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.
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.
Anomalous and topological Hall effects can be controlled by an electric field
