Terahertz-Field-Induced Changes Of Electronic States Associated With A Polarization Modulation In Bifeo3

We performed terahertz-pulse-pump second-harmonic-generation (SHG)-probe and optical-reflectivity-probe measurements on a single crystal of a displacive-type ferroelectric, BiFeO3, to achieve an ultrafast control of the optical response by a terahertz pulse. By an application of a terahertz pulse with the amplitude of about 200kV/cm along the polarization direction, the SHG intensity was modulated completely along the electric-field waveform, suggesting that the relevant ionic motions are faster than the time resolution similar to 130 fs of the measurement as well as the period of the terahertz electric field. The electric-field-induced change of the reflectivity spectrum revealed that the intensities of d-d transitions in iron ions was strongly varied, while those of the charge-transfer transitions from oxygen-2p to iron-3d bands were almost unchanged. These results demonstrate that the electric field hardly modifies the wavefunction of valence electrons, but exclusively changes the distortions of iron-oxygen octahedrons. The latter effect shows that the octahedrons' distortions play dominant roles on the ferroelectric-polarization generation in BiFeO3.