FUJII Shun

写真a

Affiliation

Faculty of Science and Technology, Department of Physics (Yagami)

Position

Research Associate/Assistant Professor/Instructor

E-mail Address

E-mail address

Telephone No.

045-566-1629

External Links
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Career 【 Display / hide 】

  • 2018.04
    -
    2020.09

    Japan Society for the Promotion of Science, 特別研究員DC1

  • 2020.01
    -
    2020.03

    Max Planck Institute for the Science of Light , Visiting Researcher

  • 2020.10
    -
    2022.08

    RIKEN, RIKEN Center for Advanced Photonics, Special Postdoctoral Researcher

  • 2020.10
    -
    2022.08

    Keio University, Visiting Researcher

  • 2022.09
    -
    Present

    Keio University, Department of Physics, Faculty of Science and Technology, Assistant Professor

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Academic Background 【 Display / hide 】

  • 2012.04
    -
    2016.03

    Keio University, 理工学部, 電子工学科

    University, Graduated

  • 2016.04
    -
    2018.03

    Keio University, 理工学研究科, 総合デザイン工学専攻

    Graduate School, Completed, Master's course

  • 2018.04
    -
    2020.09

    Keio University, 理工学研究科, 総合デザイン工学専攻

    Graduate School, Completed, Doctoral course

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Academic Degrees 【 Display / hide 】

  • Ph.D, Keio University, 2020.09

    Dispersion engineering of high-Q optical microresonators for frequency comb generation

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Research Areas 【 Display / hide 】

  • Natural Science / Semiconductors, optical properties of condensed matter and atomic physics

  • Nanotechnology/Materials / Nanomaterials

  • Nanotechnology/Materials / Optical engineering and photon science

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Research Keywords 【 Display / hide 】

  • ナノフォトニクス

  • 二次元材料

  • 光周波数コム

  • 超高速光技術

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Books 【 Display / hide 】

  • On-chip frequency comb

    T Tetsumoto, S Fujii, T Tanabe, On Chip Photonics Principles Technology and Applications, 2024,  Page： 481

    , Accepted

    　View Summary

    An on-chip frequency comb is a light source on a chip that generates coherent multifrequency light whose frequencies are equidistant in the optical frequency domain. It even suggests the possible realization of chip-scale devices with metrological precision in terms of time and frequency. In particular, a microresonator-based frequency comb is a novel on-chip frequency comb device that exhibits intriguing features as experimental platforms on which to explore unique nonlinear phenomena and tools for practical applications. In this chapter, we first provide an overview of conventional frequency combs and on-chip frequency combs. Then, we describe the fundamentals of microresonator-based frequency combs, their key features, and applications. Finally, we summarize the chapter by discussing various challenges and future perspectives in relation to this emerging on-chip frequency comb.

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Papers 【 Display / hide 】

  • Soliton-comb-based photonic microwave filter operating in the 300 GHz band

    Koya Tanikawa, Mantaro Imamura, Ayaka Yomoda, Soma Kogure, Ryo Sugano, Satoki Kawanishi, Shun Fujii, Takasumi Tanabe

    Photonics Research 13 ( 5 ) 1282 - 1289 2025.05

    Research paper (scientific journal), Joint Work,  ISSN  23279125

    　View Summary

    We present a 300 GHz photonic filter based on a soliton microcomb, addressing the increasing demand for high-frequency, low-phase-noise signals in THz-band wireless communications. Utilizing a uni-traveling-carrier photodiode (UTC-PD), our configuration enables the generation of 300 GHz waves through beat signals from adjacent longitudinal modes of the microcomb. By combining the UTC-PD with a photonic filter approach, we demonstrate that RF photonic filters with high suppression ratios are achieved in the 300 GHz band. Additionally, by mitigating the effects of higher-order dispersion, we achieve a stopband rejection exceeding 30 dB. These results underscore the potential of our photonic filter for advanced THz applications and provide a promising pathway for low-noise, high-bandwidth wireless communication systems.

  • Ultrafast movies of the photoinduced phonon and magnon propagation using dual frequency-comb technology

    Daichi Nishikawa, Kazuki Maezawa, Riku Shibata, Shun Fujii, Shinichi Watanabe

    Optics Letters 50 ( 6 ) 1929 - 1932 2025.03

    Research paper (scientific journal), Joint Work,  ISSN  01469592

    　View Summary

    The interaction between surface acoustic waves (SAWs) and magnons is an interesting phenomenon that can be used to control spins in spintronic devices, and thus the visualization of the related spatiotemporal dynamics can be useful. In this study, we used dual-frequency-comb-based asynchronous optical sampling to record the spatiotemporal evolution of the surface vibration associated with an optically excited SAW and that of the spin precession associated with the magnon under the same excitation condition at effectively 160 frames/ns. Isotropic and anisotropic propagation were observed for phonons and magnons, respectively, demonstrating the capability of measuring such dynamics with high resolution.

