SignificanceShift current is one of the bulk photovoltaic phenomena in the materials without inversion symmetry, originating from the geometric Berry phase of the constituting electron bands. This concept of photocurrent generation based on the real-space shift of the electron cloud on the short timescale of optical transition is distinct from that of conventional p–n junction photovoltaics, where the carriers are driven by the built-in Coulomb potential. We experimentally demonstrate for a ferroelectric polar semiconductor how the subpicosecond charge swing on the relevant chemical bond changes its dynamics while scanning the excitation photon energy across the bandgap. On the interband photoexcitation above the bandgap, a finite net charge flow is produced along the electrically polar direction.

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

Using a terahertz-radiation imaging, visualizations of ferroelectric domains were made in a room-temperature organic ferroelectric, croconic acid. In as-grown crystals, observed are ferroelectric domains with sizes larger than 50-μm square, which are separated by both 180° and tail-to-tail domain walls (DWs). By applying an electric field along c axis (the polarization direction), a pair of 180° DWs is generated and an each 180° DW oppositely propagates along a axis, resulting in a single domain. By cyclic applications of electric fields, a pair of 180° DWs repeatedly emerges, while no tail-to-tail DWs appear. We discuss the usefulness of the terahertz-radiation imaging as well as the observed unique DW dynamics.

We report on the observation of terahertz radiation in a non-centrosymmetric insulating oxide, paratellurite (α-TeO2) by irradiation of a femtosecond laser pulse at room temperature. In the power spectrum of the terahertz radiation, an intensity fringe pattern with a period of ∼ 0.25 THz shows up below 3 THz. It can be reproduced by taking into account the effective generation length for the terahertz radiation with a poor phase-matching condition. In addition, a temporal oscillation component appears in the radiated terahertz wave with a frequency of ∼ 3.71 THz, which is in good agreement with the center frequency of the Raman active longitudinal optical (LO) E mode. On the basis of comprehensive polarized optical and Raman spectroscopic studies, we explain the generation mechanism of the temporal oscillation component in terms of the coherent phonon generation via impulsive stimulated Raman scattering.

Linear and nonlinear optical responses in a molecular crystal, croconic acid, showing electronic-type ferroelectricity were studied by reflection and second harmonic generation spectroscopy. The second-order nonlinear susceptibility χ(2) was very large, exceeding 10−6 esu in the near-infrared region. The enhancement of χ(2) was attributed to the large dipole moment of the lowest π–π* transition and the large difference between the molecular dipole moments for the ground state and the photoexcited state. We deduced the molecular orbitals (MOs) and dipole moments responsible for the large χ(2) by comparing the experimental optical parameters and MO calculation results based upon density functional theory.