Emergent Photodynamics Research Team

Principal Investigator

PI Name Naoki Ogawa
Degree D.Eng.
Title Team Leader
Brief Resume
2004D.Eng., University of Tokyo
2004Postdoctoral Associate, University of California at Irvine
2004Research Fellow of the Japan Society for the Promotion of Science
2006Project Assistant Professor, University of Tokyo
2008Assistant Professor, University of Tokyo
2012ASI Research Scientist, RIKEN
2013Senior Research Scientist, RIKEN Center for Emergent Matter Science
2015Unit Leader, Emergent Photodynamics Research Unit, RIKEN Center for Emergent Matter Science
2017JST PRESTO Researcher
2018Team Leader, Emergent Photodynamics Research Team, RIKEN Center for Emergent Matter Science (-present)
2020Guest Professor, University of Tokyo (-present)

Outline

Our team explores novel photodynamics of electron/spin/lattice in bulk crystals and at thin-film interfaces emerging via electron-correlation, strong spin-orbit interaction, and topology. Examples are ultrafast spectroscopy of shift current, generation of spin current mediated by photoexcited Dirac/Weyl electrons, and manipulation of topological magnetic orders. With a strong command of photons, we try to realize new photo-electric/magnetic effects in solids, and visualize spatiotemporal propagation of various elementally excitations at the sub-diffraction limit.

Research Fields

Physics, Engineering, Materials Science

Keywords

Strongly correlated electron system
Spin-orbit interaction
Ultrafast/broadband spectroscopy
Spintronics
Shift current

Results

Optical diode effect in second harmonic generation

In multiferroic materials where spatial-inversion and time-reversal symmetries are simultaneously broken, the optical response can change by reversing the direction of light propagation. This nonreciprocal effect has been realized in various linear optical responses, such as transmission, emission, scattering, and refraction. We investigate the nonreciprocal effects in nonlinear optical processes, specifically second harmonic generation (SHG) in CuB2O4. Generally, nonreciprocal effects are negligibly small, because their origin is an interference between magnetic and electric dipole transitions with the former is intrinsically much smaller than the latter. We found that the magnetic dipole transition in CuB2O4 can be enhanced extremely due to magnetic resonance, leading to the magnitude comparable to that of the electric dipole transition under non-resonant processes. As a result, the two transitions interfere with each other in the same phase and amplitude, resulting in an almost perfect nonlinear nonreciprocal effect with a 97% change in the SHG intensity. We also demonstrate that the direction with the larger SHG intensity can be controlled by reversing a magnetic field of only 10 mT.

(Left) Schematic illustration of nonreciprocal SHG. (Right) Magnetic field dependence of SHG intensity.

 

Ultrafast spectral dynamics of shift current

Shift current refers to the photocurrent in materials with broken inversion symmetry, originating from the spontaneous shift of electron clouds in real-space via the topological nature of the electronic bands. It is distinct from conventional photovoltaic effect where the interfaces of semiconductors are employed; shift current emerges in bulk crystals at ultrafast time-scale without much dissipation, in many cases accompanied by large open-circuit voltage exceeding the band gap energy. We have unveiled the ultrafast spectral dynamics of the shift current by detecting THz electromagnetic waves generated through its carrier motion. The shift current is found to appear faster than the experimental time-resolution (~100 fs) with a tensor response to the incoming photon polarization, and shows distinct time profile from that of the optical rectification. Importantly, the experimental shift current nicely compares with the spectra deduced from first-principles calculations based only on the crystal structures.

Schematic illustration of the THz emission via photoexcited shift current.

 

Photoinduced dynamics in topological spin textures

High-speed magnetic memories and photonic-magnonic interconnections will be realized by using the pulsed-optical-control of spins. For this purpose, the inverse-Faraday effect, one of the optomagnetic phenomena, is promising, where circularly-polarized laser pulses at non-absorbing photon energy can induce effective magnetic fields via strong spin-orbit interactions. We demonstrated that the collective dynamics of magnetic skyrmions, topologically-protected nano-scale spin vortices, can be characterized by using the inverse-Faraday excitation and time-resolved magneto-optics with sub-picosecond time-resolution. We also found that magnetoelastic waves, coupled propagation of magnon and phonon, can be excited in iron garnet films by photoexcitation. The time-resolved microscopy on the magnetoelastic wave revealed that this traveling spin excitation shows an attractive interaction with magnetic bubbles (skyrmions) and domain walls, whose efficiency strongly depends on the curvature of the local spin texture.

(a) Schematics for the impulsive Raman excitation.
(b) Rotation dynamics of magnetic skyrmions in Cu2OSeO3.
(c) Magneto-optical microscopy on photoexcited magnetoelastic waves.
(d) Optical manipulation of a magnetic bubble domain.

Members

Naoki Ogawa

Team Leader naoki.ogawa[at]riken.jp R

Manfred Fiebig

Senior Visiting Scientist

Shingo Toyoda

Special Postdoctoral Researcher

Ziqian Wang

Postdoctoral Researcher

Tsubasa Takagi

Student Trainee

Publications

  1. S. Toyoda, M. Fiebig, T. Arima, Y. Tokura, and N. Ogawa

    Nonreciprocal second harmonic generation in a magnetoelectric material

    Sci. Adv. 7, eabe2793 (2021)
  2. M. Sotome, T. Ogaki, K. Takimiya, and N. Ogawa

    Highly-efficient terahertz emission from hydrogen-bonded single molecular crystal 4-nitro-2,5-bis(phenylethynyl)aniline

    Opt. Express 29, 10048 (2021)
  3. N. Ogawa, L. Köhler, M. Garst, S. Toyoda, S. Seki, and Y. Tokura

    Nonreciprocity of spin-waves in the conical helix state

    Proc. Natl. Acad. Sci. U.S.A. 118, e2022927118 (2021)
  4. M. Sotome, M. Nakamura, J. Fujioka, M. Ogino, Y. Kaneko, T. Morimoto, Y. Zhang, M. Kawasaki, N. Nagaosa, Y. Tokura, and N. Ogawa

    Spectral dynamics of shift current in ferroelectric semiconductor SbSI

    Proc. Natl. Acad. Sci. U.S.A. 116, 1929 (2019)
  5. N. Ogawa, R. Yoshimi, K. Yasuda, A. Tsukazaki, M. Kawasaki, and Y. Tokura

    Zero-bias photocurrent in ferromagnetic topological insulator

    Nat. Commun. 7, 12246 (2016)

Articles