  • Scalable fabrication of erbium-doped high-Q silica microtoroid resonators via sol-gel coating

    Riku Imamura, Shun Fujii, Keigo Nagashima, Takasumi Tanabe

    Optics Continuum 4 ( 3 ) 512 - 521 2025.03

    Research paper (scientific journal), Joint Work

    　View Summary

    This study explores sol-gel methods for fabricating erbium-doped silica microtoroid resonators, addressing the limitations of conventional doping techniques and enhancing device scalability. We develop a reproducible sol-gel process that yields defect-free films for photonic applications and detail common defects and troubleshooting strategies. Two fabrication methods are compared: traditional film deposition on substrates and the direct coating of prefabricated resonators. The latter enables the fabrication of larger resonator diameters (up to 450 pm) without buckling while achieving a high-g factor and a low lasing threshold of 350 pW. These erbium-doped resonators exhibit multi-mode laser oscillations at 1550 nm, revealing the sol-gel method’s potential for realizing scalable, gain-doped photonic devices.

  • Field trial of optical transmission experiment employing a microresonator frequency comb light source for low-latency, short-reach optical communication

    K Tanikawa, S Fujii, S Kogure, S Tanaka, S Tasaka, K Wada, S Kawanishi, ...

    IEICE Transactions on Electronics E108.C ( 3 ) 146 - 152 2025.03

    Research paper (scientific journal), Joint Work,  ISSN  09168524

    　View Summary

    We present first-time demonstration of short-reach and low-latency optical communication within a real network, employing a microresonator frequency comb as a light source. The modulated signal is transmitted through a 9-km single-mode optical fiber installed in a metropolitan network. This demonstration paves the way for realizing low-latency massively parallel optical communication, which is the key to beyond-5G and 6G network. For a proof-of-concept experiment, we employ an MgF<inf>2</inf> crystalline microresonator with a 20-GHz free-spectral range that could be used for dense wavelength division multiplexing communication. We generated a stable soliton comb and modulated it with simple 10-Gbps intensity modulation and direct detection to achieve a small excess delay of 3.1 µs.

  • Gyro-spintronic material science using vorticity gradient in solids

    Y Nozaki, H Sukegawa, S Watanabe, S Yunoki, T Horaguchi, ...

    Science and Technology of Advanced Materials 26 ( 1 )  2025.02

    Research paper (scientific journal), Joint Work,  ISSN  14686996

    　View Summary

    We present a novel method for generating spin currents using the gyromagnetic effect, a phenomenon discovered over a century ago. This effect, crucial for understanding the origins of magnetism, enables the coupling between various macroscopic rotational motions and electron spins. While higher rotational speeds intensify the effect, conventional mechanical rotations, typically, below 10,000 RPM, produce negligible results comparable to geomagnetic fluctuations, limiting applied research. Our studies demonstrate that spin current generation comparable to that of rare metals can be achieved through atomic rotations induced by GHz-range surface acoustic waves and the rotational motion of conduction electrons in metallic thin films with nanoscale gradient modulation of electrical conductivity. These effects, termed the acoustic gyromagnetic effect and the current-vorticity gyromagnetic effect, are significant in different contexts. The acoustic gyromagnetic effect is notable in high-conductivity materials like aluminum and copper, which are more abundant than conventional spintronics materials with strong spin–orbit interactions (SOIs). Conversely, the current-vorticity gyromagnetic effect requires a large conductivity gradient to produce current vorticity efficiently. This is achieved by using composition gradient structures from highly conductive metals to poorly conductive oxides or semiconductors. Consequently, unlike traditional strong-SOI materials, we can create highly efficient spin current generators with low energy dissipation due to reduced Joule loss.

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Papers, etc., Registered in KOARA 【 Display / hide 】

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Reviews, Commentaries, etc. 【 Display / hide 】

  • 機械駆動による高繰り返しマイクロコムの実現

    藤井　瞬

    応用物理 94 ( 7 ) 390 - 394 2025

    Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media), Single Work, Lead author, Corresponding author

  • ソリトンマイクロコムの基礎と応用

    藤井　瞬

    レーザー研究 52 ( 11 ) 572 - 576 2024

    Article, review, commentary, editorial, etc. (scientific journal), Single Work, Lead author, Corresponding author

  • 集積型光周波数コム光源とシリコンフォトニクス

    菅野 凌，藤井 瞬，木暮蒼真，ヌル・アシキン・ビンディ・ダウド，田邉孝純

    光アライアンス 34 ( 2 ) 54 - 58 2024

    Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media), Joint Work

  • 超並列光通信のための集積光周波数コム光源

    [5] 木暮蒼真，藤井 瞬，田邉孝純

    レーザー研究 50 ( 5 ) 248 - 253 2022

    Article, review, commentary, editorial, etc. (scientific journal), Joint Work

  • 超精密機械加工技術が拓くマイクロ周波数コム応用

    藤井　瞬

    フォトニクスニュース 7 ( 2 ) 60 - 64 2021

    Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media), Single Work, Lead author, Corresponding author

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Research Projects of Competitive Funds, etc. 【 Display / hide 】

  • 異種機能素子を活用した高繰り返し光周波数コム光源の深化

    2024.04
    -
    2026.03

    日本学術振興会, 科学研究費助成事業, 若手研究, Principal investigator

  • Dynamic interface engineering of atomically-thin two-dimensional meterials by ultrafast optical measurement

