Ultrafast Control of the Polarity of BiCoO3 by Orbital Excitation as Investigated by Femtosecond Spectroscopy

BiCoO3 is a perovskite-type cobalt oxide with a polar structure. We investigate the dynamics of its polar state by using photoexcitation with femtosecond visible and terahertz pulses. The intensity of the second-harmonic light, caused by a polar structure without inversion symmetry, is enhanced by more than 50% by irradiating the terahertz pulse showing an ultrafast response following the femtosecond terahertz pulse. By contrast, it is suppressed by more than 60% by exciting visible light with a 100-fs pulse width at room temperature. These results suggest not only an important role of the orbital excitation in the Co3+ ion but also a key to improving the nonlinear optical response, e.g., a figure of merit in nonlinear crystals on the time scale of a femtosecond.