Shoji; Kunitomi, Kazuhiko; Izumiya, Toru*
Nihon Genshiryoku Gakkai Wabun Rombunshi, 2(4), p.490 - 499, 2003/12
no abstracts in English
Katanishi, Shoji; Kunitomi, Kazuhiko; Takei, Masanobu; Nakata, Tetsuo; Watanabe, Takashi*; Izumiya, Toru*
Nihon Genshiryoku Gakkai Wabun Rombunshi, 1(4), p.373 - 383, 2002/12
no abstracts in English
Takei, Masanobu; Katanishi, Shoji; Nakata, Tetsuo; Oda, Takefumi*; Izumiya, Toru*; Kunitomi, Kazuhiko
JAERI-Tech 2002-089, 44 Pages, 2002/11
JAERI-Tech-2002-089.pdf:3.35MB
no abstracts in English
Yamauchi, Toshihiko; Takemoto, Ryo*; Yabuno, Masahiro*; Kanno, Yoshinori*; Kobayashi, Seiji*; Shiramizu, Miho*; Takei, Toru*; Kato, Hatsuhiro*; Nakagaki, Keita*
no journal, ,
no abstracts in English
Nishikino, Masaharu; Yamamoto, Minoru*; Hasegawa, Noboru; Tomita, Takuro*; Terakawa, Kota*; Minami, Yasuo*; Takei, Ryota*; Onishi, Ryo*; Ishino, Masahiko; Kaihori, Takeshi; et al.
no journal, ,
The X-ray reflective imaging using a soft X-ray laser was used to observe the early stage of the 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 the strongly 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.
Hasegawa, Noboru; Nishikino, Masaharu; Ochi, Yoshihiro; Kaihori, Takeshi; Ishino, Masahiko; Imazono, Takashi; Sasaki, Akira; Yamagiwa, Mitsuru; Kawachi, Tetsuya; Suemoto, Toru; et al.
no journal, ,
We 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 nm and 1.8 micron, 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 detailed dynamics, the temporal synchronization between the pump and probe pulses was improved to be 3 ps by adopting 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.
Nishikino, Masaharu; Hasegawa, Noboru; Tomita, Takuro*; Minami, Yasuo*; Takei, Ryota*; Onishi, Ryo*; Ishino, Masahiko; Yamamoto, Minoru*; Terakawa, Kota*; Kaihori, Takeshi; et al.
no journal, ,
To improve the femtosecond laser processing technique more accurate and controllable, the knowledge about the laser ablation process is important. In order to understand the dynamics of the femtosecond laser ablation process, we performed a pump and probe reflectivity imaging of the platinum surface during the femtosecond laser ablation by using the laser-driven soft X-ray laser as a probe beam. 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 the strongly excited region.
Nishikino, Masaharu; Hasegawa, Noboru; Tomita, Takuro*; Minami, Yasuo*; Takei, Ryota*; Ishino, Masahiko; Yamamoto, Minoru*; Terakawa, Kota*; Kaihori, Takeshi; Kawachi, Tetsuya; et al.
no journal, ,
In order to understand the dynamics of the femtosecond laser ablation process, we performed a pump and probe reflectivity imaging of the platinum surface during the femtosecond laser ablation by using the laser-driven soft X-ray laser as a probe beam. 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 the strongly 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.
Hasegawa, Noboru; Nishikino, Masaharu; Kaihori, Takeshi; Kawachi, Tetsuya; Yamagiwa, Mitsuru; Yamamoto, Minoru*; Tomita, Takuro*; Minami, Yasuo*; Takei, Ryota*; Suemoto, Toru
no journal, ,
We have developed the femto-second laser pump and soft X-ray laser probe system in order to observe the dynamical processes of the femto-second laser ablation. By using this system, we succeed to obtain the temporal evolution of the soft X-ray reflectivity from the laser induced Pt surface. The results lead that the rate of decrease in the reflectivity of the probe beam has a non-linear relation with the pump laser fluence.
Hasegawa, Noboru; Kawachi, Tetsuya; Nishikino, Masaharu; Ochi, Yoshihiro; Terakawa, Kota*; Takei, Ryota*; Minami, Yasuo*; Baba, Motoyoshi*; Suemoto, Toru; Yamagiwa, Mitsuru
no journal, ,
We developed the soft X-ray laser interferometer synchronized with Ti:Sapphire laser pulse for the measurement of the nanometer-scaled surface dynamics. The soft X-ray laser is suitable for probing the initial process of the surface morphological changes, because it has the suitable wavelength (= 13.9 nm), short duration (= 7 ps) and large photon numbers. By using this system, we succeed to obtain the temporal evolution of ablation front erupting from the laser induced Pt film. From this result, it was revealed that the expansion speed of the ablation front decreased continually from the center (= 2200 m/s) to the edge (= 420 m/s) of the ablation area in the initial stage of the ablation process.
Hasegawa, Noboru; Nishikino, Masaharu; Yamagiwa, Mitsuru; Kawachi, Tetsuya; Suemoto, Toru; Tomita, Takuro*; Baba, Motoyoshi*; Terakawa, Kota*; Minami, Yasuo*; Takei, Ryota*
no journal, ,
The dynamical processes of the laser-induced surface modifications such as laser ablation come to attract much attention for the micro processing by the ultra-short laser pulse. The soft X-ray laser (SXRL) is suitable for probing the initial process of the surface morphological changes, because it has the suitable wavelength (= 13.9 nm, penetration depth = 5 nm), short duration (= 7 ps) and large photon numbers (larger than 1e+10 photons/shot). In this study, we have developed the femto-second laser pump and soft X-ray laser probe system with the soft X-ray interferometer in order to observe the dynamical processes of the femto-second laser ablation. By using this system, we succeed to obtain the temporal evolution of the laser induced Pt surface. From this result, it was revealed that the expansion speed of the ablation front at the center (= 1500 m/s) of the ablation area in the initial stage of the ablation process.
Nishikino, Masaharu; Hasegawa, Noboru; Tomita, Takuro*; Takei, Ryota*; Baba, Motoyoshi*; Kawachi, Tetsuya; Yamagiwa, Mitsuru; Suemoto, Toru
no journal, ,
no abstracts in English
Nishikino, Masaharu; Hasegawa, Noboru; Tomita, Takuro*; Takei, Ryota*; Baba, Motoyoshi*; Kawachi, Tetsuya; Yamagiwa, Mitsuru; Suemoto, Toru
no journal, ,
no abstracts in English
Nishikino, Masaharu; Hasegawa, Noboru; Tomita, Takuro*; Minami, Yasuo*; Takei, Ryota*; Ishino, Masahiko; Terakawa, Kota*; Eyama, Tsuyoshi*; Takayoshi, Shodai*; Kawachi, Tetsuya; et al.
no journal, ,
no abstracts in English