    2024.04
    -
    2026.03

    日本学術振興会, 科学研究費助成事業, 学術変革領域研究(A), Principal investigator

  • 超高Q値微小光共振器を用いた二次元材料の非線形光学効果の発現

    2022.04
    -
    2024.03

    MEXT,JSPS, Grant-in-Aid for Scientific Research, 若手研究, Principal investigator

  • 微小光共振器による可視光マイクロコム発生に関する研究

    2018.04
    -
    2021.03

    日本学術振興会, 科学研究費助成事業 特別研究員奨励費, 特別研究員奨励費, No Setting

    　View Summary

    微小光共振器を用いた光周波数コム光源であるマイクロコムを可視光帯で実現することを目的としている．マイクロコムとは高Q値微小光共振器を用いて高効率に発生できる四光波混合から得られる，周波数軸上で等間隔な櫛状の光を指す．
    通信波長帯よりも短波長である可視光帯ではレイリー散乱の影響を受けやすいため，光の閉じ込め性能指標であるQ値が低下しやすい．また，マイクロコムの発生には高度な分散制御が必要とされることが知られており，Q値と分散制御を高度に両立する必要がある．そのための方策として超精密機械加工を用いることで，世界で初めて分散制御されたQ値10^8を超える微小光共振器の作製を実現した．その過程において，フッ化マグネシウムおよびフッ化カルシウムの切削加工条件を理論と実験の両面から調査した．ここで得られた知見をもとに機械切削加工を行うことで，今後微小光共振器の作製がより再現性高く可能になることが期待される．
    また，フッ化マグネシウム共振器のソリトンモード同期化とその長期安定化を達成した．モード同期は各縦モード間の位相が同期した状態であり，マイクロコムの応用において特に重要である．そこでレーザー周波数安定化に広く用いられるPound-Drever-Hall法を利用したフィードバック安定化機構を開発した．装置の都合上，実証は通信波長帯で行ったが原理的には可視光帯でも利用可能である．これらの研究成果は精密機械加工と高Q値微小光共振器作製の親和性の高さを初めて示すものであり，これからの研究進展が期待される．

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Intellectual Property Rights, etc. 【 Display / hide 】

  • 光共振器、及びこれを用いた光パルス発生装置

    Date applied： 特願2020-002729  2020.01 

    Date announced： 特開2021-110835  2021.08 

    Date issued： 第7409642号 

    Patent, Joint

  • 電磁波発生装置、及び無線送信装置

    Date applied： 特願2023-002959   

    Patent

  • 光共振器及び光パルス発生器

    Date issued： 特願2025-7081 

    Patent

  • 発信機及び発信方法

    Date applied： 特願2024-57764   

    Patent

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Awards 【 Display / hide 】

  • 理研桜舞賞

    2025.03, 理化学研究所

    Type of Award： Other

  • 第40回井上研究奨励賞

    2023.12, 井上科学振興財団

    Type of Award： Award from publisher, newspaper, foundation, etc.

  • 工作機械技術振興賞・論文賞

    葉山優花, 藤井瞬, 田邉孝純, 柿沼康弘, 2023.06, 工作機械技術振興財団, Theoretical approach on the critical depth of cut of single crystal MgF2 and its application to a microcavity

    Type of Award： Award from Japanese society, conference, symposium, etc.

  • Paper Presentation Encouragement Award

    2022.05, The Laser Soceity of Japan, 高Q値微小光共振器による高繰り返しソリトンマイクロコムの発生と精密制御

    Type of Award： Award from Japanese society, conference, symposium, etc.

  • Fujiwara Award

    2021.03, Keio University FUJIWARA Foundation

    Type of Award： Keio commendation etc.

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Courses Taught 【 Display / hide 】

  • PHYSICS LABORATORIES 2

    2025

  • PHYSICS LABORATORIES 1

    2025

  • PHYSICS A

    2025

  • LABORATORIES IN SCIENCE AND TECHNOLOGY

    2025

  • PHYSICS SEMINAR 3

    2024

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Media Coverage 【 Display / hide 】

  • 切削で最高性能の光共振器

    日経産業新聞, 2020.06

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Memberships in Academic Societies 【 Display / hide 】

  • 日本光学会, 

    2025.04
    -
    Present

  • レーザー学会, 

    2019.05
    -
    Present

  • 応用物理学会, 

    2019.05
    -
    Present

  • The Optical Society, 

    2016.01
    -
    Present
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Committee Experiences 【 Display / hide 】

  • 2024.08
    -
    Present

    Applied Physics Letters Early Career Editorial Advisory Board, AIP Publishing

  • 2024.06
    -
    Present

    「レーザー研究」編集委員, レーザー学会

  • 2024.04
    -
    Present

    応用物理学会フォトニクス分科会 幹事

  • 2023.04
    -
    Present

    KOGAKU Editorial committee, The Optical Society of Japan

  • 2021
    -
    Present

    応用物理学会ナノカーボン若手チャプター 幹事

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