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• Pulse-to-pulse ultrafast dynamics of highly photoexcited Ge₂Sb₂Te₅ thin films

  Kobayashi Masataka, Arashida Yusuke, Asakawa Kanta, Kaneshima Keisuke, Kuwahara Masashi, Konishi Ku … 全著者表示

  Japanese Journal of Applied Physics   62 ( 2 )   022001   2023年2月

  DOI Web of Science CiNii Research

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  記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：IOP Publishing   共著  

  Observing ultrafast pulse-to-pulse dynamics of highly photoexcited materials could foster a comprehensive understanding of the initial stage of irreversible photoinduced events, such as phase change, structural deformation, and laser ablation. In this study, using high-repetition-rate single-shot spectroscopy and a laser microscope, the pulse-to-pulse ultrafast dynamics of energy relaxation in Ge2Sb2Te5 thin films are revealed under high-density photoexcitation that induces sequential events involving the crystalline-to-amorphous phase transition, melt and quench processes, and formation of laser-induced periodic surface structures (LIPSSs). Above the threshold excitation density for LIPSS formation, the first excitation pulse induces the transient transmittance change of the crystalline phase in a picosecond timescale, and subsequent pulses provoke the amorphous phase energy relaxation with a long decay time of hundreds of picoseconds. We observed that the subsequent pulses gradually reduce the amplitude and decay time of the transient transmittance, leading to efficient energy relaxation and LIPSS formation in the photoinduced amorphous phase.

• Terahertz-Field-Driven Scanning Tunneling Luminescence Spectroscopy

  Kimura, K; Morinaga, Y; Imada, H; Katayama, I; Asakawa, K; Yoshioka, K; Kim, Y; Takeda, J

  ACS PHOTONICS   8 ( 4 )   982 - 987   2021年4月

  DOI Web of Science

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  記述言語：日本語   掲載種別：研究論文（学術雑誌）   共著  

• テラヘルツ電場駆動トンネル電子により誘起されるSTM発光の観測

  木村 謙介, 森永 悠太, 今田 裕, 片山 郁文, 浅川 寛太, 吉岡 克将, 金 有洙, 武田 淳

  日本表面真空学会学術講演会要旨集   2021 ( 0 )   2Cp05R   2021年

  DOI CiNii Research

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  記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：公益社団法人 日本表面真空学会   共著  

  <p>テラヘルツ(THz)パルスの光電場により超高速に走査トンネル顕微鏡(STM)を駆動させるTHz-STMとSTMのトンネル電流により誘起される発光を検出するSTM発光分光法を組み合わせることで、新規なTHz-STM発光分光法を開発した。本研究では、THz-STM発光分光法を用いて、探針とAg(111)基板間のトンネルギャップに形成される局在プラズモンからの発光を観測することに成功したので報告する。</p>

• Nanoscale phase change on Ge2Sb2Te5 thin films induced by optical near fields with photoassisted scanning tunneling microscope

  Asakawa Kanta, Kim Dang-il, Yaguchi Shotaro, Mikito Tsujii, Katsumasa Yoshioka, Kaneshima Keisuke, … 全著者表示

  Applied Physics Letters   117 ( 21 )   211102   2020年11月

  DOI Web of Science CiNii Research

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  記述言語：英語   掲載種別：研究論文（学術雑誌）   出版者・発行元：American Institute of Physics   共著  

  A scanning probe microscope coupled with either femtosecond laser pulses or terahertz pulses holds great promise not only for observing ultrafast phenomena but also for fabricating desirable structures at the nanoscale. In this study, we demonstrate that a few-nanometer-scale phase change can be non-thermally stored on the Ge2Sb2Te5 surface by a laser-driven scanning tunneling microscope (STM). An atomically flat Ge2Sb2Te5 surface was irradiated with the optical near-field generated by introducing femtosecond laser pulses to the STM tip-sample junction. The STM topographic images showed that few-nanometer-scale mounds appeared after irradiation. In addition, tunneling conductance spectra showed that the bandgap increased by 0.2 eV in the area of 5 × 5 nm2. These indicate that the nanoscale crystal-to-amorphous phase change was induced by the STM-tip-induced near field. Our approach presented here offers an unprecedented increase in the recording density of optical storage devices and is, therefore, expected to facilitate the development of next-generation information technology.

• 特集「レーザー冷却，光格子時計に関連する表面科学と真空科学」企画趣旨

  大野 真也, 浅川 寛太

  表面と真空   63 ( 10 )   512 - 513   2020年10月

  DOI CiNii Research

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  記述言語：日本語   掲載種別：研究論文（学術雑誌）   出版者・発行元：公益社団法人 日本表面真空学会   共著  

  <p>In recent years, atomic clocks have been widely recognized as a key technology in global positioning system (GPS) and other sensing systems. State-of-the-art clocks such as optical lattice clocks showed outstanding stability and accuracy, which will possibly be used for the new definition of time in the near future. This special issue introduces recent research topics on laser cooling methods and optical lattice clocks. Evidently, all the contributing articles exhibit some association with our core research fields, that is, surface science and vacuum science. Therefore, we expect that fruitful communications and collaborations will be promoted by these articles, and hopefully a new research area would be discovered.</p>

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• Development of Radio-Frequency Scanning Tunneling Microscopy for the Detection of Ferromagnetic Resonance

  F. Oguro, K. Asakawa, H.-H. Yang ,Y. Hasegawa

  ACSIN-14 & ICSPM26  

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  開催年月日： 2018年10月

  記述言語：英語   会議種別：口頭発表（一般）  

  開催地：Sendai, Japan  

  F. Oguro, K. Asakawa, H.-H. Yang and Y. Hasegawa, "Development of Radio-Frequency Scanning Tunneling Microscopy for the Detection of Ferromagnetic Resonance", ACSIN-14 & ICSPM26 (21-25 October 2018, Sendai, Japan).

• Hydrogen-Induced Electronic States Modulation on Fe3O4 (111) Surfaces

  K. Asakawa, N. Nagatsuka, Y. Miura, K. Fukutani

  ACSIN-14 & ICSPM26  

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  開催年月日： 2018年10月

  記述言語：英語   会議種別：口頭発表（一般）  

  開催地：Sendai, Japan  

  K. Asakawa, N. Nagatsuka, Y. Miura, and K. Fukutani, "Hydrogen-Induced Electronic States Modulation on Fe3O4 (111) Surfaces", ACSIN-14 & ICSPM26 (21-25 October 2018, Sendai, Japan).

• Defect-Induced Superstructure on SnSe Surfaces

  K. Asakawa, F. Oguro, Y. Yoshida, H. Sakai, N. Hanasaki, Y. Hasegawa

  ACSIN-14 & ICSPM26  

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  開催年月日： 2018年10月

  記述言語：英語   会議種別：口頭発表（一般）  

  開催地：Sendai, Japan  

  K. Asakawa, F. Oguro, Y. Yoshida, H. Sakai, N. Hanasaki, and Y. Hasegawa, "Defect-Induced Superstructure on SnSe Surfaces", ACSIN-14 & ICSPM26 (21-25 October 2018, Sendai, Japan).
  https://dora.bk.tsukuba.ac.jp/event/acsin14/