  Photoexcitation in solids brings about transitions of electrons/holes between
different electronic bands. If the solid lacks an inversion symmetry, these
electronic transitions support spontaneous photocurrent due to the topological
character of the constituting electronic bands; the Berry connection. This
photocurrent, termed shift current, is expected to emerge on the time-scale of
primary photoexcitation process. We observed ultrafast time evolution of the
shift current in a prototypical ferroelectric semiconductor by detecting
emitted terahertz electromagnetic waves. By sweeping the excitation photon
energy across the band gap, ultrafast electron dynamics as a source of
terahertz emission abruptly changes its nature, reflecting a contribution of
Berry connection upon interband optical transition. The shift excitation
carries a net charge flow, and is followed by a swing-over of the electron
cloud on the sub-picosecond time-scale of electron-phonon interaction.
Understanding these substantive characters of the shift current will pave the
way for its application to ultrafast sensors and solar cells.
