Emergent Spectroscopy Research Unit

Principal Investigator

PI Name Youtarou Takahashi
Degree Ph.D.
Title Unit Leader
Brief Resume
2007 Ph.D., The University of Tokyo
2007 Researcher, Tokura Multiferroic Project, ERATO, Japan Science and Technology Agency
2011 Lecturer, Quantum-Phase Electronics Center, School of Engineering, The University of Tokyo
2014 Associate Professor, Quantum-Phase Electronics Center, School of Engineering, The University of Tokyo
2014 Unit Leader, Emergent Spectroscopy Research Unit, RIKEN Center for Emergent Matter Science (-present)
2016 Associate Professor, Quantum-Phase Electronics Center, School of Engineering, The University of Tokyo (-present)


Light-matter interaction has been a fundamental issue for the condensed matter physics. Optical spectroscopy plays an important role for the various researches, and the emergent phenomena in condensed matter provide novel optical responses. Our unit focuses on the light-matter interaction on the strongly correlated electron systems as listed below. (1) Magnetoelectric optical effect driven by the cross-coupling between the magnetism and dielectric properties in matter. (2) Optical control of the magnetism and dielectric properties. (3) Novel optical responses derived from the topology in condensed matter. We are pushing forward scientific and technological developments with these researches.

Research Fields

Physics, Materials Science


Strongly correlated electron system
Terahertz spectroscopy
Ultrafast spectroscopy
Non-reciprocal effect


Magnetoelectric optical effect with electromagnons in helimagnet

Helical spin orders exhibit the magnetically induced ferroelectricity, resulting in the concept of multiferroics with strong magnetoelectric coupling.  In addition to the ferroelectric polarization, the helical spin orders possess the chirality; the right-handed and left-handed spin habits are distinguished in terms of chirality.  The strong magnetoelectric coupling generates the novel spin excitation referred to as electromagnon, which is the magnon endowed with the electric activity, in terahertz region.  We clarified that the strong magnetoelectric coupling of the electromagnon resonance causes the nonreciprocal optical effect in general. We also demonstrated the electric field control of chirality by using the helical spin order with ferroelectricity and chirality. On the electromagnon resonance, the reversal of the natural optical activity, which is most fundamental nature of chiral matter, is observed. The control of optical activity may lead to the novel chiral optics.


Control of natural optical activity induced by helical spin-order


Youtarou Takahashi

Unit Leader youtarou.takahashi[at]riken.jp

Yoshihiro Okamura

Visiting Scientist


  1. Y. D. Kato, Y. Okamura, S. Minami, R. Fujimura, M. Mogi, R. Yoshimi, A. Tsukazaki, K. S. Takahashi, M. Kawasaki, R. Arita, Y. Tokura, and Y. Takahashi

    Optical anomalous Hall effect enhanced by flat bands in ferromagnetic van der Waals semimetal

    npj Quantum Mater. 7, 73 (2022)
  2. Y. Okamura, T. Morimoto, N. Ogawa, Y. Kaneko, G-Y. Guo, M. Kawasaki, N. Naogaosa, Y. Tokura, and Y. Takahashi

    Photovoltaic effect by soft phonon excitation

    Proc. Natl. Acad. Sci. U.S.A. 119, e2122313119 (2022)
  3. S. Iguchi, R. Masuda, S. Seki, Y. Tokura, and Y. Takahashi

    Enhanced gyrotropic birefringence and natural optical activity on electromagnon resonance in a helimagnet

    Nat. Commun. 12, 6674 (2021)
  4. Y. Hayashi, Y. Okamura, N. Kanazawa, T. Yu, T. Koretsune, R. Arita, A. Tsukazaki, M. Ichikawa, M. Kawasaki, Y. Tokura, and Y. Takahashi

    Magneto-optical spectroscopy on Weyl nodes for anomalous and topological Hall effects in chiral MnGe

    Nat. Commun. 12, 5974 (2021)
  5. R. Masuda, Y. Kaneko, Y. Tokura, and Y. Takahashi

    Electric field control of natural optical activity in a multiferroic helimagnet

    Science 372, 496+ (2021)


  • Dec 08, 2017 RIKEN RESEARCH Solar cells with a quantum shift
    A quantum-mechanical way of generating photocurrents may help solar devices overcome existing inefficiencies
  • Oct 17, 2014 RIKEN RESEARCH Light finds a one-way street
    A multiferroic material displays a novel spin structure that allows light to travel in only one direction


The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 Japan