Research

We study ultrafast phenomena in solids through the development of novel femtosecond measurement technology and light sources. 

Below are the topics I studied so far：

1.  Control of antiferromagnetic spin systems using terahertz radiation 

We explored the nonlinear dynamics of strongly driven spin systems in antiferromagnets using high-intensity THz radiation and plasmonic techniques. 

Selected publications：

1. • • Phys. Rev. Lett. 120, 107202 (2018) ：Using intense THz radiation, we show that resonantly driven coherent magnons can break the symmetry of phase transition pathways in the spin reorientation transition of orthoferrite ErFeO3. In the context of THz magnonics, we believe that it was one of the earliest to demonstrate the macroscopic control of magnetisation by THz magnetic fields. (ISSP, The University of Tokyo) 

     • Commun Phys 6, 51, 1–6 (2023) ：Demonstrated that the second harmonic dynamics of the magnon mode can be excited by intense THz radiation and can be observed efficiently by using magneto-birefringence effect. (ISSP, The University of Tokyo) 

     • Nature Physics (2024). https://doi.org/10.1038/s41567-024-02386-3. ：Observed nonlinear coupling between distinct magnon modes using two-dimensional THz spectroscopy. (Collaboration with MIT) 

     • Nature Materials 1 (2024), https://doi.org/10.1038/s41563-024-02034-4  ：Achieved ultrafast magnetization switching in a magnetic material using ultra-intense THz magnetic fields. (Collaboration with Kyoto University) 

2. Stochastic fluctuation dynamics of spin systems at ultrafast time scales

We have pioneered and demonstrated experimental techniques to observe stochastic spin dynamics (spin noise) on the femto- to picosecond timescale. 

Selected  Publications：

2. • • Nature Communications 14, 7651 (2023) ：By statistically analyzing polarization noise induced by femtosecond probe pulses interacting with magnetic materials, we directly revealed the autocorrelation function of thermally excited spin fluctuations at room temperature. This is among the first studies to reveal stochastic dynamics of solid-state excitations at ultrafast timescales. (University of Konstanz) 

     • Review of Scientific Instruments 95, 083005 (2024) ：A technical paper detailing the experimental setup of femtosecond noise correlation spectroscopy using subharmonic lock-in method, including theoretical background. (Collaborative work with University of Konstanz)

     •  arXiv:2501.17531. ：Developed a methodology to quantitatively evaluate the femtosecond noise correlation signals. (Under review at Phys. Rev. Applied) 

3. R&D of ultrashort-pulse laser sources

We also develop few-cycle laser sources for the nonlinear optical experiments such as THz, MIR and attosecond sciences.

Selected publications：

3. • • Optica 5, Issue 11, pp. 1474-1477 (2018) ：Irradiation of PW laser pulses on metallic film causes strong radiation of THz transient. By symmetrically collecting and focusing them onto a EOS crystal, we showed that a longitudinally (Z-) polarized THz pulse can be realized. (Collaborative work with University of Jena) 

     • Optics Letters 47, 3552-3555 (2022).  ：Using the ultrafast Er:fiber technology, we generated extremely short (∼4 fs) femtosecond pulses in the 0.9–2.2 μm region and observed quantum interference currents in GaAs. (University of Konstanz)

     • Opt. Express, 31(7), 11649–11658 (2023). ：Starting from a 100 kHz Yb:KGW regenerative amplifier, we constructed a degenerate OPA that covers a broadband mid-infrared region (1.7–2.4 μm) and compressed down to ~2 optical cycles. (ISSP, The University of Tokyo) 

     • Applied Physics Express 17, 122006 (2024). ：We developed an OPA driven by a 10 mJ-class Ti:sapphire CPA system to produce several hundred μJ mid-IR pulses (∼3 μm), and discovered that high-order harmonic generation from water films is enhanced by pre-pulse irradiation. (ISSP, The University of Tokyo) 

     • Applied Physics Express, 2025：Visible OPA  requires pumping at ultraviolet such as 343 nm (third harmonic of a Yb:KGW laser), which is sometimes hard to handle because the relatively strong 2-photon absorption can damage the optics and nonlinear crystals in the system. We constructed a noncollinear OPA using a LiB₃O₅ crystal which has wide transparent window in the UV, thereby avoiding this problem and realizing the visible femtosecond pulse generation with a pulse duration as short as 11 fs. (ISSP, The University of Tokyo) 

(Last update: 19 April 2025)