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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 geometric phase of the constituting electronic bands: the Berry connection. This photocurrent, termed shift current, is expected to emerge on the timescale of primary photoexcitation process. We observe ultrafast evolution of the shift current in a prototypical ferroelectric semiconductor antimony sulfur iodide (SbSI) by detecting emitted terahertz electromagnetic waves. By sweeping the excitation photon energy across the bandgap, ultrafast electron dynamics as a source of terahertz emission abruptly changes its nature, reflecting a contribution of Berry connection on interband optical transition. The shift excitation carries a net charge flow and is followed by a swing over of the electron cloud on a subpicosecond timescale. Understanding these substantive characters of the shift current with the help of first-principles calculation will pave the way for its application to ultrafast sensors and solar cells. [ABSTRACT FROM AUTHOR]

A spontaneous photocurrent, termed a shift current, can flow in noncentrosymmetric bulk crystals due to the topological nature of the constituting electronic bands. A shift current with a less dissipative character may have remarkable advantages over the conventional drift photocurrent driven by a built-in potential or external electric field. We revisit the generation and transport of the shift current in a prototypical ferroelectric semiconductor SbSI near its band-gap energy. It is revealed that a switchable shift current is steadily generated by photoexcitation down to low temperatures, appears over a distance of millimeter range in a highly insulating bulk without noticeable decay, and largely exceeds the polarization charge in the sample, reflecting its Berry phase origin. [ABSTRACT FROM AUTHOR]